When should I use Arrow functions in ECMAScript 6?

Question

The question is directed at people who have thought about code style in the context of the upcoming ECMAScript 6 (Harmony) and who have already worked with the language.

With () => {} and function () {} we are getting two very similar ways to write functions in ES6. In other languages lambda functions often distinguish themselves by being anonymous, but in ECMAScript any function can be anonymous. Each of the two types have unique usage domains (namely when this needs to either be bound explicitly or explicitly not be bound). Between those domains there is a vast number of cases where either notation will do.

Arrow functions in ES6 have at least two limitations:

• Don't work with new
• Fixed this bound to scope at initialisation

These two limitations aside, arrow functions could theoretically replace regular functions almost anywhere. What is the right approach using them in practice? Should arrow functions be used e.g.:

• "everywhere they work", i.e. everywhere a function does not have to be agnostic about the this variable and we are not creating an object.
• only "everywhere they are needed", i.e. event listeners, timeouts, that need to be bound to a certain scope
• with 'short' functions but not with 'long' functions
• only with functions that do not contain another arrow function

What I am looking for is a guideline to selecting the appropriate function notation in the future version of ECMAScript. The guideline will need to be clear, so that it can be taught to developers in a team, and to be consistent so that it does not require constant refactoring back and forth from one function notation to another.

Answer 1

Arrow functions were created to simplify function scope and solving the this keyword by making it more simpler. They utilize the => syntax, which looks like an arrow.

Note: It does not replace the existing functions. If you replace every function syntax with arrow functions, its not going to work in all cases.

Let's have a look at the existing ES5 syntax, If the this keyword were inside an object’s method (a function that belongs to an object), what would it refer to?

var Actor = {
  name: 'RajiniKanth',
  getName: function() {
     console.log(this.name);
  }
};
Actor.getName();

The above snippet would refer to an object and print out the name "RajiniKanth". Let's explore the below snippet and see what would this point out here.

var Actor = {
  name: 'RajiniKanth',
  movies: ['Kabali', 'Sivaji', 'Baba'],
  showMovies: function() {
   this.movies.forEach(function(movie) {
     alert(this.name + " has acted in " + movie);
   });
  }
};

Actor.showMovies();

Now what about if the this keyword were inside of method’s function?

Here this would refer to window object than the inner function as its fallen out of scope. Because this, always references the owner of the function it is in, for this case — since it is now out of scope — the window/global object.

When it is inside of an object’s method — the function’s owner is the object. Thus the this keyword is bound to the object. Yet when it is inside of a function, either stand alone or within another method, it will always refer to the window/global object.

var fn = function(){
  alert(this);
}

fn(); // [object Window]

There are ways to solve this problem in our ES5 itself, let us look into that before diving into ES6 arrow functions on how solve it.

Typically you would, create a variable outside of the method’s inner function. Now the ‘forEach’ method gains access to this and thus the object’s properties and their values.

var Actor = {
  name: 'RajiniKanth',
  movies: ['Kabali', 'Sivaji', 'Baba'],
  showMovies: function() {
   var _this = this;
   this.movies.forEach(function(movie) {
     alert(_this.name + " has acted in " + movie);
   });
  }
};

Actor.showMovies();

using bind to attach the this keyword that refers to the method to the method’s inner function.

var Actor = {
  name: 'RajiniKanth',
  movies: ['Kabali', 'Sivaji', 'Baba'],
  showMovies: function() {
   this.movies.forEach(function(movie) {
     alert(_this.name + " has acted in " + movie);
   }).bind(this);
  }
};

Actor.showMovies();

Now with ES6 arrow function, we can deal with lexical scoping issue in a simpler way.

var Actor = {
  name: 'RajiniKanth',
  movies: ['Kabali', 'Sivaji', 'Baba'],
  showMovies: function() {
   this.movies.forEach((movie) => {
     alert(this.name + " has acted in " + movie);
   });
  }
};

Actor.showMovies();

Arrow functions are more like function statements, except that they bind the this to parent scope. If the arrow function is in top scope, this argument will refer to window/global scope, while an arrow function inside a regular function will have its this argument the same as its outer function.

