In PHP you can do amazing/horrendous things like this:
$a = 1;
$b = 2;
$c = 3;
$name = 'a';
echo $$name;
// prints 1
Is there any way of doing something like this with Javascript?
E.g. if I have a var name = 'the name of the variable'; can I get a reference to the variable with name name?
Since ECMA-/Javascript is all about Objects and Contexts (which, are also somekind of Object), every variable is stored in a such called Variable- (or in case of a Function, Activation Object).
So if you create variables like this:
var a = 1,
b = 2,
c = 3;
In the Global scope (= NO function context), you implicitly write those variables into the Global object (= window in a browser).
Those can get accessed by using the "dot" or "bracket" notation:
var name = window.a;
or
var name = window['a'];
This only works for the global object in this particular instance, because the Variable Object of the Global Object is the window object itself. Within the Context of a function, you don't have direct access to the Activation Object. For instance:
function foobar() {
this.a = 1;
this.b = 2;
var name = window['a']; // === undefined
console.log(name);
name = this['a']; // === 1
console.log(name);
}
new foobar();
new creates a new instance of a self-defined object (context). Without new the scope of the function would be also global (=window). This example would alert undefined and 1 respectively. If we would replace this.a = 1; this.b = 2 with:
var a = 1,
b = 2;
Both alert outputs would be undefined. In that scenario, the variables a and b would get stored in the Activation Object from foobar, which we cannot access (of course we could access those directly by calling a and b).
eval is one option.
var a = 1;
var name = 'a';
document.write(eval(name)); // 1
Warning: Note that using the eval() function is not recommended if you don't know what you are doing, since it brings multiple security issues. Use something else unless absolutely necessary. See the MDN page for eval for more info.
You can use the window object to get at it .
window['myVar']
window has a reference to all global variables and global functions you are using.
Just don't know what a bad answer gets so many votes. It's quite easy answer but you make it complex.
// If you want to get article_count
// var article_count = 1000;
var type = 'article';
this[type+'_count'] = 1000; // in a function we use "this";
alert(article_count);
This is an example :
for(var i=0; i<=3; i++) {
window['p'+i] = "hello " + i;
}
alert(p0); // hello 0
alert(p1); // hello 1
alert(p2); // hello 2
alert(p3); // hello 3
Another example :
var myVariable = 'coco';
window[myVariable] = 'riko';
alert(coco); // display : riko
So, the value "coco" of myVariable becomes a variable coco.
Because all the variables in the global scope are properties of the Window object.
a = 'varname';
str = a+' = '+'123';
eval(str)
alert(varname);
Try this...
In Javascript you can use the fact that all properties are key value pairs. jAndy already mentioned this but I don't think his answer show how it can be exploited.
Usually you are not trying to create a variable to hold a variable name but are trying to generate variable names and then use them. PHP does it with $$var notation but Javascript doesn't need to because property keys are interchangeable with array keys.
var id = "abc";
var mine = {};
mine[id] = 123;
console.log(mine.abc);
gives 123. Usually you want to construct the variable which is why there is the indirection so you can also do it the other way around.
var mine = {};
mine.abc = 123;
console.log(mine["a"+"bc"]);
If you don't want to use a global object like window or global (node), you can try something like this:
var obj = {};
obj['whatever'] = 'There\'s no need to store even more stuff in a global object.';
console.log(obj['whatever']);
2019
TL;DR
eval operator can run string expression in the context it called and return variables from that context;
literal object theoretically can do that by write:{[varName]}, but it blocked by definition.
So I come across this question and everyone here just play around without bringing a real solution. but #Axel Heider has a good approaching.
The solution is eval.
almost most forgotten operator. ( think most one is with() )
eval operator can dynamically run expression in the context it called. and return the result of that expression. we can use that to dynamically return a variable's value in function's context.
example:
function exmaple1(){
var a = 1, b = 2, default = 3;
var name = 'a';
return eval(name)
}
example1() // return 1
function example2(option){
var a = 1, b = 2, defaultValue = 3;
switch(option){
case 'a': name = 'a'; break;
case 'b': name = 'b'; break;
default: name = 'defaultValue';
}
return eval (name);
}
example2('a') // return 1
example2('b') // return 2
example2() // return 3
Note that I always write explicitly the expression eval will run.
