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return Keyword

Site: Saylor Academy
Course: PRDV401: Introduction to JavaScript I
Book: return Keyword
Printed by: Guest user
Date: Sunday, May 19, 2024, 2:25 AM

Table of contents

return Keyword

The return statement ends function execution and specifies a value to be returned to the function caller.

Try It

Click here to try an example using Code Runner.

Syntax

return [expression];

expression
The expression whose value is to be returned. If omitted, undefined is returned instead.

Description

When a return statement is used in a function body, the execution of the function is stopped. If specified, a given value is returned to the function caller. For example, the following function returns the square of its argument, x, where x is a number. 

function square(x) {
  return x * x;
}
const demo = square(3);
// demo will equal 9

If the value is omitted, undefined is returned instead.

The following return statements all break the function execution:

return;
return true;
return false;
return x;
return x + y / 3;


Automatic Semicolon Insertion

The return statement is affected by automatic semicolon insertion (ASI). No line terminator is allowed between the return keyword and the expression. 

return
a + b;

is transformed by ASI into:

return;
a + b;

The console will warn "unreachable code after return statement".

Note: Starting with Firefox 40, a warning is shown in the console if unreachable code is found after a return statement.

To avoid this problem (to prevent ASI), you could use parentheses:

return (
  a + b
);

Examples

Interrupt a function

A function immediately stops at the point where return is called.

function counter() {
  for (let count = 1; ; count++) {  // infinite loop
    console.log(`${count}A`); // until 5
    if (count === 5) {
      return;
    }
    console.log(`${count}B`);  // until 4
  }
  console.log(`${count}C`);  // never appears
}

counter();

// Output:
// 1A
// 1B
// 2A
// 2B
// 3A
// 3B
// 4A
// 4B
// 5A


Returning a function

See also the article about Closures.

function magic() {
  return function calc(x) { return x * 42; };
}

const answer = magic();
answer(1337); // 56154