With arrow functions this is bound to the enclosing scope at creation time and cannot be changed. The new operator, bind, call, and apply have no effect on this.

var asyncFunction = (param, callback) => {
  window.setTimeout(() => {
  callback(param);
  }, 1);
};

// With a traditional function if we don't control
// the context then can we lose control of `this`.
var o = {
  doSomething: function () {
  // Here we pass `o` into the async function,
  // expecting it back as `param`
  asyncFunction(o, function (param) {
  // We made a mistake of thinking `this` is
  // the instance of `o`.
  console.log('param === this?', param === this);
  });
  }
};

o.doSomething(); // param === this? false

In the above example, we lost the control of this. We can solve the above example by using a variable reference of this or using bind. With ES6, it becomes easier in managing the this as its bound to lexical scoping.

var asyncFunction = (param, callback) => {
  window.setTimeout(() => {
  callback(param);
  }, 1);
};

var o = {
  doSomething: function () {
  // Here we pass `o` into the async function,
  // expecting it back as `param`.
  //
  // Because this arrow function is created within
  // the scope of `doSomething` it is bound to this
  // lexical scope.
  asyncFunction(o, (param) => {
  console.log('param === this?', param === this);
  });
  }
};

o.doSomething(); // param === this? true

When not to Arrow functions

Inside an object literal.

var Actor = {
  name: 'RajiniKanth',
  movies: ['Kabali', 'Sivaji', 'Baba'],
  getName: () => {
     alert(this.name);
  }
};

Actor.getName();

Actor.getName is defined with an arrow function, but on invocation it alerts undefined because this.name is undefined as the context remains to window.

It happens because the arrow function binds the context lexically with the window object... i.e outer scope. Executing this.name is equivalent to window.name, which is undefined.

Object prototype

The same rule applies when defining methods on a prototype object. Instead of using an arrow function for defining sayCatName method, which brings an incorrect context window:

function Actor(name) {
  this.name = name;
}
Actor.prototype.getName = () => {
  console.log(this === window); // => true
  return this.name;
};
var act = new Actor('RajiniKanth');
act.getName(); // => undefined

Invoking constructors

this in a construction invocation is the newly created object. When executing new Fn(), the context of the constructor Fn is a new object: this instanceof Fn === true.

this is setup from the enclosing context, i.e the outer scope which makes it not assigned to newly created object.

var Message = (text) => {
  this.text = text;
};
// Throws "TypeError: Message is not a constructor"
var helloMessage = new Message('Hello World!');

Callback with dynamic context

Arrow function binds the context statically on declaration and is not possible to make it dynamic. Attaching event listeners to DOM elements is a common task in client side programming. An event triggers the handler function with this as the target element.

var button = document.getElementById('myButton');
button.addEventListener('click', () => {
  console.log(this === window); // => true
  this.innerHTML = 'Clicked button';
});

this is window in an arrow function that is defined in the global context. When a click event happens, browser tries to invoke the handler function with button context, but arrow function does not change its pre-defined context. this.innerHTML is equivalent to window.innerHTML and has no sense.

You have to apply a function expression, which allows to change this depending on the target element:

var button = document.getElementById('myButton');
button.addEventListener('click', function() {
  console.log(this === button); // => true
  this.innerHTML = 'Clicked button';
});

When user clicks the button, this in the handler function is button. Thus this.innerHTML = 'Clicked button' modifies correctly the button text to reflect clicked status.

References: https://rainsoft.io/when-not-to-use-arrow-functions-in-javascript/

Answer 2

I prefer to use arrow functions at all times where access to local this is not needed, because arrow function do not bind their own this, arguments, super, or new.target.

Answer 3

Arrow functions - most widely used ES6 feature so far ...

