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What does apply and concat means in below nodejs function, and how to use it?

How to understand apply and concat in below code.
I view https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/concat
the concat syntax
is const new_array = old_array.concat([value1[, value2[, ...[, valueN]]]])
Parameters.
But what is purpose of 0 and last parenthesis （）？
return extend.apply(undefined, [methodTxObject].concat((0, _toConsumableArray3.default)(extendArgs)));
var ContractFactory = function ContractFactory(extend) {
return function (contractABI) {
var output = {};
output.at = function atContract(address) {
function Contract() {
var self = this;
self.abi = contractABI || [];
self.address = address || '0x';
getCallableMethodsFromABI(contractABI).forEach(function (methodObject) {
self[methodObject.name] = function contractMethod() {
if (methodObject.constant === true) {
throw new Error('A call does not return the txobject, no transaction necessary.');
}
if (methodObject.type === 'event') {
throw new Error('An event does not return the txobject, events not supported');
}
var providedTxObject = {};
var methodArgs = [].slice.call(arguments);
if (methodObject.type === 'function') {
if (hasTransactionObject(methodArgs)) providedTxObject = methodArgs.pop();
var methodTxObject = (0, _assign2.default)({}, providedTxObject, {
to: self.address
});
methodTxObject.function = encodeMethodReadable(methodObject, methodArgs);
if (!extend) return methodTxObject;
var extendArgs = methodArgs.slice(methodObject.inputs.length);
return extend.apply(undefined, [methodTxObject].concat((0, _toConsumableArray3.default)(extendArgs)));
}
};
});
}
return new Contract();
};
return output;
};
};

An object/array containing functions. It is just a different way to call
the function. instead of assigning and then calling, just get and call immediate.
const object = {
fn: function(name) {
console.log("My name is: " + name)
}
}
const fn = object["fn"]
fn("Slim Sheddy") // one way of calling
// OR:
object["fn"]("Slim Sheddy") // other way of calling

I don't understand the purpose of the (0, _fn) nomenclature, but that's just a function call. The '0,' part somehow changes 'this', and then the function after the comma is called with the arguments. It's basically this:
var append = (0, _toConsumableArray3.default)(extendArgs)
// -> something like: _toConsumableArray3.default.call(0?, extendArgs)
var args = [methodTxObject].concat(append)
return extend.apply(undefined, args);
It may be a way to avoid prototype inheritance. See:
> (0,console.log)('test')
test
> (console.log)('test')
test
> (Object.toString)('test')
'function Object() { [native code] }'
> (0,Object.toString)('test')
Thrown:
TypeError: Function.prototype.toString requires that 'this' be a Function
at toString (<anonymous>)

About Javascript Module Architecture (Class VS Object literal)

// Class style
class class_sample {
constructor(){
}
get_something (key) {
result = do_something (key);
return result
}
}
let class_sample = new class_sample();
module.exports = class_sample;
// Object literal style
let literal_sample = {
get_something : () => {
result = do_something (key);
return result
}
}
module.exports = literal_sample;
let class_sample = require("/class_sample");
let literal_sample = require("literal_sample");
class_sample.get_something(key);
literal_sample.get_something(key);
I found there is a two type to make a module. The first is using Class, the second is Object literal.
Which one has better performance to make module?(about resource use and speed)

How do I know who has instantiated a function? [duplicate]

I am creating a JavaScript code and I had a situation where I want to read the object name (string) in the object method. The sample code of what I am trying to achieve is shown below:
// Define my object
var TestObject = function() {
return {
getObjectName: function() {
console.log( /* Get the Object instance name */ );
}
};
}
// create instance
var a1 = TestObject();
var a2 = TestObject();
a1.getObjectName(); // Here I want to get the string name "a1";
a2.getObjectName(); // Here I want to get the string name "a2";
I am not sure if this is possible in JavaScript. But in case it is, I would love to hear from you guys how to achieve this.

