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Arrow Function: Basics

bhagatparwinder profile image Parwinder 👨🏻‍💻 ・1 min read

Introduction

Arrow functions introduced in ES6 is a concise way of creating functions compared to function expressions.

The name arrow function comes from the use of =>.

Syntax:

const functionName = (arg1, arg2, ... argN) => {
    return value;
}

Example

const multiply = (a, b) => {
    return a * b;
}

console.log(multiply(7, 8)); // 56
console.log(multiply(3, 2)); // 6

Key Features

  • Arrow functions are anonymous function until they are assigned to a variable.
  • If there is only 1 argument, we can skip parenthesis.
   const square = x => {
       return x * x;
   }

   console.log(square(2)); // 4
   console.log(square(7)); // 49

The only caveat to this rule is that if the 1 argument is destructured.

const foo = ({name = "New User"}) => name;

console.log(foo({})); // New User
console.log(foo({name: "Parwinder"})); // Parwinder
  • If there are no arguments, we need to have the parenthesis
   const greeting = () => {
       return "Hello World!";
   }

   console.log(greeting()); // Hello World!
  • If the function body is an expression, it will return the expression, we can remove the brackets and the return keyword.
   const greeting = () => "Hello World!";
   console.log(greeting()); // Hello World

Now that we know all these key features, let us rewrite the example to get the square of a number:

const square = x => x * x;
console.log(square(4)); // 16

Discussion (7)

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pentacular profile image
pentacular • Edited

If there is only 1 argument, we can skip parenthesis

Unless there's one argument which is destructured.

const q = ({ a = 1 }) => a;

If the function has only one statement, and the statement returns a value, we can remove the brackets and the return keyword.

Rather, if the function body is an expression, the value of the expression is returned.

Consider why the following is not legal -- it is one statement, but it is not a valid expression.

const r = () => return;

This is also why you need parentheses in the following case

const s = () => ({ a: 1 });
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bhagatparwinder profile image
Parwinder 👨🏻‍💻 Author

I do consider both examples as edge cases but they are correct and valid. I have updated the article to showcase them. I appreciate the feedback.

Thanks for reading! ♥️

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pentacular profile image
pentacular

If the function has only one statement, and the statement returns a value, we can remove the brackets and the return keyword.

const greeting = () => "Hello World!";
console.log(greeting()); // Hello World

"Hello World!" is not a statement, and if it were, it would not return.

It is an expression, and it evaluates.

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bhagatparwinder profile image
Parwinder 👨🏻‍💻 Author

Yep agreed. Corrected the post as I mentioned above.

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pentacular profile image
pentacular • Edited

Looking better, although

If the function body is an expression, it will return the expression, we can remove the brackets and the return keyword.

If it has {} or return, then the function body isn't an expression. :)

I think the simplest way to express it is that there are two distinct forms.

  1. () => expression;
  2. () => { ...; return value; }; // block of statements
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bhagatparwinder profile image
Parwinder 👨🏻‍💻 Author

Your bullet point 2 is generally how folks write functions if they are not using shorthand. That is why my blog post starts with the example of an arrow function that has {} and return.

The shorthand is omitting the braces and return when there is one expression, Which is reflected by the fourth bullet point (or the part you just quoted). So at this point I am covering both the points you mentioned.

Thanks for the feedback.

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pentacular profile image
pentacular

Sure. The main thing is not to conflate expressions and statements. :)

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