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Is it possible to convert a double quoted string with template literal to tilde string to assign values? [duplicate]

Is it possible to create a template string as a usual string,
let a = "b:${b}";
and then convert it into a template string,
let b = 10;
console.log(a.template()); // b:10
without eval, new Function and other means of dynamic code generation?

In my project I've created something like this with ES6:
String.prototype.interpolate = function(params) {
const names = Object.keys(params);
const vals = Object.values(params);
return new Function(...names, `return \`${this}\`;`)(...vals);
}
const template = 'Example text: ${text}';
const result = template.interpolate({
text: 'Foo Boo'
});
console.log(result);

As your template string must get reference to the b variable dynamically (in runtime), so the answer is: NO, it's impossible to do it without dynamic code generation.
But, with eval it's pretty simple:
let tpl = eval('`'+a+'`');

No, there is not a way to do this without dynamic code generation.
However, I have created a function which will turn a regular string into a function which can be provided with a map of values, using template strings internally.
Generate Template String Gist
/**
* Produces a function which uses template strings to do simple interpolation from objects.
*
* Usage:
* var makeMeKing = generateTemplateString('${name} is now the king of ${country}!');
*
* console.log(makeMeKing({ name: 'Bryan', country: 'Scotland'}));
* // Logs 'Bryan is now the king of Scotland!'
*/
var generateTemplateString = (function(){
var cache = {};
function generateTemplate(template){
var fn = cache[template];
if (!fn){
// Replace ${expressions} (etc) with ${map.expressions}.
var sanitized = template
.replace(/\$\{([\s]*[^;\s\{]+[\s]*)\}/g, function(_, match){
return `\$\{map.${match.trim()}\}`;
})
// Afterwards, replace anything that's not ${map.expressions}' (etc) with a blank string.
.replace(/(\$\{(?!map\.)[^}]+\})/g, '');
fn = Function('map', `return \`${sanitized}\``);
}
return fn;
}
return generateTemplate;
})();
Usage:
var kingMaker = generateTemplateString('${name} is king!');
console.log(kingMaker({name: 'Bryan'}));
// Logs 'Bryan is king!' to the console.
Hope this helps somebody. If you find a problem with the code, please be so kind as to update the Gist.

What you're asking for here:
//non working code quoted from the question
let b=10;
console.log(a.template());//b:10
is exactly equivalent (in terms of power and, er, safety) to eval: the ability to take a string containing code and execute that code; and also the ability for the executed code to see local variables in the caller's environment.
There is no way in JS for a function to see local variables in its caller, unless that function is eval(). Even Function() can't do it.
When you hear there's something called "template strings" coming to JavaScript, it's natural to assume it's a built-in template library, like Mustache. It isn't. It's mainly just string interpolation and multiline strings for JS. I think this is going to be a common misconception for a while, though. :(

There are many good solutions posted here, but none yet which utilizes the ES6 String.raw method. Here is my contriubution. It has an important limitation in that it will only accept properties from a passed in object, meaning no code execution in the template will work.
function parseStringTemplate(str, obj) {
let parts = str.split(/\$\{(?!\d)[\wæøåÆØÅ]*\}/);
let args = str.match(/[^{\}]+(?=})/g) || [];
let parameters = args.map(argument => obj[argument] || (obj[argument] === undefined ? "" : obj[argument]));
return String.raw({ raw: parts }, ...parameters);
}
let template = "Hello, ${name}! Are you ${age} years old?";
let values = { name: "John Doe", age: 18 };
parseStringTemplate(template, values);
// output: Hello, John Doe! Are you 18 years old?
Split string into non-argument textual parts. See regex.
parts: ["Hello, ", "! Are you ", " years old?"]
Split string into property names. Empty array if match fails.
args: ["name", "age"]
Map parameters from obj by property name. Solution is limited by shallow one level mapping. Undefined values are substituted with an empty string, but other falsy values are accepted.
parameters: ["John Doe", 18]
Utilize String.raw(...) and return result.

TLDR:
https://jsfiddle.net/bj89zntu/1/
Everyone seems to be worried about accessing variables. Why not just pass them? I'm sure it won't be too hard to get the variable context in the caller and pass it down. Use
ninjagecko's answer to get the props from obj.
function renderString(str,obj){
return str.replace(/\$\{(.+?)\}/g,(match,p1)=>{return index(obj,p1)})
}
Here is the full code:
function index(obj,is,value) {
if (typeof is == 'string')
is=is.split('.');
if (is.length==1 && value!==undefined)
return obj[is[0]] = value;
else if (is.length==0)
return obj;
else
return index(obj[is[0]],is.slice(1), value);
}
function renderString(str,obj){
return str.replace(/\$\{.+?\}/g,(match)=>{return index(obj,match)})
}
renderString('abc${a}asdas',{a:23,b:44}) //abc23asdas
renderString('abc${a.c}asdas',{a:{c:22,d:55},b:44}) //abc22asdas

The issue here is to have a function that has access to the variables of its caller. This is why we see direct eval being used for template processing. A possible solution would be to generate a function taking formal parameters named by a dictionary's properties, and calling it with the corresponding values in the same order. An alternative way would be to have something simple as this:
var name = "John Smith";
var message = "Hello, my name is ${name}";
console.log(new Function('return `' + message + '`;')());
And for anyone using Babel compiler we need to create closure which remembers the environment in which it was created:
console.log(new Function('name', 'return `' + message + '`;')(name));

I liked s.meijer's answer and wrote my own version based on his:
function parseTemplate(template, map, fallback) {
return template.replace(/\$\{[^}]+\}/g, (match) =>
match
.slice(2, -1)
.trim()
.split(".")
.reduce(
(searchObject, key) => searchObject[key] || fallback || match,
map
)
);
}

Similar to Daniel's answer (and s.meijer's gist) but more readable:
const regex = /\${[^{]+}/g;
export default function interpolate(template, variables, fallback) {
return template.replace(regex, (match) => {
const path = match.slice(2, -1).trim();
return getObjPath(path, variables, fallback);
});
}
//get the specified property or nested property of an object
function getObjPath(path, obj, fallback = '') {
return path.split('.').reduce((res, key) => res[key] || fallback, obj);
}
Note: This slightly improves s.meijer's original, since it won't match things like ${foo{bar} (the regex only allows non-curly brace characters inside ${ and }).
UPDATE: I was asked for an example using this, so here you go:
const replacements = {
name: 'Bob',
age: 37
}
interpolate('My name is ${name}, and I am ${age}.', replacements)

#Mateusz Moska, solution works great, but when i used it in React Native(build mode), it throws an error: Invalid character '`', though it works when i run it in debug mode.
So i wrote down my own solution using regex.
String.prototype.interpolate = function(params) {
let template = this
for (let key in params) {
template = template.replace(new RegExp('\\$\\{' + key + '\\}', 'g'), params[key])
}
return template
}
const template = 'Example text: ${text}',
result = template.interpolate({
text: 'Foo Boo'
})
console.log(result)
Demo: https://es6console.com/j31pqx1p/
NOTE: Since I don't know the root cause of an issue, i raised a ticket in react-native repo, https://github.com/facebook/react-native/issues/14107, so that once they can able to fix/guide me about the same :)

You can use the string prototype, for example
String.prototype.toTemplate=function(){
return eval('`'+this+'`');
}
//...
var a="b:${b}";
var b=10;
console.log(a.toTemplate());//b:10
But the answer of the original question is no way.

