return;
return expression;

expression Optional

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

The return statement can only be used within function bodies. When a return statement is used in a function body, the execution of the function is stopped. The return statement has different effects when placed in different functions:

• In a plain function, the call to that function evaluates to the return value.
• In an async function, the produced promise is resolved with the returned value.
• In a generator function, the produced generator object's next() method returns { done: true, value: returnedValue }.
• In an async generator function, the produced async generator object's next() method returns a promise fulfilled with { done: true, value: returnedValue }.

If a return statement is executed within a try block, its finally block, if present, is first executed, before the value is actually returned.

The syntax forbids line terminators between the return keyword and the expression to be returned.

return
a + b;

The code above is transformed by automatic semicolon insertion (ASI) into:

return;
a + b;

This makes the function return undefined and the a + b expression is never evaluated. This may generate a warning in the console.

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

return (
  a + b
);

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

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

counter();

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

See also the article about Closures.

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

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

• Functions
• Closures