ES6++: 5- Arrow Functions

Arrow Functions

Arrow functions are a new way to write functions in ES6. They are a shorter syntax for writing functions and they do not have their own this and arguments. They also do not have a prototype property.

How to use arrow functions?

To use arrow functions you need to use the => operator instead of the function keyword.

// normal function
function add(a, b) {
  return a + b;
}

// arrow function
const add = (a, b) => {
  return a + b;
};

Actually, we can shorten that even more:

const add = (a, b) => a + b;

What we did here is that, when we have only a return statement in the function body, we can remove the curly braces and the return keyword and the function will return the value of the expression.

Let's see couple more examples:

// normal function
function isPositive(number) {
  return number >= 0;
}

// arrow function

const isPositive = (number) => number >= 0;

// normal function
function randomString() {
  return Math.random().toString(36).substring(2);
}

// arrow function

const randomString = () => Math.random().toString(36).substring(2);

More Shorter Syntax

If the arrow function has only one parameter you can remove the parentheses:

// normal function

function double(number) {
  return number * 2;
}

// arrow function

const double = (number) => {
  return number * 2;
};

// shorter syntax
const double = (number) => number * 2;

// more shorter syntax
const double = number => number * 2;

No Parameters

If the arrow function has no parameters you need to use empty parentheses:

// normal function

function getPi() {
  return Math.PI;
}

// arrow function

const getPi = () => {
  return Math.PI;
};

// shorter syntax
const getPi = () => Math.PI;

Arrow Functions and this

Arrow functions do not have their own this, which means that the this keyword inside an arrow function will refer to the this keyword in the outer scope.

// normal function
function Person() {
  this.age = 0;

  setTimeout(function growUp() {
    this.age++; // The this refers to the global object, which is window
  }, 1000);
}

let p = new Person();

In the above example, the this keyword inside the growUp function refers to the global object, which is window. To fix this we can use the bind method:

// normal function
function Person() {
  this.age = 0;

  setTimeout(
    function growUp() {
      this.age++; // The this refers to the Person object
    }.bind(this),
    1000
  );
}

let p = new Person();

The bind method returns a new function, where the this keyword is bound to the Person object.

In ES6 we can use arrow functions to solve this problem:

// normal function
function Person() {
  this.age = 0;

  setTimeout(() => {
    this.age++; // The this refers to the Person object
  }, 1000);
}

let p = new Person();

Arrow Functions and arguments

Arrow functions do not have their own arguments object. The arguments object is an array-like object that contains the arguments passed to the function.

// normal function
function sum() {
  let total = 0;
  for (let i = 0; i < arguments.length; i++) {
    total += arguments[i];
  }
  return total;
}

console.log(sum(1, 2, 3, 4, 5)); // 15

In the above example, the arguments object contains the arguments passed to the sum function. We can use the arguments object to loop through the arguments and add them to the total variable.

In ES6 we can use the rest parameter to solve this problem:

// normal function
function sum(...args) {
  let total = 0;
  for (let i = 0; i < args.length; i++) {
    total += args[i];
  }
  return total;
}

console.log(sum(1, 2, 3, 4, 5)); // 15

Arrow Functions and prototype

Arrow functions do not have a prototype property. This means that you cannot use the new keyword with arrow functions.

const Person = (name) => {
  this.name = name;
};

const p = new Person("John"); // TypeError: Person is not a constructor

Best Practices

You'll end up in your real projects using most of the time Arrow functions, they are shorter and easier, also with the help of the rest parameter you can pass as many arguments as you want.

So use Arrow functions if you want to get rid of the this headache and if you want to write shorter functions.

🎨 Conclusion

In this article, we learned about arrow functions in ES6. We learned how to use arrow functions and how they are different from normal functions. We also learned about the this keyword and the arguments object in arrow functions.

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Comments

[Frank Wisniewski]

check for positive number

let a = 5
console.log(Math.sign(a)) // 1

arrow function for random ??

const randomNumber = Math.random()

Both randomNumber functions return an function not an value

why getPi() Math.PI´s typeof is a Number

When using ES6, use it consequent

const sum = (...args) => 
  args.reduce( (a, b) => a + b, 0 )

console.log(sum(1, 2, 3, 4, 5)); // 15

[Hasan Zohdy]

The purpose of that example is to getting know what is the replacements for arguments in arrow functions, not about the code written inside the function itself, also the tutorial is mainly for people who are new with ES6 features, that's why i didn't write complex code so everyone can get understand it clearly.

Thanks for your comment and your feedback.

[Frank Wisniewski]

However, one should not use impractical examples.
and please change to Math.random()

[Hasan Zohdy]

Ah okay, i didn't see that i forgot the parenthesis, thanks for the hint, i updated the post.