I required this method with support for Internet Explorer. It turned out the back ticks aren't supported by even IE11. Also; using eval or it's equivalent Function doesn't feel right.
For the one that notice; I also use backticks, but these ones are removed by compilers like babel. The methods suggested by other ones, depend on them on run-time. As said before; this is an issue in IE11 and lower.
So this is what I came up with:
function get(path, obj, fb = `$\{${path}}`) {
return path.split('.').reduce((res, key) => res[key] || fb, obj);
}
function parseTpl(template, map, fallback) {
return template.replace(/\$\{.+?}/g, (match) => {
const path = match.substr(2, match.length - 3).trim();
return get(path, map, fallback);
});
}
Example output:
const data = { person: { name: 'John', age: 18 } };
parseTpl('Hi ${person.name} (${person.age})', data);
// output: Hi John (18)
parseTpl('Hello ${person.name} from ${person.city}', data);
// output: Hello John from ${person.city}
parseTpl('Hello ${person.name} from ${person.city}', data, '-');
// output: Hello John from -

I currently can't comment on existing answers so I am unable to directly comment on Bryan Raynor's excellent response. Thus, this response is going to update his answer with a slight correction.
In short, his function fails to actually cache the created function, so it will always recreate, regardless of whether it's seen the template before. Here is the corrected code:
/**
* Produces a function which uses template strings to do simple interpolation from objects.
*
* Usage:
* var makeMeKing = generateTemplateString('${name} is now the king of ${country}!');
*
* console.log(makeMeKing({ name: 'Bryan', country: 'Scotland'}));
* // Logs 'Bryan is now the king of Scotland!'
*/
var generateTemplateString = (function(){
var cache = {};
function generateTemplate(template){
var fn = cache[template];
if (!fn){
// Replace ${expressions} (etc) with ${map.expressions}.
var sanitized = template
.replace(/\$\{([\s]*[^;\s\{]+[\s]*)\}/g, function(_, match){
return `\$\{map.${match.trim()}\}`;
})
// Afterwards, replace anything that's not ${map.expressions}' (etc) with a blank string.
.replace(/(\$\{(?!map\.)[^}]+\})/g, '');
fn = cache[template] = Function('map', `return \`${sanitized}\``);
}
return fn;
};
return generateTemplate;
})();

Still dynamic but seems more controlled than just using a naked eval:
const vm = require('vm')
const moment = require('moment')
let template = '### ${context.hours_worked[0].value} \n Hours worked \n #### ${Math.abs(context.hours_worked_avg_diff[0].value)}% ${fns.gt0(context.hours_worked_avg_diff[0].value, "more", "less")} than usual on ${fns.getDOW(new Date())}'
let context = {
hours_worked:[{value:10}],
hours_worked_avg_diff:[{value:10}],
}
function getDOW(now) {
return moment(now).locale('es').format('dddd')
}
function gt0(_in, tVal, fVal) {
return _in >0 ? tVal: fVal
}
function templateIt(context, template) {
const script = new vm.Script('`'+template+'`')
return script.runInNewContext({context, fns:{getDOW, gt0 }})
}
console.log(templateIt(context, template))
https://repl.it/IdVt/3

I made my own solution doing a type with a description as a function
export class Foo {
...
description?: Object;
...
}
let myFoo:Foo = {
...
description: (a,b) => `Welcome ${a}, glad to see you like the ${b} section`.
...
}
and so doing:
let myDescription = myFoo.description('Bar', 'bar');

I came up with this implementation and it works like a charm.
function interpolateTemplate(template: string, args: any): string {
return Object.entries(args).reduce(
(result, [arg, val]) => result.replace(`$\{${arg}}`, `${val}`),
template,
)
}
const template = 'This is an example: ${name}, ${age} ${email}'
console.log(interpolateTemplate(template,{name:'Med', age:'20', email:'example#abc.com'}))
You could raise an error if arg is not found in template

This solution works without ES6:
function render(template, opts) {
return new Function(
'return new Function (' + Object.keys(opts).reduce((args, arg) => args += '\'' + arg + '\',', '') + '\'return `' + template.replace(/(^|[^\\])'/g, '$1\\\'') + '`;\'' +
').apply(null, ' + JSON.stringify(Object.keys(opts).reduce((vals, key) => vals.push(opts[key]) && vals, [])) + ');'
)();
}
render("hello ${ name }", {name:'mo'}); // "hello mo"
Note: the Function constructor is always created in the global scope, which could potentially cause global variables to be overwritten by the template, e.g. render("hello ${ someGlobalVar = 'some new value' }", {name:'mo'});

You should try this tiny JS module, by Andrea Giammarchi, from github :
https://github.com/WebReflection/backtick-template
/*! (C) 2017 Andrea Giammarchi - MIT Style License */
function template(fn, $str, $object) {'use strict';
var
stringify = JSON.stringify,
hasTransformer = typeof fn === 'function',
str = hasTransformer ? $str : fn,
object = hasTransformer ? $object : $str,
i = 0, length = str.length,
strings = i < length ? [] : ['""'],
values = hasTransformer ? [] : strings,
open, close, counter
;
while (i < length) {
open = str.indexOf('${', i);
if (-1 < open) {
strings.push(stringify(str.slice(i, open)));
open += 2;
close = open;
counter = 1;
while (close < length) {
switch (str.charAt(close++)) {
case '}': counter -= 1; break;
case '{': counter += 1; break;
}
if (counter < 1) {
values.push('(' + str.slice(open, close - 1) + ')');
break;
}
}
i = close;
} else {
strings.push(stringify(str.slice(i)));
i = length;
}
}
if (hasTransformer) {
str = 'function' + (Math.random() * 1e5 | 0);
if (strings.length === values.length) strings.push('""');
strings = [
str,
'with(this)return ' + str + '([' + strings + ']' + (
values.length ? (',' + values.join(',')) : ''
) + ')'
];
} else {
strings = ['with(this)return ' + strings.join('+')];
}
return Function.apply(null, strings).apply(
object,
hasTransformer ? [fn] : []
);
}
template.asMethod = function (fn, object) {'use strict';
return typeof fn === 'function' ?
template(fn, this, object) :
template(this, fn);
};
Demo (all the following tests return true):
const info = 'template';
// just string
`some ${info}` === template('some ${info}', {info});
// passing through a transformer
transform `some ${info}` === template(transform, 'some ${info}', {info});
// using it as String method
String.prototype.template = template.asMethod;
`some ${info}` === 'some ${info}'.template({info});
transform `some ${info}` === 'some ${info}'.template(transform, {info});

Faz assim (This way):
let a = 'b:${this.b}'
let b = 10
function template(templateString, templateVars) {
return new Function('return `' + templateString + '`').call(templateVars)
}
result.textContent = template(a, {b})
<b id=result></b>

Since we're reinventing the wheel on something that would be a lovely feature in javascript.
I use eval(), which is not secure, but javascript is not secure. I readily admit that I'm not excellent with javascript, but I had a need, and I needed an answer so I made one.
I chose to stylize my variables with an # rather than an $, particularly because I want to use the multiline feature of literals without evaluating til it's ready. So variable syntax is #{OptionalObject.OptionalObjectN.VARIABLE_NAME}
I am no javascript expert, so I'd gladly take advice on improvement but...
var prsLiteral, prsRegex = /\#\{(.*?)(?!\#\{)\}/g
for(i = 0; i < myResultSet.length; i++) {
prsLiteral = rt.replace(prsRegex,function (match,varname) {
return eval(varname + "[" + i + "]");
// you could instead use return eval(varname) if you're not looping.
})
console.log(prsLiteral);
}
A very simple implementation follows
myResultSet = {totalrecords: 2,
Name: ["Bob", "Stephanie"],
Age: [37,22]};
rt = `My name is #{myResultSet.Name}, and I am #{myResultSet.Age}.`
var prsLiteral, prsRegex = /\#\{(.*?)(?!\#\{)\}/g
for(i = 0; i < myResultSet.totalrecords; i++) {
prsLiteral = rt.replace(prsRegex,function (match,varname) {
return eval(varname + "[" + i + "]");
// you could instead use return eval(varname) if you're not looping.
})
console.log(prsLiteral);
}
In my actual implementation, I choose to use #{{variable}}. One more set of braces. Absurdly unlikely to encounter that unexpectedly. The regex for that would look like /\#\{\{(.*?)(?!\#\{\{)\}\}/g
To make that easier to read
\#\{\{ # opening sequence, #{{ literally.
(.*?) # capturing the variable name
# ^ captures only until it reaches the closing sequence
(?! # negative lookahead, making sure the following
# ^ pattern is not found ahead of the current character
\#\{\{ # same as opening sequence, if you change that, change this
)
\}\} # closing sequence.
If you're not experienced with regex, a pretty safe rule is to escape every non-alphanumeric character, and don't ever needlessly escape letters as many escaped letters have special meaning to virtually all flavors of regex.