This is not possible in JavaScript. A variable is just a reference to an object, and the same object can be referenced by multiple variables. There is no way to tell which variable was used to gain access to your object. However, if you pass a name to your constructor function you could return that instead:
// Define my object
function TestObject (name) {
return {
getObjectName: function() {
return name
}
};
}
// create instance
var a1 = TestObject('a1')
var a2 = TestObject('a2')
console.log(a1.getObjectName()) //=> 'a1'
console.log(a2.getObjectName()) //=> 'a2'

This is definitely possible but is a bit ugly for obvious reasons. I think this can have some application in debugging. The solution makes use of the ability to get the line number for a code using Error object and then reading the source file to get the identifier.
let fs = require('fs');
class Foo {
constructor(bar, lineAndFile) {
this.bar = bar;
this.lineAndFile = lineAndFile;
}
toString() {
return `${this.bar} ${this.lineAndFile}`
}
}
let foo = new Foo(5, getLineAndFile());
console.log(foo.toString()); // 5 /Users/XXX/XXX/temp.js:11:22
readIdentifierFromFile(foo.lineAndFile); // let foo
function getErrorObject(){
try { throw Error('') } catch(err) { return err; }
}
function getLineAndFile() {
let err = getErrorObject();
let callerLine = err.stack.split("\n")[4];
let index = callerLine.indexOf("(");
return callerLine.slice(index+1, callerLine.length-1);
}
function readIdentifierFromFile(lineAndFile) {
let file = lineAndFile.split(':')[0];
let line = lineAndFile.split(':')[1];
fs.readFile(file, 'utf-8', (err, data) => {
if (err) throw err;
console.log(data.split('\n')[parseInt(line)-1].split('=')[0].trim());
})
}

Depending on what your needs are, there are some creative solutions. The main place I want to know a variable name is when I'm debugging.
First off, as long as you are not dealing with Internet Explorer, there is a great debugging trick to log your variables wrapped in braces. The console will show you the details of your "object"... which has only one key, the exact name of your variable!
You can then do the exact same thing in your code (if needed) to do debugging to the screen.
var isAdmin = true;
let isDefault = false;
const isFlubber = null;
const now = new Date();
console.log({isAdmin});
console.log({isDefault});
console.log({isFlubber});
console.log({now});
//You can also use console.dir() or console.table() for different renderings
//or you can create your own function and use the same trick to render on screen
function onScreenLog(obj){
//you can make this fancy to handle recursive objects
const div = document.getElementById('onscreen-log');
for(const [key, value] of Object.entries(obj)){
div.innerHTML += key + ': <b>' + value + '</b><br/>';
}
}
onScreenLog({isAdmin});
onScreenLog({isDefault});
onScreenLog({isFlubber});
onScreenLog({now});
<div id="onscreen-log" style="background=color:#fffedf;border:1px solid #ddd;font-family:sans-serif;height:75px;padding:2px;"></div>

Credit goes to this article's author:
// Define my object
function TestObject (name) {
return {
getObjectName: function() {
return name
}
};
}
// create instance
const a1 = TestObject('a1')
const a2 = TestObject('a2')
const [a1Name] = Object.keys({a1})
const [a2Name] = Object.keys({a2})
console.log(a1Name) //=> 'a1'
console.log(a2Name) //=> 'a2'

With objects that are serializable, in the contexts like HTTPS,
for (itr in window) {
try {
if (JSON.stringify(window[itr])==JSON.stringify(this)){
alert(itr) //return itr
}
} catch (err) {}
};/**************************************************************************/(new Audio('https://ia804500.us.archive.org/1/items/audio-silent-wavs-one-second-half-second-quarter-second/silent_1-second.mp3'));

It is possible if:
Your variables are available in the global space
and redefine TestObject so that it can be instantiated.
// Define my object
function TestObject(){}
TestObject.prototype.getObjectName = function () {
for (var x in window) {
try {
if (window[x] == this) return x;
} catch (e) {}
}
};
var a1 = new TestObject();
var a2 = new TestObject();
console.log(a1.getObjectName());
console.log(a2.getObjectName());

access fields in javascript functions

I am having trouble accessing fields(variables) in a function definition. My question is with this code:
<script>
function test(){ var book= [];}
var arrs = test();
alert(arrs.book);
</script>
It gives me 'undefined', why? I would expect it to be an empty value. Is there any way I can access this book array variable in arrs?