Usage : All ES5 functions should be replaced with ES6 arrow functions except in following scenarios:

Arrow functions should NOT be used:

1. When we want function hoisting
   • as arrow functions are anonymous.
2. When we want to use this/arguments in a function
   • as arrow functions do not have this/arguments of their own, they depend upon their outer context.
3. When we want to use named function
   • as arrow functions are anonymous.
4. When we want to use function as a constructor
   • as arrow functions do not have their own this.
5. When we want to add function as a property in object literal and use object in it
   • as we can not access this (which should be object itself).

Let us understand some of the variants of arrow functions to understand better:

Variant 1: When we want to pass more than one argument to a function and return some value from it.

ES5 version:

var multiply = function (a,b) {
    return a*b;
};
console.log(multiply(5,6)); //30

ES6 version:

var multiplyArrow = (a,b) => a*b;
console.log(multiplyArrow(5,6)); //30

Note: function keyword is NOT required. => is required. {} are optional, when we do not provide {} return is implicitly added by JavaScript and when we do provide {} we need to add return if we need it.

Variant 2: When we want to pass ONLY one argument to a function and return some value from it.

ES5 version:

var double = function(a) {
    return a*2;
};
console.log(double(2)); //4

ES6 version:

var doubleArrow  = a => a*2;
console.log(doubleArrow(2)); //4

Note: When passing only one argument we can omit parenthesis ().

Variant 3: When we do NOT want to pass any argument to a function and do NOT want to return any value.

ES5 version:

var sayHello = function() {
    console.log("Hello");
};
sayHello(); //Hello

ES6 version:

var sayHelloArrow = () => {console.log("sayHelloArrow");}
sayHelloArrow(); //sayHelloArrow

Variant 4: When we want to explicitly return from arrow functions.

ES6 version:

var increment = x => {
  return x + 1;
};
console.log(increment(1)); //2

Variant 5: When we want to return an object from arrow functions.

ES6 version:

var returnObject = () => ({a:5});
console.log(returnObject());

Note: We need to wrap the object in parenthesis () otherwise JavaScript cannot differentiate between a block and an object.

Variant 6: Arrow functions do NOT have arguments (an array like object) of their own they depend upon outer context for arguments.

ES6 version:

function foo() {
  var abc = i => arguments[0];
  console.log(abc(1));
};    
foo(2); // 2

Note: foo is an ES5 function, with an arguments array like object and an argument passed to it is 2 so arguments[0] for foo is 2.

abc is an ES6 arrow function since it does NOT have it's own arguments hence it prints arguments[0] of foo it's outer context instead.

Variant 7: Arrow functions do NOT have this of their own they depend upon outer context for this

ES5 version:

var obj5 = {
  greet: "Hi, Welcome ",
  greetUser : function(user) {
        setTimeout(function(){
        console.log(this.greet + ": " +  user); // "this" here is undefined.
        });
     }
};

obj5.greetUser("Katty"); //undefined: Katty

Note: The callback passed to setTimeout is an ES5 function and it has it's own this which is undefined in use-strict environment hence we get output:

undefined: Katty

ES6 version:

var obj6 = {
  greet: "Hi, Welcome ",
  greetUser : function(user) {
    setTimeout(() => console.log(this.greet + ": " +  user)); 
      // this here refers to outer context
   }
};

obj6.greetUser("Katty"); //Hi, Welcome: Katty

Note: The callback passed to setTimeout is an ES6 arrow function and it does NOT have it's own this so it takes it from it's outer context that is greetUser which has this that is obj6 hence we get output:

Hi, Welcome: Katty

Miscellaneous: We cannot use new with arrow functions. Arrow functions do Not have prototype property. We do NOT have binding of this when arrow function is invoked through apply or call.

Answer 4

In a simple way,

var a =20; function a(){this.a=10; console.log(a);} 
//20, since the context here is window.

Another instance:

var a = 20;
function ex(){
this.a = 10;
function inner(){
console.log(this.a); //can you guess the output of this line.
}
inner();
}
var test = new ex();

Ans: The console would print 20.

The reason being whenever a function is executed its own stack is created, in this example ex function is executed with the new operator so a context will be created, and when inner is executed it JS would create a new stack and execute the inner function a global context though there is a local context.