You can refer to this solution
const interpolate = (str) =>
new Function(`return \`${new String(str)}\`;`)();
const foo = 'My';
const obj = {
text: 'Hanibal Lector',
firstNum: 1,
secondNum: 2
}
const str = "${foo} name is : ${obj.text}. sum = ${obj.firstNum} + ${obj.secondNum} = ${obj.firstNum + obj.secondNum}";
console.log(interpolate(str));

I realize I am late to the game, but you could:
const a = (b) => `b:${b}`;
let b = 10;
console.log(a(b)); // b:10

Javascript string interpolation not working from text input [duplicate]

Is it possible to create a template string as a usual string,
let a = "b:${b}";
and then convert it into a template string,
let b = 10;
console.log(a.template()); // b:10
without eval, new Function and other means of dynamic code generation?

In my project I've created something like this with ES6:
String.prototype.interpolate = function(params) {
const names = Object.keys(params);
const vals = Object.values(params);
return new Function(...names, `return \`${this}\`;`)(...vals);
}
const template = 'Example text: ${text}';
const result = template.interpolate({
text: 'Foo Boo'
});
console.log(result);

As your template string must get reference to the b variable dynamically (in runtime), so the answer is: NO, it's impossible to do it without dynamic code generation.
But, with eval it's pretty simple:
let tpl = eval('`'+a+'`');

No, there is not a way to do this without dynamic code generation.
However, I have created a function which will turn a regular string into a function which can be provided with a map of values, using template strings internally.
Generate Template String Gist
/**
* Produces a function which uses template strings to do simple interpolation from objects.
*
* Usage:
* var makeMeKing = generateTemplateString('${name} is now the king of ${country}!');
*
* console.log(makeMeKing({ name: 'Bryan', country: 'Scotland'}));
* // Logs 'Bryan is now the king of Scotland!'
*/
var generateTemplateString = (function(){
var cache = {};
function generateTemplate(template){
var fn = cache[template];
if (!fn){
// Replace ${expressions} (etc) with ${map.expressions}.
var sanitized = template
.replace(/\$\{([\s]*[^;\s\{]+[\s]*)\}/g, function(_, match){
return `\$\{map.${match.trim()}\}`;
})
// Afterwards, replace anything that's not ${map.expressions}' (etc) with a blank string.
.replace(/(\$\{(?!map\.)[^}]+\})/g, '');
fn = Function('map', `return \`${sanitized}\``);
}
return fn;
}
return generateTemplate;
})();
Usage:
var kingMaker = generateTemplateString('${name} is king!');
console.log(kingMaker({name: 'Bryan'}));
// Logs 'Bryan is king!' to the console.
Hope this helps somebody. If you find a problem with the code, please be so kind as to update the Gist.

What you're asking for here:
//non working code quoted from the question
let b=10;
console.log(a.template());//b:10
is exactly equivalent (in terms of power and, er, safety) to eval: the ability to take a string containing code and execute that code; and also the ability for the executed code to see local variables in the caller's environment.
There is no way in JS for a function to see local variables in its caller, unless that function is eval(). Even Function() can't do it.
When you hear there's something called "template strings" coming to JavaScript, it's natural to assume it's a built-in template library, like Mustache. It isn't. It's mainly just string interpolation and multiline strings for JS. I think this is going to be a common misconception for a while, though. :(

There are many good solutions posted here, but none yet which utilizes the ES6 String.raw method. Here is my contriubution. It has an important limitation in that it will only accept properties from a passed in object, meaning no code execution in the template will work.
function parseStringTemplate(str, obj) {
let parts = str.split(/\$\{(?!\d)[\wæøåÆØÅ]*\}/);
let args = str.match(/[^{\}]+(?=})/g) || [];
let parameters = args.map(argument => obj[argument] || (obj[argument] === undefined ? "" : obj[argument]));
return String.raw({ raw: parts }, ...parameters);
}
let template = "Hello, ${name}! Are you ${age} years old?";
let values = { name: "John Doe", age: 18 };
parseStringTemplate(template, values);
// output: Hello, John Doe! Are you 18 years old?
Split string into non-argument textual parts. See regex.
parts: ["Hello, ", "! Are you ", " years old?"]
Split string into property names. Empty array if match fails.
args: ["name", "age"]
Map parameters from obj by property name. Solution is limited by shallow one level mapping. Undefined values are substituted with an empty string, but other falsy values are accepted.
parameters: ["John Doe", 18]
Utilize String.raw(...) and return result.

TLDR:
https://jsfiddle.net/bj89zntu/1/
Everyone seems to be worried about accessing variables. Why not just pass them? I'm sure it won't be too hard to get the variable context in the caller and pass it down. Use
ninjagecko's answer to get the props from obj.
function renderString(str,obj){
return str.replace(/\$\{(.+?)\}/g,(match,p1)=>{return index(obj,p1)})
}
Here is the full code:
function index(obj,is,value) {
if (typeof is == 'string')
is=is.split('.');
if (is.length==1 && value!==undefined)
return obj[is[0]] = value;
else if (is.length==0)
return obj;
else
return index(obj[is[0]],is.slice(1), value);
}
function renderString(str,obj){
return str.replace(/\$\{.+?\}/g,(match)=>{return index(obj,match)})
}
renderString('abc${a}asdas',{a:23,b:44}) //abc23asdas
renderString('abc${a.c}asdas',{a:{c:22,d:55},b:44}) //abc22asdas

The issue here is to have a function that has access to the variables of its caller. This is why we see direct eval being used for template processing. A possible solution would be to generate a function taking formal parameters named by a dictionary's properties, and calling it with the corresponding values in the same order. An alternative way would be to have something simple as this:
var name = "John Smith";
var message = "Hello, my name is ${name}";
console.log(new Function('return `' + message + '`;')());
And for anyone using Babel compiler we need to create closure which remembers the environment in which it was created:
console.log(new Function('name', 'return `' + message + '`;')(name));

I liked s.meijer's answer and wrote my own version based on his:
function parseTemplate(template, map, fallback) {
return template.replace(/\$\{[^}]+\}/g, (match) =>
match
.slice(2, -1)
.trim()
.split(".")
.reduce(
(searchObject, key) => searchObject[key] || fallback || match,
map
)
);
}

Similar to Daniel's answer (and s.meijer's gist) but more readable:
const regex = /\${[^{]+}/g;
export default function interpolate(template, variables, fallback) {
return template.replace(regex, (match) => {
const path = match.slice(2, -1).trim();
return getObjPath(path, variables, fallback);
});
}
//get the specified property or nested property of an object
function getObjPath(path, obj, fallback = '') {
return path.split('.').reduce((res, key) => res[key] || fallback, obj);
}
Note: This slightly improves s.meijer's original, since it won't match things like ${foo{bar} (the regex only allows non-curly brace characters inside ${ and }).
UPDATE: I was asked for an example using this, so here you go:
const replacements = {
name: 'Bob',
age: 37
}
interpolate('My name is ${name}, and I am ${age}.', replacements)

#Mateusz Moska, solution works great, but when i used it in React Native(build mode), it throws an error: Invalid character '`', though it works when i run it in debug mode.
So i wrote down my own solution using regex.
String.prototype.interpolate = function(params) {
let template = this
for (let key in params) {
template = template.replace(new RegExp('\\$\\{' + key + '\\}', 'g'), params[key])
}
return template
}
const template = 'Example text: ${text}',
result = template.interpolate({
text: 'Foo Boo'
})
console.log(result)
Demo: https://es6console.com/j31pqx1p/
NOTE: Since I don't know the root cause of an issue, i raised a ticket in react-native repo, https://github.com/facebook/react-native/issues/14107, so that once they can able to fix/guide me about the same :)

You can use the string prototype, for example
String.prototype.toTemplate=function(){
return eval('`'+this+'`');
}
//...
var a="b:${b}";
var b=10;
console.log(a.toTemplate());//b:10
But the answer of the original question is no way.