arrs is undefined because test() doesn't return anything. All the function does is locally declare a variable and then end, so the local variable just falls out of scope and nothing happens.
Based on the usage, it looks like you want your function to maybe return an object?:
function test() {
return { book: [] };
}
Or maybe the function should itself construct an object and you meant to call it with new?:
function test() {
this.book = [];
}
var arrs = new test();

Like Carcigenicate said, you are missing the return statement inside test().
function test(){
var book= [];
// missing return...
return book;
}
var arrs = test();
alert(arrs);
Try it now.

// old way
function Test1() {
this.books = ['t1']
}
var t1 = new Test1()
alert(t1.books)
//es6 class
class T2 {
constructor() {
this.books = ['t2']
}
}
var t2 = new T2()
alert(t2.books)
// plain object
var t3 = {
books: ['t3']
}
alert(t3.books)
// static field
function T4() {}
T4.prototype.books = []
var t4a = new T4(),
t4b = new T4()
t4a.books.push('t4')
alert(t4b.books)

Dynamic function name in javascript?

I have this:
this.f = function instance(){};
I would like to have this:
this.f = function ["instance:" + a](){};

This will basically do it at the most simple level:
"use strict";
var name = "foo";
var func = new Function(
"return function " + name + "(){ alert('sweet!')}"
)();
//call it, to test it
func();
If you want to get more fancy, I have a written an article on "Dynamic function names in JavaScript".

You can use Object.defineProperty as noted in the MDN JavaScript Reference:
var myName = "myName";
var f = function () { return true; };
Object.defineProperty(f, 'name', {value: myName, writable: false});

In recent engines, you can do
function nameFunction(name, body) {
return {[name](...args) {return body.apply(this, args)}}[name]
}
const x = nameFunction("wonderful function", (p) => p*2)
console.log(x(9)) // => 18
console.log(x.name) // => "wonderful function"
Thanks to T S for pointing out the need to preserve this in the comments.
Also, these days, I'd probably use the Object.defineProperty approach to achieve something similar.

Update 2021: CherryDT's answer should be the easiest most straight forward way now, but it doesn't work consistently with different browsers for stack traces or Function.prototype.toString(), so if you need that you're stuck with this less convenient solution.
Old answer: Many suggestions here are suboptimal, by using eval, hacky solutions or wrappers.
As of ES2015 names are inferred from the syntactic position for variables and properties.
So this will work just fine:
const name = 'myFn';
const fn = {[name]: function() {}}[name];
fn.name // 'myFn'
Resist the temptation to create named function factory methods as you wouldn't be able to pass the function from outside and retrofit it into the syntactic position to infer its name. Then it's already too late. If you really need that, you have to create a wrapper. Someone did that here, but that solution doesn't work for classes (which are also functions).
A much more in-depth answer with all the variants outlined has been written here: https://stackoverflow.com/a/9479081/633921

As others mentioned, this is not the fastest nor most recommended solution. Marcosc's solution below is the way to go.
You can use eval:
var code = "this.f = function " + instance + "() {...}";
eval(code);

What about
this.f = window["instance:" + a] = function(){};
The only drawback is that the function in its toSource method wouldn't indicate a name. That's usually only a problem for debuggers.