To avoid unnecessary surprises in the code. eval is very strong
But I'm sure you know that already
BTW, if it was legal we could use literal object to capture the variable name and value, but we can’t combine computed property names and property value shorthand, sadly, is invalid
functopn example( varName ){
var var1 = 'foo', var2 ='bar'
var capture = {[varName]}
}
example('var1') //trow 'Uncaught SyntaxError: Unexpected token }`
I needed to draw multiple FormData on the fly and object way worked well
var forms = {}
Then in my loops whereever i needed to create a form data i used
forms["formdata"+counter]=new FormData();
forms["formdata"+counter].append(var_name, var_value);
This is an alternative for those who need to export a dynamically named variable
export {
[someVariable]: 'some value',
[anotherVariable]: 'another value',
}
// then.... import from another file like this:
import * as vars from './some-file'
Another alternative is to simply create an object whose keys are named dynamically
const vars = { [someVariable]: 1, [otherVariable]: 2 };
// consume it like this
vars[someVariable];
use Object is great too.
var a=123
var b=234
var temp = {"a":a,"b":b}
console.log(temp["a"],temp["b"]);
Although this have an accepted answer I would like to add an observation:
In ES6 using let doesn't work:
/*this is NOT working*/
let t = "skyBlue",
m = "gold",
b = "tomato";
let color = window["b"];
console.log(color);
However using var works
/*this IS working*/
var t = "skyBlue",
m = "gold",
b = "tomato";
let color = window["b"];
console.log(color);
I hope this may be useful to some.
This will do exactly what you done in php:
var a = 1;
var b = 2;
var ccc = 3;
var name = 'a';
console.log( window[name] ); // 1
Simplest solution : Create an array of objects that every object has two field (variableName,variableValue)
let allVariables = [];
for (let i = 0; i < 5; i++)
allVariables.push({ variableName: 'variable' + i, variableValue: i * 10 });
for (let i = 0; i < allVariables.length; i++)
console.log(allVariables[i].variableName + ' is ' + allVariables[i].variableValue);
OutPut :
variable0 is 0
variable1 is 10
variable2 is 20
variable3 is 30
variable4 is 40
console.log(allVariables) json :
[
{
"variableName": "variable0",
"variableValue": 0
},
{
"variableName": "variable1",
"variableValue": 10
},
{
"variableName": "variable2",
"variableValue": 20
},
{
"variableName": "variable3",
"variableValue": 30
},
{
"variableName": "variable4",
"variableValue": 40
}
]
what they mean is no, you can't.
there is no way to get it done.
so it was possible you could do something like this
function create(obj, const){
// where obj is an object and const is a variable name
function const () {}
const.prototype.myProperty = property_value;
// .. more prototype
return new const();
}
having a create function just like the one implemented in ECMAScript 5.
eval() did not work in my tests. But adding new JavaScript code to the DOM tree is possible. So here is a function that adds a new variable:
function createVariable(varName,varContent)
{
var scriptStr = "var "+varName+"= \""+varContent+"\""
var node_scriptCode = document.createTextNode( scriptStr )
var node_script = document.createElement("script");
node_script.type = "text/javascript"
node_script.appendChild(node_scriptCode);
var node_head = document.getElementsByTagName("head")[0]
node_head.appendChild(node_script);
}
createVariable("dynamicVar", "some content")
console.log(dynamicVar)
Here's pure javascript solution which is not dependant on the global this of the runtime environment. Simple to achieve using object destructuring.
const dynamicVar = (nameValue, value) => {
const dynamicVarObj = {
[nameValue]: value
}
return dynamicVarObj;
}
const nameToUse = "myVar";
const value = 55;
const { myVar } = dynamicVar(nameToUse, value);
console.log(myVar); // prints 55
It is always better to use create a namespace and declare a variable in it instead of adding it to the global object. We can also create a function to get and set the value
See the below code snippet:
//creating a namespace in which all the variables will be defined.
var myObjects={};
//function that will set the name property in the myObjects namespace
function setName(val){
myObjects.Name=val;
}
//function that will return the name property in the myObjects namespace
function getName(){
return myObjects.Name;
}
//now we can use it like:
setName("kevin");
var x = getName();
var y = x;
console.log(y) //"kevin"
var z = "y";
console.log(z); //"y"
console.log(eval(z)); //"kevin"
In this similar way, we can declare and use multiple variables. Although this will increase the line of code but the code will be more robust and less error-prone.
I want to create an object that has some keys which come from a variable parameter.
Let's say for example that prod_id below is a variable containing some value... I want to create an object which has an attribute with key of that 'prod_id' and value of 1. However this does not work? Is this possible to achieve? if so, how? thanks a heaps
var cart_obj;
cart_obj = {
prod_id : 1
};
localStorage.setItem("cart", JSON.stringify(cart_obj));
You can't do it with a simple object literal. You can do this however:
var cart_obj = {};
cart_obj[prod_id] = 1;
JavaScript object literal syntax makes no provisions for expressions on the left side of a property declaration stanza.
var cart_obj = {};
cart_obj.prod_id = 1;
or
var cart_obj = {};
cart_obj[prod_id] = 1;
This question already has answers here:
Return multiple values in JavaScript?