I required this method with support for Internet Explorer. It turned out the back ticks aren't supported by even IE11. Also; using eval or it's equivalent Function doesn't feel right.
For the one that notice; I also use backticks, but these ones are removed by compilers like babel. The methods suggested by other ones, depend on them on run-time. As said before; this is an issue in IE11 and lower.
So this is what I came up with:
function get(path, obj, fb = `$\{${path}}`) {
return path.split('.').reduce((res, key) => res[key] || fb, obj);
}
function parseTpl(template, map, fallback) {
return template.replace(/\$\{.+?}/g, (match) => {
const path = match.substr(2, match.length - 3).trim();
return get(path, map, fallback);
});
}
Example output:
const data = { person: { name: 'John', age: 18 } };
parseTpl('Hi ${person.name} (${person.age})', data);
// output: Hi John (18)
parseTpl('Hello ${person.name} from ${person.city}', data);
// output: Hello John from ${person.city}
parseTpl('Hello ${person.name} from ${person.city}', data, '-');
// output: Hello John from -

I currently can't comment on existing answers so I am unable to directly comment on Bryan Raynor's excellent response. Thus, this response is going to update his answer with a slight correction.
In short, his function fails to actually cache the created function, so it will always recreate, regardless of whether it's seen the template before. Here is the corrected code:
/**
* Produces a function which uses template strings to do simple interpolation from objects.
*
* Usage:
* var makeMeKing = generateTemplateString('${name} is now the king of ${country}!');
*
* console.log(makeMeKing({ name: 'Bryan', country: 'Scotland'}));
* // Logs 'Bryan is now the king of Scotland!'
*/
var generateTemplateString = (function(){
var cache = {};
function generateTemplate(template){
var fn = cache[template];
if (!fn){
// Replace ${expressions} (etc) with ${map.expressions}.
var sanitized = template
.replace(/\$\{([\s]*[^;\s\{]+[\s]*)\}/g, function(_, match){
return `\$\{map.${match.trim()}\}`;
})
// Afterwards, replace anything that's not ${map.expressions}' (etc) with a blank string.
.replace(/(\$\{(?!map\.)[^}]+\})/g, '');
fn = cache[template] = Function('map', `return \`${sanitized}\``);
}
return fn;
};
return generateTemplate;
})();

Still dynamic but seems more controlled than just using a naked eval:
const vm = require('vm')
const moment = require('moment')
let template = '### ${context.hours_worked[0].value} \n Hours worked \n #### ${Math.abs(context.hours_worked_avg_diff[0].value)}% ${fns.gt0(context.hours_worked_avg_diff[0].value, "more", "less")} than usual on ${fns.getDOW(new Date())}'
let context = {
hours_worked:[{value:10}],
hours_worked_avg_diff:[{value:10}],
}
function getDOW(now) {
return moment(now).locale('es').format('dddd')
}
function gt0(_in, tVal, fVal) {
return _in >0 ? tVal: fVal
}
function templateIt(context, template) {
const script = new vm.Script('`'+template+'`')
return script.runInNewContext({context, fns:{getDOW, gt0 }})
}
console.log(templateIt(context, template))
https://repl.it/IdVt/3

I made my own solution doing a type with a description as a function
export class Foo {
...
description?: Object;
...
}
let myFoo:Foo = {
...
description: (a,b) => `Welcome ${a}, glad to see you like the ${b} section`.
...
}
and so doing:
let myDescription = myFoo.description('Bar', 'bar');

I came up with this implementation and it works like a charm.
function interpolateTemplate(template: string, args: any): string {
return Object.entries(args).reduce(
(result, [arg, val]) => result.replace(`$\{${arg}}`, `${val}`),
template,
)
}
const template = 'This is an example: ${name}, ${age} ${email}'
console.log(interpolateTemplate(template,{name:'Med', age:'20', email:'example#abc.com'}))
You could raise an error if arg is not found in template

This solution works without ES6:
function render(template, opts) {
return new Function(
'return new Function (' + Object.keys(opts).reduce((args, arg) => args += '\'' + arg + '\',', '') + '\'return `' + template.replace(/(^|[^\\])'/g, '$1\\\'') + '`;\'' +
').apply(null, ' + JSON.stringify(Object.keys(opts).reduce((vals, key) => vals.push(opts[key]) && vals, [])) + ');'
)();
}
render("hello ${ name }", {name:'mo'}); // "hello mo"
Note: the Function constructor is always created in the global scope, which could potentially cause global variables to be overwritten by the template, e.g. render("hello ${ someGlobalVar = 'some new value' }", {name:'mo'});

You should try this tiny JS module, by Andrea Giammarchi, from github :
https://github.com/WebReflection/backtick-template
/*! (C) 2017 Andrea Giammarchi - MIT Style License */
function template(fn, $str, $object) {'use strict';
var
stringify = JSON.stringify,
hasTransformer = typeof fn === 'function',
str = hasTransformer ? $str : fn,
object = hasTransformer ? $object : $str,
i = 0, length = str.length,
strings = i < length ? [] : ['""'],
values = hasTransformer ? [] : strings,
open, close, counter
;
while (i < length) {
open = str.indexOf('${', i);
if (-1 < open) {
strings.push(stringify(str.slice(i, open)));
open += 2;
close = open;
counter = 1;
while (close < length) {
switch (str.charAt(close++)) {
case '}': counter -= 1; break;
case '{': counter += 1; break;
}
if (counter < 1) {
values.push('(' + str.slice(open, close - 1) + ')');
break;
}
}
i = close;
} else {
strings.push(stringify(str.slice(i)));
i = length;
}
}
if (hasTransformer) {
str = 'function' + (Math.random() * 1e5 | 0);
if (strings.length === values.length) strings.push('""');
strings = [
str,
'with(this)return ' + str + '([' + strings + ']' + (
values.length ? (',' + values.join(',')) : ''
) + ')'
];
} else {
strings = ['with(this)return ' + strings.join('+')];
}
return Function.apply(null, strings).apply(
object,
hasTransformer ? [fn] : []
);
}
template.asMethod = function (fn, object) {'use strict';
return typeof fn === 'function' ?
template(fn, this, object) :
template(this, fn);
};
Demo (all the following tests return true):
const info = 'template';
// just string
`some ${info}` === template('some ${info}', {info});
// passing through a transformer
transform `some ${info}` === template(transform, 'some ${info}', {info});
// using it as String method
String.prototype.template = template.asMethod;
`some ${info}` === 'some ${info}'.template({info});
transform `some ${info}` === 'some ${info}'.template(transform, {info});

Faz assim (This way):
let a = 'b:${this.b}'
let b = 10
function template(templateString, templateVars) {
return new Function('return `' + templateString + '`').call(templateVars)
}
result.textContent = template(a, {b})
<b id=result></b>

Since we're reinventing the wheel on something that would be a lovely feature in javascript.
I use eval(), which is not secure, but javascript is not secure. I readily admit that I'm not excellent with javascript, but I had a need, and I needed an answer so I made one.
I chose to stylize my variables with an # rather than an $, particularly because I want to use the multiline feature of literals without evaluating til it's ready. So variable syntax is #{OptionalObject.OptionalObjectN.VARIABLE_NAME}
I am no javascript expert, so I'd gladly take advice on improvement but...
var prsLiteral, prsRegex = /\#\{(.*?)(?!\#\{)\}/g
for(i = 0; i < myResultSet.length; i++) {
prsLiteral = rt.replace(prsRegex,function (match,varname) {
return eval(varname + "[" + i + "]");
// you could instead use return eval(varname) if you're not looping.
})
console.log(prsLiteral);
}
A very simple implementation follows
myResultSet = {totalrecords: 2,
Name: ["Bob", "Stephanie"],
Age: [37,22]};
rt = `My name is #{myResultSet.Name}, and I am #{myResultSet.Age}.`
var prsLiteral, prsRegex = /\#\{(.*?)(?!\#\{)\}/g
for(i = 0; i < myResultSet.totalrecords; i++) {
prsLiteral = rt.replace(prsRegex,function (match,varname) {
return eval(varname + "[" + i + "]");
// you could instead use return eval(varname) if you're not looping.
})
console.log(prsLiteral);
}
In my actual implementation, I choose to use #{{variable}}. One more set of braces. Absurdly unlikely to encounter that unexpectedly. The regex for that would look like /\#\{\{(.*?)(?!\#\{\{)\}\}/g
To make that easier to read
\#\{\{ # opening sequence, #{{ literally.
(.*?) # capturing the variable name
# ^ captures only until it reaches the closing sequence
(?! # negative lookahead, making sure the following
# ^ pattern is not found ahead of the current character
\#\{\{ # same as opening sequence, if you change that, change this
)
\}\} # closing sequence.
If you're not experienced with regex, a pretty safe rule is to escape every non-alphanumeric character, and don't ever needlessly escape letters as many escaped letters have special meaning to virtually all flavors of regex.