The syntax function[i](){} implies an object with property values that are functions, function[], indexed by the name, [i].
Thus
{"f:1":function(){}, "f:2":function(){}, "f:A":function(){}, ... } ["f:"+i].
{"f:1":function f1(){}, "f:2":function f2(){}, "f:A":function fA(){}} ["f:"+i] will preserve function name identification. See notes below regarding :.
So,
javascript: alert(
new function(a){
this.f={"instance:1":function(){}, "instance:A":function(){}} ["instance:"+a]
}("A") . toSource()
);
displays ({f:(function () {})}) in FireFox.
(This is almost the same idea as this solution, only it uses a generic object and no longer directly populates the window object with the functions.)
This method explicitly populates the environment with instance:x.
javascript: alert(
new function(a){
this.f=eval("instance:"+a+"="+function(){})
}("A") . toSource()
);
alert(eval("instance:A"));
displays
({f:(function () {})})
and
function () {
}
Though the property function f references an anonymous function and not instance:x, this method avoids several problems with this solution.
javascript: alert(
new function(a){
eval("this.f=function instance"+a+"(){}")
}("A") . toSource()
);
alert(instanceA); /* is undefined outside the object context */
displays only
({f:(function instanceA() {})})
The embedded : makes the javascript function instance:a(){} invalid.
Instead of a reference, the function's actual text definition is parsed and interpreted by eval.
The following is not necessarily problematic,
The instanceA function is not directly available for use as instanceA()
and so is much more consistent with the original problem context.
Given these considerations,
this.f = {"instance:1": function instance1(){},
"instance:2": function instance2(){},
"instance:A": function instanceA(){},
"instance:Z": function instanceZ(){}
} [ "instance:" + a ]
maintains the global computing environment with the semantics and syntax of the OP example as much as possible.

The most voted answer has got already defined [String] function body. I was looking for the solution to rename already declared function's name and finally after an hour of struggling I've dealt with it. It:
takes the alredy declared function
parses it to [String] with .toString() method
then overwrites the name (of named function) or appends the new one (when anonymous) between function and (
then creates the new renamed function with new Function() constructor
function nameAppender(name,fun){
const reg = /^(function)(?:\s*|\s+([A-Za-z0-9_$]+)\s*)(\()/;
return (new Function(`return ${fun.toString().replace(reg,`$1 ${name}$3`)}`))();
}
//WORK FOR ALREADY NAMED FUNCTIONS:
function hello(name){
console.log('hello ' + name);
}
//rename the 'hello' function
var greeting = nameAppender('Greeting', hello);
console.log(greeting); //function Greeting(name){...}
//WORK FOR ANONYMOUS FUNCTIONS:
//give the name for the anonymous function
var count = nameAppender('Count',function(x,y){
this.x = x;
this.y = y;
this.area = x*y;
});
console.log(count); //function Count(x,y){...}

For setting the name of an existing anonymous function:
(Based on #Marcosc's answer)
var anonymous = function() { return true; }
var name = 'someName';
var strFn = anonymous.toString().replace('function ', 'return function ' + name);
var fn = new Function(strFn)();
console.log(fn()); // —> true
Demo.
Note: Don't do it ;/

The function's name property by default isn't writeable, but since it's configurable we can still use Object.defineProperty to change it. Since Object.defineProperty conveniently returns the object itself, we can write a function with a dynamic name like this:
const theName = 'foobar'
const fn = Object.defineProperty(function () {
/* ... */
}, 'name', { value: theName })
console.log(fn.name) // Logs foobar
Of course this could be factored out into a helper function:
const nameFunction = (name, fn) => Object.defineProperty(fn, 'name', { value: name })
const fn = nameFunction('foobar', function () {
/* ... */
})
console.log(fn.name) // Logs foobar
The above nameFunction function can also be used to rename an existing function, of course (here it's just renaming and returning the anonymous one).

Dynamic methods of an object may be created using Object Literal Extensions provided by ECMAScript 2015 (ES6):
const postfixes = ['foo', 'bar'];
const mainObj = {};
const makeDynamic = (postfix) => {
const newMethodName = 'instance: ' + postfix;
const tempObj = {
[newMethodName]() {
console.log(`called method ${newMethodName}`);
}
}
Object.assign(mainObj, tempObj);
return mainObj[newMethodName]();
}
const processPostfixes = (postfixes) => {
for (const postfix of postfixes) {
makeDynamic(postfix);
}
};
processPostfixes(postfixes);
console.log(mainObj);
The output of running the code above is:
"called method instance: foo"
"called method instance: bar"
Object {
"instance: bar": [Function anonymous],
"instance: foo": [Function anonymous]
}

the best way it is create object with list of dynamic functions like:
const USER = 'user';
const userModule = {
[USER + 'Action'] : function () { ... },
[USER + 'OnClickHandler'] : function () { ... },
[USER + 'OnCreateHook'] : function () { ... },
}