So, if we want inner function to have a local context which is ex then we need to bind the context to inner function.

Arrows solve this problem, instead of taking the Global context they take the local context if exists any. In the given example, it will take new ex() as this.

So, in all cases where binding is explicit Arrows solve the problem by defaults.

Answer 5

According to the proposal, arrows aimed "to address and resolve several common pain points of traditional Function Expression." They intended to improve matters by binding this lexically and offering terse syntax.

However,

• One cannot consistently bind this lexically
• Arrow function syntax is delicate and ambiguous

Therefore, arrow functions create opportunities for confusion and errors, and should be excluded from a JavaScript programmer's vocabulary, replaced with function exclusively.

Regarding lexical this

this is problematic:

function Book(settings) {
    this.settings = settings;
    this.pages = this.createPages();
}
Book.prototype.render = function () {
    this.pages.forEach(function (page) {
        page.draw(this.settings);
    }, this);
};

Arrow functions intend to fix the problem where we need to access a property of this inside a callback. There are already several ways to do that: One could assign this to a variable, use bind, or use the 3rd argument available on the Array aggregate methods. Yet arrows seem to be the simplest workaround, so the method could be refactored like this:

this.pages.forEach(page => page.draw(this.settings));

However, consider if the code used a library like jQuery, whose methods bind this specially. Now, there are two this values to deal with:

Book.prototype.render = function () {
    var book = this;
    this.$pages.each(function (index) {
        var $page = $(this);
        book.draw(book.currentPage + index, $page);
    });
};

We must use function in order for each to bind this dynamically. We can't use an arrow function here.

Dealing with multiple this values can also be confusing, because it's hard to know which this an author was talking about:

function Reader() {
    this.book.on('change', function () {
        this.reformat();
    });
}

Did the author actually intend to call Book.prototype.reformat? Or did he forget to bind this, and intend to call Reader.prototype.reformat? If we change the handler to an arrow function, we will similarly wonder if the author wanted the dynamic this, yet chose an arrow because it fit on one line:

function Reader() {
    this.book.on('change', () => this.reformat());
}

One may pose: "Is it exceptional that arrows could sometimes be the wrong function to use? Perhaps if we only rarely need dynamic this values, then it would still be okay to use arrows most of the time."

But ask yourself this: "Would it be 'worth it' to debug code and find that the result of an error was brought upon by an 'edge case?'" I'd prefer to avoid trouble not just most of the time, but 100% of the time.

There is a better way: Always use function (so this can always be dynamically bound), and always reference this via a variable. Variables are lexical and assume many names. Assigning this to a variable will make your intentions clear:

function Reader() {
    var reader = this;
    reader.book.on('change', function () {
        var book = this;
        book.reformat();
        reader.reformat();
    });
}

Furthermore, always assigning this to a variable (even when there is a single this or no other functions) ensures one's intentions remain clear even after the code is changed.

Also, dynamic this is hardly exceptional. jQuery is used on over 50 million websites (as of this writing in February 2016). Here are other APIs binding this dynamically:

• Mocha (~120k downloads yesterday) exposes methods for its tests via this.
• Grunt (~63k downloads yesterday) exposes methods for build tasks via this.
• Backbone (~22k downloads yesterday) defines methods accessing this.
• Event APIs (like the DOM's) refer to an EventTarget with this.
• Prototypal APIs that are patched or extended refer to instances with this.

(Stats via http://trends.builtwith.com/javascript/jQuery and https://www.npmjs.com.)

You are likely to require dynamic this bindings already.

A lexical this is sometimes expected, but sometimes not; just as a dynamic this is sometimes expected, but sometimes not. Thankfully, there is a better way, which always produces and communicates the expected binding.

Regarding terse syntax

Arrow functions succeeded in providing a "shorter syntactical form" for functions. But will these shorter functions make you more successful?