You can refer to this solution
const interpolate = (str) =>
new Function(`return \`${new String(str)}\`;`)();
const foo = 'My';
const obj = {
text: 'Hanibal Lector',
firstNum: 1,
secondNum: 2
}
const str = "${foo} name is : ${obj.text}. sum = ${obj.firstNum} + ${obj.secondNum} = ${obj.firstNum + obj.secondNum}";
console.log(interpolate(str));

I realize I am late to the game, but you could:
const a = (b) => `b:${b}`;
let b = 10;
console.log(a(b)); // b:10

Using template literals with JSON [duplicate]

Is it possible to create a template string as a usual string,
let a = "b:${b}";
and then convert it into a template string,
let b = 10;
console.log(a.template()); // b:10
without eval, new Function and other means of dynamic code generation?

In my project I've created something like this with ES6:
String.prototype.interpolate = function(params) {
const names = Object.keys(params);
const vals = Object.values(params);
return new Function(...names, `return \`${this}\`;`)(...vals);
}
const template = 'Example text: ${text}';
const result = template.interpolate({
text: 'Foo Boo'
});
console.log(result);

As your template string must get reference to the b variable dynamically (in runtime), so the answer is: NO, it's impossible to do it without dynamic code generation.
But, with eval it's pretty simple:
let tpl = eval('`'+a+'`');

No, there is not a way to do this without dynamic code generation.
However, I have created a function which will turn a regular string into a function which can be provided with a map of values, using template strings internally.
Generate Template String Gist
/**
* Produces a function which uses template strings to do simple interpolation from objects.
*
* Usage:
* var makeMeKing = generateTemplateString('${name} is now the king of ${country}!');
*
* console.log(makeMeKing({ name: 'Bryan', country: 'Scotland'}));
* // Logs 'Bryan is now the king of Scotland!'
*/
var generateTemplateString = (function(){
var cache = {};
function generateTemplate(template){
var fn = cache[template];
if (!fn){
// Replace ${expressions} (etc) with ${map.expressions}.
var sanitized = template
.replace(/\$\{([\s]*[^;\s\{]+[\s]*)\}/g, function(_, match){
return `\$\{map.${match.trim()}\}`;
})
// Afterwards, replace anything that's not ${map.expressions}' (etc) with a blank string.
.replace(/(\$\{(?!map\.)[^}]+\})/g, '');
fn = Function('map', `return \`${sanitized}\``);
}
return fn;
}
return generateTemplate;
})();
Usage:
var kingMaker = generateTemplateString('${name} is king!');
console.log(kingMaker({name: 'Bryan'}));
// Logs 'Bryan is king!' to the console.
Hope this helps somebody. If you find a problem with the code, please be so kind as to update the Gist.

What you're asking for here:
//non working code quoted from the question
let b=10;
console.log(a.template());//b:10
is exactly equivalent (in terms of power and, er, safety) to eval: the ability to take a string containing code and execute that code; and also the ability for the executed code to see local variables in the caller's environment.
There is no way in JS for a function to see local variables in its caller, unless that function is eval(). Even Function() can't do it.
When you hear there's something called "template strings" coming to JavaScript, it's natural to assume it's a built-in template library, like Mustache. It isn't. It's mainly just string interpolation and multiline strings for JS. I think this is going to be a common misconception for a while, though. :(

There are many good solutions posted here, but none yet which utilizes the ES6 String.raw method. Here is my contriubution. It has an important limitation in that it will only accept properties from a passed in object, meaning no code execution in the template will work.
function parseStringTemplate(str, obj) {
let parts = str.split(/\$\{(?!\d)[\wæøåÆØÅ]*\}/);
let args = str.match(/[^{\}]+(?=})/g) || [];
let parameters = args.map(argument => obj[argument] || (obj[argument] === undefined ? "" : obj[argument]));
return String.raw({ raw: parts }, ...parameters);
}
let template = "Hello, ${name}! Are you ${age} years old?";
let values = { name: "John Doe", age: 18 };
parseStringTemplate(template, values);
// output: Hello, John Doe! Are you 18 years old?
Split string into non-argument textual parts. See regex.
parts: ["Hello, ", "! Are you ", " years old?"]
Split string into property names. Empty array if match fails.
args: ["name", "age"]
Map parameters from obj by property name. Solution is limited by shallow one level mapping. Undefined values are substituted with an empty string, but other falsy values are accepted.
parameters: ["John Doe", 18]
Utilize String.raw(...) and return result.

TLDR:
https://jsfiddle.net/bj89zntu/1/
Everyone seems to be worried about accessing variables. Why not just pass them? I'm sure it won't be too hard to get the variable context in the caller and pass it down. Use
ninjagecko's answer to get the props from obj.
function renderString(str,obj){
return str.replace(/\$\{(.+?)\}/g,(match,p1)=>{return index(obj,p1)})
}
Here is the full code:
function index(obj,is,value) {
if (typeof is == 'string')
is=is.split('.');
if (is.length==1 && value!==undefined)
return obj[is[0]] = value;
else if (is.length==0)
return obj;
else
return index(obj[is[0]],is.slice(1), value);
}
function renderString(str,obj){
return str.replace(/\$\{.+?\}/g,(match)=>{return index(obj,match)})
}
renderString('abc${a}asdas',{a:23,b:44}) //abc23asdas
renderString('abc${a.c}asdas',{a:{c:22,d:55},b:44}) //abc22asdas

The issue here is to have a function that has access to the variables of its caller. This is why we see direct eval being used for template processing. A possible solution would be to generate a function taking formal parameters named by a dictionary's properties, and calling it with the corresponding values in the same order. An alternative way would be to have something simple as this:
var name = "John Smith";
var message = "Hello, my name is ${name}";
console.log(new Function('return `' + message + '`;')());
And for anyone using Babel compiler we need to create closure which remembers the environment in which it was created:
console.log(new Function('name', 'return `' + message + '`;')(name));

I liked s.meijer's answer and wrote my own version based on his:
function parseTemplate(template, map, fallback) {
return template.replace(/\$\{[^}]+\}/g, (match) =>
match
.slice(2, -1)
.trim()
.split(".")
.reduce(
(searchObject, key) => searchObject[key] || fallback || match,
map
)
);
}

Similar to Daniel's answer (and s.meijer's gist) but more readable:
const regex = /\${[^{]+}/g;
export default function interpolate(template, variables, fallback) {
return template.replace(regex, (match) => {
const path = match.slice(2, -1).trim();
return getObjPath(path, variables, fallback);
});
}
//get the specified property or nested property of an object
function getObjPath(path, obj, fallback = '') {
return path.split('.').reduce((res, key) => res[key] || fallback, obj);
}
Note: This slightly improves s.meijer's original, since it won't match things like ${foo{bar} (the regex only allows non-curly brace characters inside ${ and }).
UPDATE: I was asked for an example using this, so here you go:
const replacements = {
name: 'Bob',
age: 37
}
interpolate('My name is ${name}, and I am ${age}.', replacements)

#Mateusz Moska, solution works great, but when i used it in React Native(build mode), it throws an error: Invalid character '`', though it works when i run it in debug mode.
So i wrote down my own solution using regex.
String.prototype.interpolate = function(params) {
let template = this
for (let key in params) {
template = template.replace(new RegExp('\\$\\{' + key + '\\}', 'g'), params[key])
}
return template
}
const template = 'Example text: ${text}',
result = template.interpolate({
text: 'Foo Boo'
})
console.log(result)
Demo: https://es6console.com/j31pqx1p/
NOTE: Since I don't know the root cause of an issue, i raised a ticket in react-native repo, https://github.com/facebook/react-native/issues/14107, so that once they can able to fix/guide me about the same :)

You can use the string prototype, for example
String.prototype.toTemplate=function(){
return eval('`'+this+'`');
}
//...
var a="b:${b}";
var b=10;
console.log(a.toTemplate());//b:10
But the answer of the original question is no way.