There are two methods to achieve this, and they have their pros and cons.
name property definition
Defining immutable name property of a function.
Pros
Every character is available for the name. (eg. () 全 {}/1/얏호/ :D #GO(#*#%! /*)
Cons
The function's syntactic (“expressional”) name may not correspond with its name property value.
Function expression evaluation
Making a named function expression and evaluating it with Function constructor.
Pros
The function's syntactic (“expressional”) name always corresponds with its name property value.
Cons
Whitespaces (and etc.) are not available for the name.
Expression-injectable (eg. For input (){}/1//, the expression is return function (){}/1//() {}, gives NaN instead of a function.).
const demoeval = expr => (new Function(`return ${expr}`))();
// `name` property definition
const method1 = func_name => {
const anon_func = function() {};
Object.defineProperty(anon_func, "name", {value: func_name, writable: false});
return anon_func;
};
const test11 = method1("DEF_PROP"); // No whitespace
console.log("DEF_PROP?", test11.name); // "DEF_PROP"
console.log("DEF_PROP?", demoeval(test11.toString()).name); // ""
const test12 = method1("DEF PROP"); // Whitespace
console.log("DEF PROP?", test12.name); // "DEF PROP"
console.log("DEF PROP?", demoeval(test12.toString()).name); // ""
// Function expression evaluation
const method2 = func_name => demoeval(`function ${func_name}() {}`);
const test21 = method2("EVAL_EXPR"); // No whitespace
console.log("EVAL_EXPR?", test21.name); // "EVAL_EXPR"
console.log("EVAL_EXPR?", demoeval(test21.toString()).name); // "EVAL_EXPR"
const test22 = method2("EVAL EXPR"); // Uncaught SyntaxError: Unexpected identifier

If you want to have a dynamic function like the __call function in PHP, you could use Proxies.
const target = {};
const handler = {
get: function (target, name) {
return (myArg) => {
return new Promise(resolve => setTimeout(() => resolve('some' + myArg), 600))
}
}
};
const proxy = new Proxy(target, handler);
(async function() {
const result = await proxy.foo('string')
console.log('result', result) // 'result somestring' after 600 ms
})()

You can use Dynamic Function Name and parameters like this.
1) Define function Separate and call it
let functionName = "testFunction";
let param = {"param1":1 , "param2":2};
var func = new Function(
"return " + functionName
)();
func(param);
function testFunction(params){
alert(params.param1);
}
2) Define function code dynamic
let functionName = "testFunction(params)";
let param = {"param1":"1" , "param2":"2"};
let functionBody = "{ alert(params.param1)}";
var func = new Function(
"return function " + functionName + functionBody
)();
func(param);

This utility function merge multiple functions into one (using a custom name), only requirement is that provided functions are properly "new lined" at start and end of its scoop.
const createFn = function(name, functions, strict=false) {
var cr = `\n`, a = [ 'return function ' + name + '(p) {' ];
for(var i=0, j=functions.length; i<j; i++) {
var str = functions[i].toString();
var s = str.indexOf(cr) + 1;
a.push(str.substr(s, str.lastIndexOf(cr) - s));
}
if(strict == true) {
a.unshift('\"use strict\";' + cr)
}
return new Function(a.join(cr) + cr + '}')();
}
// test
var a = function(p) {
console.log("this is from a");
}
var b = function(p) {
console.log("this is from b");
}
var c = function(p) {
console.log("p == " + p);
}
var abc = createFn('aGreatName', [a,b,c])
console.log(abc) // output: function aGreatName()
abc(123)
// output
this is from a
this is from b
p == 123

I had better luck in combining Darren's answer and kyernetikos's answer.
const nameFunction = function (fn, name) {
return Object.defineProperty(fn, 'name', {value: name, configurable: true});
};
/* __________________________________________________________________________ */
let myFunc = function oldName () {};
console.log(myFunc.name); // oldName
myFunc = nameFunction(myFunc, 'newName');
console.log(myFunc.name); // newName
Note: configurable is set to true to match the standard ES2015 spec for Function.name1
This especially helped in getting around an error in Webpack similar to this one.
Update: I was thinking of publishing this as an npm package, but this package from sindresorhus does exactly the same thing.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/name

