Are 'Arrow Functions' and 'Functions' equivalent / exchangeable?

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Arrow functions in ES2015 provide a more concise syntax.

  • Can I replace all my function declarations / expressions with arrow functions now?
  • What do I have to look out for?

Examples:

Constructor function

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

// vs

const User = name => {
  this.name = name;
};

Prototype methods

User.prototype.getName = function() {
  return this.name;
};

// vs

User.prototype.getName = () => this.name;

Object (literal) methods

const obj = {
  getName: function() {
    // ...
  }
};

// vs

const obj = {
  getName: () => {
    // ...
  }
};

Callbacks

setTimeout(function() {
  // ...
}, 500);

// vs

setTimeout(() => {
  // ...
}, 500);

Variadic functions

function sum() {
  let args = [].slice.call(arguments);
  // ...
}

// vs
const sum = (...args) => {
  // ...
};

tl;dr: No! Arrow functions and function declarations / expressions are not equivalent and cannot be replaced blindly. If the function you want to replace does not use this, arguments and is not called with new, then yes.


As so often: it depends. Arrow functions have different behavior than function declarations / expressions, so let's have a look at the differences first:

1. Lexical this and arguments

Arrow functions don't have their own this or arguments binding. Instead, those identifiers are resolved in the lexical scope like any other variable. That means that inside an arrow function, this and arguments refer to the values of this and arguments in the environment the arrow function is defined in (i.e. "outside" the arrow function):

// Example using a function expression
function createObject() {
  console.log('Inside `createObject`:', this.foo);
  return {
    foo: 42,
    bar: function() {
      console.log('Inside `bar`:', this.foo);
    },
  };
}

createObject.call({foo: 21}).bar(); // override `this` inside createObject

Are dictionary definition, of are is a plural, present tense conjugation of the verb be. An example of usage of the word "are" is to change the following phrase to present tense: “We will be warm.” Are definition, present indicative plural and 2nd person singular of be. See more.

Arrow functions => best ES6 feature so far. They are a tremendously powerful addition to ES6, that I use constantly.

Wait, you can't use arrow function everywhere in your code, its not going to work in all cases like this where arrow functions are not usable. Without a doubt, the arrow function is a great addition it brings code simplicity.

But you can’t use an arrow function when a dynamic context is required: defining methods, create objects with constructors, get the target from this when handling events.

Arrow functions should NOT be used because:
  1. They do not have this

    It uses "lexical scoping" to figure out what the value of "this" should be. In simple word lexical scoping it uses "this" from the inside the function’s body.

  2. They do not have arguments

    Arrow functions don’t have an arguments object. But the same functionality can be achieved using rest parameters.

    let sum = (...args) => args.reduce((x, y) => x + y, 0) sum(3, 3, 1) // output - 7 `

  3. They cannot be used with new

    Arrow functions can't be construtors because they do not have a prototype property.

When to use arrow function and when not:
  1. Don't use to add function as a property in object literal because we can not access this.
  2. Function expressions are best for object methods. Arrow functions are best for callbacks or methods like map, reduce, or forEach.
  3. Use function declarations for functions you’d call by name (because they’re hoisted).
  4. Use arrow functions for callbacks (because they tend to be terser).

Are definition and meaning, Easy Learning GrammarBeThe verb be is used as an auxiliary verb and it can also be used as a main verb. are 1 (är) v. Second person singular and plural and first and third person plural present indicative of be. [Middle English aren, from Old English aron, earon; see er- in

To use arrow functions with function.prototype.call, I made a helper function on the object prototype:

  // Using
  // @func = function() {use this here} or This => {use This here}
  using(func) {
    return func.call(this, this);
  }

usage

  var obj = {f:3, a:2}
  .using(This => This.f + This.a) // 5

Edit

You don't NEED a helper. You could do:

var obj = {f:3, a:2}
(This => This.f + This.a).call(undefined, obj); // 5

Is "Is" a Verb? Is the Word Is a Verb?, be. It takes the third person singular present form. Are, basic unit of area in the metric system, equal to 100 square metres and the equivalent of 0.0247 acre. Its multiple, the hectare (equal to 100 ares), is the principal unit of land measurement for most of the

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Are, Are is the second-person singular and plural forms of the verb "to be", the copula of the English language. From Middle English aren, from Old English earun, earon (“are”), reinforced by Old Norse plural forms in er- (displacing alternative Old English sind and bēoþ), from Proto-Germanic *arun (“(they) are”), from Proto-Germanic *esi / *izi (a form of Proto-Germanic *wesaną (“to be”)), from Proto-Indo-European *h₁ésti (“is”).

ARE, Are definition is - present tense second-person singular and present tense plural of be. When deciding whether to use is or are, look at whether the noun is plural or singular. If the noun is singular, use is. If it is plural or there is more than one noun, use are. The cat is eating all of his food.

Comments
  • Similar questions about arrow functions have come up more and more with ES2015 becoming more popular. I didn't feel like there was a good canonical question/answer for this issue so I created this one. If you think that there already is a good one, please let me know and I will close this one as duplicate or delete it. Feel free to improve the examples or add new ones.
  • What about JavaScript ecma6 change normal function to arrow function? Of course, a normal question can never be as good and generic as one specifically written to be a canonical.
  • Look at this Plnkr example The variable this is very different timesCalled increments only by 1 each time the button is called. Which answers my personal question: .click( () => { } ) and .click(function() { }) both create the same number of functions when used in a loop as you can see from the Guid count in the Plnkr.
  • Related post - When should I use Arrow functions in ECMAScript 6?
  • Possibly worth mentioning that the lexical this also affects super and that they have no .prototype.
  • It would also be good to mention that they aren't syntactically interchangeable -- an arrow function (AssignmentExpression) can't just be dropped in everywhere a function expression (PrimaryExpression) can and it trips people up fairly frequently (especially since there've been parsing errors in major JS implementations).
  • @JMM: "it trips people up fairly frequently" can you provide a concrete example? Skimming over the spec, it seems that the places where you can put a FE but not an AF would result in runtime errors anyway...
  • Sure, I mean stuff like trying to immediately invoke an arrow function like a function expression (() => {}()) or do something like x || () => {}. That's what I mean: runtime (parse) errors. (And even though that's the case, fairly frequently people think the error is in error.) Are you just trying to cover logic errors that would go unnoticed because they don't necessarily error when parsed or executed? new'ing one is a runtime error right?
  • Here are some links of it coming up in the wild: substack/node-browserify#1499, babel/babel-eslint#245 (this is an async arrow, but I think it's the same basic issue), and a bunch of issues on Babel that are hard to find now, but here's one T2847.
  • the 2. They do not have arguments, I am sorry isn't true, one can have argument without use the ... operator, maybe you want to say that thy do not have array as argument
  • @CarmineTambascia Read about the special arguments object which is not available in arrow functions here: developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…