I required this method with support for Internet Explorer. It turned out the back ticks aren't supported by even IE11. Also; using eval or it's equivalent Function doesn't feel right.
For the one that notice; I also use backticks, but these ones are removed by compilers like babel. The methods suggested by other ones, depend on them on run-time. As said before; this is an issue in IE11 and lower.
So this is what I came up with:
function get(path, obj, fb = `$\{${path}}`) {
return path.split('.').reduce((res, key) => res[key] || fb, obj);
}
function parseTpl(template, map, fallback) {
return template.replace(/\$\{.+?}/g, (match) => {
const path = match.substr(2, match.length - 3).trim();
return get(path, map, fallback);
});
}
Example output:
const data = { person: { name: 'John', age: 18 } };
parseTpl('Hi ${person.name} (${person.age})', data);
// output: Hi John (18)
parseTpl('Hello ${person.name} from ${person.city}', data);
// output: Hello John from ${person.city}
parseTpl('Hello ${person.name} from ${person.city}', data, '-');
// output: Hello John from -

I currently can't comment on existing answers so I am unable to directly comment on Bryan Raynor's excellent response. Thus, this response is going to update his answer with a slight correction.
In short, his function fails to actually cache the created function, so it will always recreate, regardless of whether it's seen the template before. Here is the corrected code:
/**
* Produces a function which uses template strings to do simple interpolation from objects.
*
* Usage:
* var makeMeKing = generateTemplateString('${name} is now the king of ${country}!');
*
* console.log(makeMeKing({ name: 'Bryan', country: 'Scotland'}));
* // Logs 'Bryan is now the king of Scotland!'
*/
var generateTemplateString = (function(){
var cache = {};
function generateTemplate(template){
var fn = cache[template];
if (!fn){
// Replace ${expressions} (etc) with ${map.expressions}.
var sanitized = template
.replace(/\$\{([\s]*[^;\s\{]+[\s]*)\}/g, function(_, match){
return `\$\{map.${match.trim()}\}`;
})
// Afterwards, replace anything that's not ${map.expressions}' (etc) with a blank string.
.replace(/(\$\{(?!map\.)[^}]+\})/g, '');
fn = cache[template] = Function('map', `return \`${sanitized}\``);
}
return fn;
};
return generateTemplate;
})();

Still dynamic but seems more controlled than just using a naked eval:
const vm = require('vm')
const moment = require('moment')
let template = '### ${context.hours_worked[0].value} \n Hours worked \n #### ${Math.abs(context.hours_worked_avg_diff[0].value)}% ${fns.gt0(context.hours_worked_avg_diff[0].value, "more", "less")} than usual on ${fns.getDOW(new Date())}'
let context = {
hours_worked:[{value:10}],
hours_worked_avg_diff:[{value:10}],
}
function getDOW(now) {
return moment(now).locale('es').format('dddd')
}
function gt0(_in, tVal, fVal) {
return _in >0 ? tVal: fVal
}
function templateIt(context, template) {
const script = new vm.Script('`'+template+'`')
return script.runInNewContext({context, fns:{getDOW, gt0 }})
}
console.log(templateIt(context, template))
https://repl.it/IdVt/3

I made my own solution doing a type with a description as a function
export class Foo {
...
description?: Object;
...
}
let myFoo:Foo = {
...
description: (a,b) => `Welcome ${a}, glad to see you like the ${b} section`.
...
}
and so doing:
let myDescription = myFoo.description('Bar', 'bar');

I came up with this implementation and it works like a charm.
function interpolateTemplate(template: string, args: any): string {
return Object.entries(args).reduce(
(result, [arg, val]) => result.replace(`$\{${arg}}`, `${val}`),
template,
)
}
const template = 'This is an example: ${name}, ${age} ${email}'
console.log(interpolateTemplate(template,{name:'Med', age:'20', email:'example#abc.com'}))
You could raise an error if arg is not found in template

This solution works without ES6:
function render(template, opts) {
return new Function(
'return new Function (' + Object.keys(opts).reduce((args, arg) => args += '\'' + arg + '\',', '') + '\'return `' + template.replace(/(^|[^\\])'/g, '$1\\\'') + '`;\'' +
').apply(null, ' + JSON.stringify(Object.keys(opts).reduce((vals, key) => vals.push(opts[key]) && vals, [])) + ');'
)();
}
render("hello ${ name }", {name:'mo'}); // "hello mo"
Note: the Function constructor is always created in the global scope, which could potentially cause global variables to be overwritten by the template, e.g. render("hello ${ someGlobalVar = 'some new value' }", {name:'mo'});

You should try this tiny JS module, by Andrea Giammarchi, from github :
https://github.com/WebReflection/backtick-template
/*! (C) 2017 Andrea Giammarchi - MIT Style License */
function template(fn, $str, $object) {'use strict';
var
stringify = JSON.stringify,
hasTransformer = typeof fn === 'function',
str = hasTransformer ? $str : fn,
object = hasTransformer ? $object : $str,
i = 0, length = str.length,
strings = i < length ? [] : ['""'],
values = hasTransformer ? [] : strings,
open, close, counter
;
while (i < length) {
open = str.indexOf('${', i);
if (-1 < open) {
strings.push(stringify(str.slice(i, open)));
open += 2;
close = open;
counter = 1;
while (close < length) {
switch (str.charAt(close++)) {
case '}': counter -= 1; break;
case '{': counter += 1; break;
}
if (counter < 1) {
values.push('(' + str.slice(open, close - 1) + ')');
break;
}
}
i = close;
} else {
strings.push(stringify(str.slice(i)));
i = length;
}
}
if (hasTransformer) {
str = 'function' + (Math.random() * 1e5 | 0);
if (strings.length === values.length) strings.push('""');
strings = [
str,
'with(this)return ' + str + '([' + strings + ']' + (
values.length ? (',' + values.join(',')) : ''
) + ')'
];
} else {
strings = ['with(this)return ' + strings.join('+')];
}
return Function.apply(null, strings).apply(
object,
hasTransformer ? [fn] : []
);
}
template.asMethod = function (fn, object) {'use strict';
return typeof fn === 'function' ?
template(fn, this, object) :
template(this, fn);
};
Demo (all the following tests return true):
const info = 'template';
// just string
`some ${info}` === template('some ${info}', {info});
// passing through a transformer
transform `some ${info}` === template(transform, 'some ${info}', {info});
// using it as String method
String.prototype.template = template.asMethod;
`some ${info}` === 'some ${info}'.template({info});
transform `some ${info}` === 'some ${info}'.template(transform, {info});

Faz assim (This way):
let a = 'b:${this.b}'
let b = 10
function template(templateString, templateVars) {
return new Function('return `' + templateString + '`').call(templateVars)
}
result.textContent = template(a, {b})
<b id=result></b>