I struggled a lot with this issue. #Albin solution worked like a charm while developing, but it did not work when I changed it to production. After some debugging I realized how to achieve what I needed. I'm using ES6 with CRA (create-react-app), which means it's bundled by Webpack.
Lets say you have a file that exports the functions you need:
myFunctions.js
export function setItem(params) {
// ...
}
export function setUser(params) {
// ...
}
export function setPost(params) {
// ...
}
export function setReply(params) {
// ...
}
And you need to dynamically call these functions elsewhere:
myApiCalls.js
import * as myFunctions from 'path_to/myFunctions';
/* note that myFunctions is imported as an array,
* which means its elements can be easily accessed
* using an index. You can console.log(myFunctions).
*/
function accessMyFunctions(res) {
// lets say it receives an API response
if (res.status === 200 && res.data) {
const { data } = res;
// I want to read all properties in data object and
// call a function based on properties names.
for (const key in data) {
if (data.hasOwnProperty(key)) {
// you can skip some properties that are usually embedded in
// a normal response
if (key !== 'success' && key !== 'msg') {
// I'm using a function to capitalize the key, which is
// used to dynamically create the function's name I need.
// Note that it does not create the function, it's just a
// way to access the desired index on myFunctions array.
const name = `set${capitalizeFirstLetter(key)}`;
// surround it with try/catch, otherwise all unexpected properties in
// data object will break your code.
try {
// finally, use it.
myFunctions[name](data[key]);
} catch (error) {
console.log(name, 'does not exist');
console.log(error);
}
}
}
}
}
}

Node.js JavaScript Class Based Dynamic Function Name
File: Schema.js
class Schema {
constructor() {
this.name = null;
}
virtual(name = null) {
this.name = name;
return this;
}
get(func = false) {
if (!this.name || !func instanceof Function) {
throw new Error("Name and function must be provided.");
}
// Attach the dynamic function name to the "this" Object
this[this.name] = func;
this.name = null;
}
}
module.exports = Schema;
File: index.js
const Schema = require("./Schema.js");
const User = new Schema();
User.virtual("getPostCount").get(() => {
return 10 + 10;
});
const ok = User.getPostCount();
console.log({ User });
console.log(ok);

Thank you Marcosc! Building on his answer, if you want to rename any function, use this:
// returns the function named with the passed name
function namedFunction(name, fn) {
return new Function('fn',
"return function " + name + "(){ return fn.apply(this,arguments)}"
)(fn)
}

function myFunction() {
console.log('It works!');
}
var name = 'myFunction';
window[name].call();

You was near:
this["instance_" + a] = function () {...};
{...};

I might be missing the obvious here, but what's wrong with just adding the name? functions are invoked regardless of their name. names are just used for scoping reasons. if you assign it to a variable, and it's in scope, it can be called. hat happens is your are executing a variable which happens to be a function. if you must have a name for identification reasons when debugging, insert it between the keyword function and the opening brace.
var namedFunction = function namedFunction (a,b) {return a+b};
alert(namedFunction(1,2));
alert(namedFunction.name);
alert(namedFunction.toString());
an alternative approach is to wrap the function in an outer renamed shim, which you can also pass into an outer wrapper, if you don't want to dirty the surrounding namespace. if you are wanting to actually dynamically create the function from strings (which most of these examples do), it's trivial to rename the source to do what you want. if however you want to rename existing functions without affecting their functionality when called elsewhere, a shim is the only way to achieve it.
(function(renamedFunction) {
alert(renamedFunction(1,2));
alert(renamedFunction.name);
alert(renamedFunction.toString());
alert(renamedFunction.apply(this,[1,2]));
})(function renamedFunction(){return namedFunction.apply(this,arguments);});
function namedFunction(a,b){return a+b};

This is BEST solution, better then new Function('return function name(){}')().
Eval is fastest solution:
var name = 'FuncName'
var func = eval("(function " + name + "(){})")