(20 answers)
Closed 8 years ago.
Consider the following code (demo):
function test(){
var h = 'Hello';
var w = 'World';
return (h, w);
}
var test = test();
alert(test);
On execution the function test only returns the second value (i.e. 'World'). How do I make it return multiple values?
You cannot explicitly return two variables from a single function, but there are various ways you could concatenate the two variables in order to return them.
If you don't need to keep the variables separated, you could just concatenate them directly like this:
function test(){
var h = 'Hello';
var w = 'World';
var hw = h+w
return (hw);
}
var test = test();
alert(test);
This would alert "HelloWorld". (If you wanted a space in there, you should use var hw = h+" "+w instead.
If you need to keep the two variables separated, you can place them into an array like so:
function test(){
var h = "Hello";
var w = "World";
var hw=[h,w];
return hw;
}
var test = test();
alert(test);
This allows the h and w values to still be accessed individually as test[0] and test[1], respectively. However, alert(test) here will display "Hello,World" because of the way alert() handles arrays (that is, it prints a comma-separated list of each element in the array sequentially). If you wanted to produce the same output as your example code, you would need use something like join(). join() will construct a string from an array, it takes one argument which serves as a separator between the elements. To reproduce the two alerts from my first example, you would need to use alert(test.join("")) and alert(test.join(" "), respectively.
My example could be shortened slightly by skipping the creation of the hw variable and just returning an array directly. In that case, test() would look like this:
function test(){
var h="Hello";
var w="World";
return [h, w];
}
This could also be done as an object with return { h : h, w : w };, in which case you would access the individual variables as test.h and test.w, respectively.
function test(){
var h = 'Hello';
var w = 'World';
return {h:h,w:w}
}
var test = test();
alert(test.h);
alert(test.w);
one simple way to do is to return Object containing multiple key value pairs.
The comma operator evaluates each operand and then returns the value of the last one.
You'd need to either return an array:
return [h, w];
...or an object:
return { h : h, w : w };
Which you'd then use as:
var test = test();
alert(test[0]); // "hello" - in the case of the array version
...or:
var test = test();
alert(test.w); // "world" in the case of the object version
You can return an array or a new object.
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Javascript use variable as object name
How do I get JS to treat a string as a reference to a previously defined object? Simplified:
var myObject = new MyObject();
var myString = "myObject";
var wantThisToWork = myString.myproperty;
If the variable is in the global scope, you can access it as a property of the global object
var a = "hello world";
var varName = "a";
console.log( window[varName] ); // outputs hello world
console.log( this[varName] ); // also works (this === window) in this case
However, if it's a local variable, the only way is to use eval (disclaimer)
function () {
var a = "hello world";
var varName = "a";
console.log( this[varName] ); // won't work
console.log( eval(varName) ); // Does work
}
Unless you can put your dynamic variables into an object and access it like a property
function () {
var scope = {
a: "hello world";
};
var varName = "a";
console.log( scope[varName] ); // works
}
You can use the eval function.
eval(myString).myproperty
Careful with eval, though, if this is something the user is inputting, it will execute any javascript code!
The only way, as it seems to me, would be to use eval. But as they say, eval is evil - but not in controlled environments. This is the way it is possible, but i don't recommend using eval, unless it is absolutely necessary.
var myObject = new MyObject();
var myString = "myObject";
var wantThisToWork = eval(myString).myproperty;
Use eval()
var myObject = {};
myObject.myproperty = "Hello";
var myString = "myObject";
var wantThisToWork = eval(myString).myproperty;
Is it possible to create get/set function for undefined properties,
like in PHP __get() and __set() ?
You can access JavaScript object properties values using array access notation, you can also create a new property at any time using this notation or regular assignment notation.
var myObject = {};
myObject.Name = "Luis";
alert(myObject.Name);
alert(myObject["Name"]);
myObject["Name"] = "Dany";
alert(myObject.Name);
You can do
function ClassName(arg) {
var v = arg;
this.getter = function {
return v;
};
this.setter = function(val) {
v = val;
};
}
when you use it
var cn = new ClassName('a');
cn.setter('b');
alert(cn.getter()); /* alerts value 'b' */
Note that this uses the Constructor Invocation Pattern. By convention, you need to declare the function/class name with capital letter to indicate that this function/class need to be declared with the 'new' keyword.
Hope this helps