Is x => x * x "easier to read" than function (x) { return x * x; }? Maybe it is, because it's more likely to produce a single, short line of code. Accoring to Dyson's The influence of reading speed and line length on the effectiveness of reading from screen,

A medium line length (55 characters per line) appears to support effective reading at normal and fast speeds. This produced the highest level of comprehension . . .

Similar justifications are made for the conditional (ternary) operator, and for single-line if statements.

However, are you really writing the simple mathematical functions advertised in the proposal? My domains are not mathematical, so my subroutines are rarely so elegant. Rather, I commonly see arrow functions break a column limit, and wrap to another line due to the editor or style guide, which nullifies "readability" by Dyson's definition.

One might pose, "How about just using the short version for short functions, when possible?" But now a stylistic rule contradicts a language constraint: "Try to use the shortest function notation possible, keeping in mind that sometimes only the longest notation will bind this as expected." Such conflation makes arrows particularly prone to misuse.

There are numerous issues with arrow function syntax:

const a = x =>
    doSomething(x);

const b = x =>
    doSomething(x);
    doSomethingElse(x);

Both of these functions are syntactically valid. But doSomethingElse(x); is not in the body of b, it is just a poorly-indented, top-level statement.

When expanding to the block form, there is no longer an implicit return, which one could forget to restore. But the expression may only have been intended to produce a side-effect, so who knows if an explicit return will be necessary going forward?

const create = () => User.create();

const create = () => {
    let user;
    User.create().then(result => {
        user = result;
        return sendEmail();
    }).then(() => user);
};

const create = () => {
    let user;
    return User.create().then(result => {
        user = result;
        return sendEmail();
    }).then(() => user);
};

What may be intended as a rest parameter can be parsed as the spread operator:

processData(data, ...results => {}) // Spread
processData(data, (...results) => {}) // Rest

Assignment can be confused with default arguments:

const a = 1;
let x;
const b = x => {}; // No default
const b = x = a => {}; // "Adding a default" instead creates a double assignment
const b = (x = a) => {}; // Remember to add parens

Blocks look like objects:

(id) => id // Returns `id`
(id) => {name: id} // Returns `undefined` (it's a labeled statement)
(id) => ({name: id}) // Returns an object

What does this mean?

() => {}

Did the author intend to create a no-op, or a function that returns an empty object? (With this in mind, should we ever place { after =>? Should we restrict ourselves to the expression syntax only? That would further reduce arrows' frequency.)

=> looks like <= and >=:

x => 1 ? 2 : 3
x <= 1 ? 2 : 3

if (x => 1) {}
if (x >= 1) {}

To invoke an arrow function expression immediately, one must place () on the outside, yet placing () on the inside is valid and could be intentional.

(() => doSomething()()) // Creates function calling value of `doSomething()`
(() => doSomething())() // Calls the arrow function

Although, if one writes (() => doSomething()()); with the intention of writing an immediately-invoked function expression, simply nothing will happen.

It's hard to argue that arrow functions are "more understandable" with all the above cases in mind. One could learn all the special rules required to utilize this syntax. Is it really worth it?

The syntax of function is unexceptionally generalized. To use function exclusively means the language itself prevents one from writing confusing code. To write procedures that should be syntactically understood in all cases, I choose function.

Regarding a guideline

You request a guideline that needs to be "clear" and "consistent." Using arrow functions will eventually result in syntactically-valid, logically-invalid code, with both function forms intertwined, meaningfully and arbitrarily. Therefore, I offer the following:

Guideline for Function Notation in ES6:

• Always create procedures with function.
• Always assign this to a variable. Do not use () => {}.

Answer 6

A while ago our team migrated all its code (a mid-sized AngularJS app) to JavaScript compiled using Traceur Babel. I'm now using the following rule of thumb for functions in ES6 and beyond:

• Use function in the global scope and for Object.prototype properties.
• Use class for object constructors.
• Use => everywhere else.

Why use arrow functions almost everywhere?