Since we're reinventing the wheel on something that would be a lovely feature in javascript.
I use eval(), which is not secure, but javascript is not secure. I readily admit that I'm not excellent with javascript, but I had a need, and I needed an answer so I made one.
I chose to stylize my variables with an # rather than an $, particularly because I want to use the multiline feature of literals without evaluating til it's ready. So variable syntax is #{OptionalObject.OptionalObjectN.VARIABLE_NAME}
I am no javascript expert, so I'd gladly take advice on improvement but...
var prsLiteral, prsRegex = /\#\{(.*?)(?!\#\{)\}/g
for(i = 0; i < myResultSet.length; i++) {
prsLiteral = rt.replace(prsRegex,function (match,varname) {
return eval(varname + "[" + i + "]");
// you could instead use return eval(varname) if you're not looping.
})
console.log(prsLiteral);
}
A very simple implementation follows
myResultSet = {totalrecords: 2,
Name: ["Bob", "Stephanie"],
Age: [37,22]};
rt = `My name is #{myResultSet.Name}, and I am #{myResultSet.Age}.`
var prsLiteral, prsRegex = /\#\{(.*?)(?!\#\{)\}/g
for(i = 0; i < myResultSet.totalrecords; i++) {
prsLiteral = rt.replace(prsRegex,function (match,varname) {
return eval(varname + "[" + i + "]");
// you could instead use return eval(varname) if you're not looping.
})
console.log(prsLiteral);
}
In my actual implementation, I choose to use #{{variable}}. One more set of braces. Absurdly unlikely to encounter that unexpectedly. The regex for that would look like /\#\{\{(.*?)(?!\#\{\{)\}\}/g
To make that easier to read
\#\{\{ # opening sequence, #{{ literally.
(.*?) # capturing the variable name
# ^ captures only until it reaches the closing sequence
(?! # negative lookahead, making sure the following
# ^ pattern is not found ahead of the current character
\#\{\{ # same as opening sequence, if you change that, change this
)
\}\} # closing sequence.
If you're not experienced with regex, a pretty safe rule is to escape every non-alphanumeric character, and don't ever needlessly escape letters as many escaped letters have special meaning to virtually all flavors of regex.

You can refer to this solution
const interpolate = (str) =>
new Function(`return \`${new String(str)}\`;`)();
const foo = 'My';
const obj = {
text: 'Hanibal Lector',
firstNum: 1,
secondNum: 2
}
const str = "${foo} name is : ${obj.text}. sum = ${obj.firstNum} + ${obj.secondNum} = ${obj.firstNum + obj.secondNum}";
console.log(interpolate(str));

I realize I am late to the game, but you could:
const a = (b) => `b:${b}`;
let b = 10;
console.log(a(b)); // b:10

How to write a function that takes a string and object and interpolates that object in the string?

Hi everyone I need to write a function that takes a string and object and interpolates that object in the string so something like this
// interpolate("Its {weather} outside", {weather: 'damn Hot'})
// returns 'It's damn hot outside'
// interpolate( "Hi my name is {name}", {name: 'John'});
// returns 'Hi my name is John'
It should also no matter how deep the object goes so a case like this
// interpolate("Hi my name is {contact.name}", {contact: {name: 'john'}});
Im a little stuck and at first I tried splitting up the string into an array then trying to fit the object value in the array then join that back together but that has not worked for all test cases, can someone help me write this function and explain their solution please? Thankyou
so I tried something like this but does not really work for all test cases, this is a template literal but the function should just work giving those values as arguments on its own, otherwise I'm pretty stuck . .
function interpolate(strings, ...keys) {
return (function(...values) {
var dict = values[values.length - 1] || {};
var result = [strings[0]];
keys.forEach(function(key, i) {
var value = Number.isInteger(key) ? values[key] : dict[key];
result.push(value, strings[i + 1]);
});
return result.join('');
});
}
var t1Closure = interpolate`${0}${1}${0}!`;
t1Closure('Y', 'A'); // "YAY!"
var t2Closure = interpolate`${0} ${'foo'}!`;
console.log(t2Closure('Hello', {foo: 'World'})); // "Hello World!"
ok I'm getting closer so I separated the problem into two functions and need to combine them, the only thing is I'm not sure how to get the last use case to work without template literals
var something = "something";
var sub = function(str) {
return str.replace(/#\{(.*?)\}/g,
function(whole, expr) {
return eval(expr)
})
}
console.log(sub("hello #{something}"));
var objectValue = function(str, obj){
for(key in obj) {
if(obj.hasOwnProperty(key)) {
var value = obj[key];
return str + value;
}
}
}
console.log(objectValue("hi my name is ", {contact: {name: 'john'}}));

Using the dreaded eval
If you control the string that you pass and consider it safe, you can use eval:
function interpolate (str, obj) {
return str.replace(/\{(.*?)\}/g, function (_, ref) {
return eval('obj.' + ref);
});
}
var output = interpolate("Its {weather} outside", {weather: 'damn Hot'});
console.log(output);
output = interpolate("Hi my name is {contact.name}", {contact: {name: 'john'}});
console.log(output);
output = interpolate("The highest bid is {rank[0].bid}", {rank: [{bid: 900}, {bid:600}]});
console.log(output);
Be aware that if given a string like '{x;alert("hi")}', the above function would execute that alert, or any code that is put instead. So this is not a good solution if the string is provided (or can be altered) by the user.
Template Literals
It does not follow your function descriptor, but template literals already offer the functionality you are looking for:
var weather = 'damn Hot';
var output = `It's ${weather} outside`;
console.log(output);
var contact = {name: 'john'};
var output = `Hi my name is ${contact.name}`;
console.log(output);

Here you go:
'use strict';
function interpolate(str, obj) {
for (let prop in obj) {
if (obj.hasOwnProperty(prop)) {
str = str.replace(new RegExp(`{${prop}}`, 'g'), obj[prop]);
}
}
return str;
}
console.log(interpolate("Its {weather} outside", {weather: 'damn Hot'}));
// Its damn Hot outside

Try renderjs.
$("<p>Its {{:weather}} outside</p>").render({weather: 'damn Hot'})
-> <p>Its damn Hot outside</p>

Or lodash templates. Lodash already has a lot of handy features you end up using a lot of in my opinion.
var str = _.template('Its <%= weather %> outside')({weather: 'damn Hot'});
// If you wanted to use the {{:var}} syntax.
_.templateSettings.interpolate = /{{:([\s\S]+?)}}/g;
var str = _.template('Its {{:weather}} outside')({weather: 'damn Hot'});

A little late to the party, but in case you cannot (don't want to) use any external libraries or ES6, here is the idea:
function getMatches(s) {
var regExp = /{([^}]*)}/g,
matches = [],
match;
while ((match = regExp.exec(s)) != null) {
matches.push(match[1]);
}
return matches;
}
function objValue(obj, i) {
return obj[i];
}
function interpolate(s, obj) {
var matches = getMatches(s),
result = s;
matches.forEach(function (match) {
var props = match.split('.'),
value = props.reduce(objValue, obj) || '';
result = result.replace('{' + match + '}', value);
});
return result;
}
Usage
interpolate("Its {weather} outside", { weather: 'damn Hot' });
JSFiddle.

Defer execution for ES6 Template Literals

I am playing with the new ES6 Template Literals feature and the first thing that came to my head was a String.format for JavaScript so I went about implementing a prototype:
String.prototype.format = function() {
var self = this;
arguments.forEach(function(val,idx) {
self["p"+idx] = val;
});
return this.toString();
};
console.log(`Hello, ${p0}. This is a ${p1}`.format("world", "test"));
ES6Fiddle
However, the Template Literal is evaluated before it's passed to my prototype method. Is there any way I can write the above code to defer the result until after I have dynamically created the elements?

