Return values

Learn to return a value from a function with return.

Return value

The return keyword sends a value back from a function and returns control to the caller. When return runs, the rest of the function does not; the function ends.

Example: function double(n) { return n * 2; }. Here return n * 2 gives the caller twice n. The call double(5) returns 10; you can assign this result to a variable or print it with console.log.

If you omit return, the function still runs but produces no value; the result is undefined. Forgetting return in a computing function is a common mistake.

Without return, undefined

If a function has no return, or return is followed by no value, JavaScript returns undefined. This is not an error; it runs silently. For side-effect functions (printing, saving to a file) this is normal.

Example: function log(msg) { console.log(msg); }. This function returns nothing; in const result = log('hello'), result is undefined. But 'hello' is still printed.

Forgetting return in a computing function is one of the most common mistakes. If you expected a value but got undefined, first check whether return is there.

Exercise Compute the sum

Define a function add(a, b) whose body returns a + b. Print the result of add(15, 27) with console.log.

Show example solution

function add(a, b) {
  return a + b;
}
console.log(add(15, 27));

Test yourself

Add the return

function double(n) { ____ n * 2; }

Show answer

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

What does a function without return give?
- null
- An empty string
- 0
- undefined
Show answer

undefined — without return JavaScript gives undefined.
Match the return concepts

return n * 2

Expression that returns a value
undefined

Result when return is missing
double(5)

Call returning 10
function double

Function definition

Next lesson: Arrow functions

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