1. Scope safety: When arrow functions are used consistently, everything is guaranteed to use the same thisObject as the root. If even a single standard function callback is mixed in with a bunch of arrow functions there's a chance the scope will become messed up.
2. Compactness: Arrow functions are easier to read and write. (This may seem opinionated so I will give a few examples further on).
3. Clarity: When almost everything is an arrow function, any regular function immediately sticks out for defining the scope. A developer can always look up the next-higher function statement to see what the thisObject is.

Why always use regular functions on the global scope or module scope?

1. To indicate a function that should not access the thisObject.
2. The window object (global scope) is best addressed explicitly.
3. Many Object.prototype definitions live in the global scope (think String.prototype.truncate etc.) and those generally have to be of type function anyway. Consistently using function on the global scope helps avoid errors.
4. Many functions in the global scope are object constructors for old-style class definitions.
5. Functions can be named¹. This has two benefits: (1) It is less awkward to writefunction foo(){} than const foo = () => {} — in particular outside other function calls. (2) The function name shows in stack traces. While it would be tedious to name every internal callback, naming all the public functions is probably a good idea.
6. Function declarations are hoisted, (meaning they can be accessed before they are declared), which is a useful attribute in a static utility function.

Object constructors

Attempting to instantiate an arrow function throws an exception:

var x = () => {};
new x(); // TypeError: x is not a constructor

One key advantage of functions over arrow functions is therefore that functions double as object constructors:

function Person(name) {
    this.name = name;
}

However, the functionally identical² ES Harmony draft class definition is almost as compact:

class Person {
    constructor(name) {
        this.name = name;
    }
}

I expect that use of the former notation will eventually be discouraged. The object constructor notation may still be used by some for simple anonymous object factories where objects are programmatically generated, but not for much else.

Where an object constructor is needed one should consider converting the function to a class as shown above. The syntax works with anonymous functions/classes as well.

Readability of arrow functions

The probably best argument for sticking to regular functions - scope safety be damned - would be that arrow functions are less readable than regular functions. If your code is not functional in the first place, then arrow functions may not seem necessary, and when arrow functions are not used consistently they look ugly.

ECMAScript has changed quite a bit since ECMAScript 5.1 gave us the functional Array.forEach, Array.map and all of these functional programming features that have us use functions where for-loops would have been used before. Asynchronous JavaScript has taken off quite a bit. ES6 will also ship a Promise object, which means even more anonymous functions. There is no going back for functional programming. In functional JavaScript, arrow functions are preferable over regular functions.

Take for instance this (particularly confusing) piece of code³:

function CommentController(articles) {
    this.comments = [];

    articles.getList()
        .then(articles => Promise.all(articles.map(article => article.comments.getList())))
        .then(commentLists => commentLists.reduce((a, b) => a.concat(b)));
        .then(comments => {
            this.comments = comments;
        })
}

The same piece of code with regular functions:

function CommentController(articles) {
    this.comments = [];

    articles.getList()
        .then(function (articles) {
            return Promise.all(articles.map(function (article) { 
                return article.comments.getList();
            }));
        })
        .then(function (commentLists) {
            return commentLists.reduce(function (a, b) {
                return a.concat(b); 
            });
        })
        .then(function (comments) {
            this.comments = comments;
        }.bind(this));
}

While any one of the arrow functions can be replaced by a standard function, there would be very little to gain from doing so. Which version is more readable? I would say the first one.

I think the question whether to use arrow functions or regular functions will become less relevant over time. Most functions will either become class methods, which make away with the function keyword, or they will become classes. Functions will remain in use for patching classes through the Object.prototype. In the mean time I suggest reserving the function keyword for anything that should really be a class method or a class.

---

Notes

1. Named arrow functions have been deferred in the ES6 spec. They might still be added a future version.
2. According to the draft specification "Class declarations/expressions create a constructor function/prototype pair exactly as for function declarations" as long as a class does not use the extend keyword. A minor difference is that class declarations are constants, whereas function declarations are not.
3. Note on blocks in single statement arrow functions: I like to use a block wherever an arrow function is called for the side effect alone (e.g. assignment). That way it is clear that the return value can be discarded.