I can see three ways around this:
Use template strings like they were designed to be used, without any format function:
console.log(`Hello, ${"world"}. This is a ${"test"}`);
// might make more sense with variables:
var p0 = "world", p1 = "test";
console.log(`Hello, ${p0}. This is a ${p1}`);
or even function parameters for actual deferral of the evaluation:
const welcome = (p0, p1) => `Hello, ${p0}. This is a ${p1}`;
console.log(welcome("world", "test"));
Don't use a template string, but a plain string literal:
String.prototype.format = function() {
var args = arguments;
return this.replace(/\$\{p(\d)\}/g, function(match, id) {
return args[id];
});
};
console.log("Hello, ${p0}. This is a ${p1}".format("world", "test"));
Use a tagged template literal. Notice that the substitutions will still be evaluated without interception by the handler, so you cannot use identifiers like p0 without having a variable named so. This behavior may change if a different substitution body syntax proposal is accepted (Update: it was not).
function formatter(literals, ...substitutions) {
return {
format: function() {
var out = [];
for(var i=0, k=0; i < literals.length; i++) {
out[k++] = literals[i];
out[k++] = arguments[substitutions[i]];
}
out[k] = literals[i];
return out.join("");
}
};
}
console.log(formatter`Hello, ${0}. This is a ${1}`.format("world", "test"));
// Notice the number literals: ^ ^

Extending #Bergi 's answer, the power of tagged template strings reveals itself when you realize you can return anything as a result, not only plain strings. In his example, the tag constructs and returns an object with a closure and function property format.
In my favorite approach, I return a function value by itself, that you can call later and pass new parameters to fill the template. Like this:
function fmt([fisrt, ...rest], ...tags) {
return values => rest.reduce((acc, curr, i) => {
return acc + values[tags[i]] + curr;
}, fisrt);
}
Or, for the code golfers:
let fmt=([f,...r],...t)=>v=>r.reduce((a,c,i)=>a+v[t[i]]+c,f)
Then you construct your templates and defer the substitutions:
> fmt`Test with ${0}, ${1}, ${2} and ${0} again`(['A', 'B', 'C']);
// 'Test with A, B, C and A again'
> template = fmt`Test with ${'foo'}, ${'bar'}, ${'baz'} and ${'foo'} again`
> template({ foo:'FOO', bar:'BAR' })
// 'Test with FOO, BAR, undefined and FOO again'
Another option, closer to what you wrote, would be to return a object extended from a string, to get duck-typing out of the box and respect interface. An extension to the String.prototype wouldn't work because you'd need the closure of the template tag to resolve the parameters later.
class FormatString extends String {
// Some other custom extensions that don't need the template closure
}
function fmt([fisrt, ...rest], ...tags) {
const str = new FormatString(rest.reduce((acc, curr, i) => `${acc}\${${tags[i]}}${curr}`, fisrt));
str.format = values => rest.reduce((acc, curr, i) => {
return acc + values[tags[i]] + curr;
}, fisrt);
return str;
}
Then, in the call-site:
> console.log(fmt`Hello, ${0}. This is a ${1}.`.format(["world", "test"]));
// Hello, world. This is a test.
> template = fmt`Hello, ${'foo'}. This is a ${'bar'}.`
> console.log(template)
// { [String: 'Hello, ${foo}. This is a ${bar}.'] format: [Function] }
> console.log(template.format({ foo: true, bar: null }))
// Hello, true. This is a null.
You can refer to more information and applications in this other answer.

AFAIS, the useful feature "deferred execution of string templates" is still not available. Using a lambda is an expressive, readable and short solution, however:
var greetingTmpl = (...p)=>`Hello, ${p[0]}. This is a ${p[1]}`;
console.log( greetingTmpl("world","test") );
console.log( greetingTmpl("#CodingIntrigue","try") );

You can inject values into string using below function
let inject = (str, obj) => str.replace(/\${(.*?)}/g, (x,g)=> obj[g]);
let inject = (str, obj) => str.replace(/\${(.*?)}/g, (x,g)=> obj[g]);
// --- Examples ---
// parameters in object
let t1 = 'My name is ${name}, I am ${age}. My brother name is also ${name}.';
let r1 = inject(t1, {name: 'JOHN',age: 23} );
console.log("OBJECT:", r1);
// parameters in array
let t2 = "Today ${0} saw ${2} at shop ${1} times - ${0} was haapy."
let r2 = inject(t2, {...['JOHN', 6, 'SUsAN']} );
console.log("ARRAY :", r2);

I also like the idea of the String.format function, and being able to explicitly define the variables for resolution.
This is what I came up with... basically a String.replace method with a deepObject lookup.
const isUndefined = o => typeof o === 'undefined'
const nvl = (o, valueIfUndefined) => isUndefined(o) ? valueIfUndefined : o
// gets a deep value from an object, given a 'path'.
const getDeepValue = (obj, path) =>
path
.replace(/\[|\]\.?/g, '.')
.split('.')
.filter(s => s)
.reduce((acc, val) => acc && acc[val], obj)
// given a string, resolves all template variables.
const resolveTemplate = (str, variables) => {
return str.replace(/\$\{([^\}]+)\}/g, (m, g1) =>
nvl(getDeepValue(variables, g1), m))
}
// add a 'format' method to the String prototype.
String.prototype.format = function(variables) {
return resolveTemplate(this, variables)
}
// setup variables for resolution...
var variables = {}
variables['top level'] = 'Foo'
variables['deep object'] = {text:'Bar'}
var aGlobalVariable = 'Dog'
// ==> Foo Bar <==
console.log('==> ${top level} ${deep object.text} <=='.format(variables))
// ==> Dog Dog <==
console.log('==> ${aGlobalVariable} ${aGlobalVariable} <=='.format(this))
// ==> ${not an object.text} <==
console.log('==> ${not an object.text} <=='.format(variables))
Alternatively, if you want more than just variable resolution (e.g. the behaviour of template literals), you can use the following.
N.B. eval is considered 'evil' - consider using a safe-eval alternative.
// evalutes with a provided 'this' context.
const evalWithContext = (string, context) => function(s){
return eval(s);
}.call(context, string)
// given a string, resolves all template variables.
const resolveTemplate = function(str, variables) {
return str.replace(/\$\{([^\}]+)\}/g, (m, g1) => evalWithContext(g1, variables))
}
// add a 'format' method to the String prototype.
String.prototype.format = function(variables) {
return resolveTemplate(this, variables)
}
// ==> 5Foobar <==
console.log('==> ${1 + 4 + this.someVal} <=='.format({someVal: 'Foobar'}))

I posted an answer to a similar question that gives two approaches where the execution of the template literal is delayed. When the template literal is in a function, the template literal is only evaluated when the function is called, and it is evaluated using the scope of the function.
https://stackoverflow.com/a/49539260/188963

Although this question is already answered, here I have a simple implementations I use when I load configuration files (the code is typescript, but it is very easy to convert into JS, just remove the typings):
/**
* This approach has many limitations:
* - it does not accept variable names with numbers or other symbols (relatively easy to fix)
* - it does not accept arbitrary expressions (quite difficult to fix)
*/
function deferredTemplateLiteral(template: string, env: { [key: string]: string | undefined }): string {
const varsMatcher = /\${([a-zA-Z_]+)}/
const globalVarsmatcher = /\${[a-zA-Z_]+}/g
const varMatches: string[] = template.match(globalVarsmatcher) ?? []
const templateVarNames = varMatches.map(v => v.match(varsMatcher)?.[1] ?? '')
const templateValues: (string | undefined)[] = templateVarNames.map(v => env[v])
const templateInterpolator = new Function(...[...templateVarNames, `return \`${template}\`;`])
return templateInterpolator(...templateValues)
}
// Usage:
deferredTemplateLiteral("hello ${thing}", {thing: "world"}) === "hello world"
Although it's possible to make this stuff more powerful & flexible, it introduces too much complexity and risk without much benefit.
Here a link to the gist: https://gist.github.com/castarco/94c5385539cf4d7104cc4d3513c14f55

(see #Bergi's very similar answer above)
function interpolate(strings, ...positions) {
var errors = positions.filter(pos=>~~pos!==pos);
if (errors.length) {
throw "Invalid Interpolation Positions: " + errors.join(', ');
}
return function $(...vals) {
var output = '';
for (let i = 0; i < positions.length; i ++) {
output += (strings[i] || '') + (vals[positions[i] - 1] || '');
}
output += strings[strings.length - 1];
return output;
};
}
var iString = interpolate`This is ${1}, which is pretty ${2} and ${3}. Just to reiterate, ${1} is ${2}! (nothing ${0} ${100} here)`;
// Sets iString to an interpolation function
console.log(iString('interpolation', 'cool', 'useful', 'extra'));
// Substitutes the values into the iString and returns:
// 'This is interpolation, which is pretty cool and useful.
// Just to reiterate, interpolation is cool! (nothing here)'
The main difference between this and #Bergi's answer is how errors are handled (silently vs not).
It should be easy enough to expand this idea into a syntax that accepts named arguments:
interpolate`This is ${'foo'}, which is pretty ${'bar'}.`({foo: 'interpolation', bar: 'cool'});
https://github.com/spikesagal/es6interpolate/blob/main/src/interpolate.js