The return keyword

Use return to specify the value that the function should produce as a final result. When the interpreter reaches a return statement, the function that contains that statement immediately ends, and the specified value is returned to the context where the function was called:

const myFunction = function() {
   return 2 + 2;
}

myFunction();
> 4

A function that returns a value can be effectively treated as the data it contains, similar to a variable:

const myFunction = function() {
   return 2 + 2;
}

myFunction() + myFunction();
> 8

A return statement without an expression ends the function and returns undefined:

const myFunction = function() {
   return;
}

myFunction();
> undefined

Because the return keyword signals the end of a function, any code that follows an encountered return isn't executed:

const myFunction = function() {
   return true;
   console.log( "This is a string." );
}

myFunction();
> true

Additionally, code following an encountered return statement might result in a warning (but not an error) in some browsers' development consoles:

const myFunction = function() {
   return true;
   console.log( "This is a string." );
}
> unreachable code after return statement

myFunction();
> true

Again, this only applies to a return statement encountered during the execution of the function, not any code that follows a return statement sequentially:

const myFunction = function( myParameter ) {
   if( myParameter === undefined ) {
    return "This is the result.";
   }
   return "This is the alternate result.";
}

myFunction();
> "This is the result."

myFunction( true );
> "This is the alternate result."

"Short circuiting" a function using an early return can allow for more concise code than a single return statement at the end of a function. For example, the following function determines whether a passed value is a string containing five or more characters. If the passed value isn't a string literal, the code that counts the characters is unnecessary, and the function can return a false result immediately:

function myFunction( myString ) {
   if( typeof myString !== "string" ) {
    return false;
   }
   if( myString.length >= 5 ) {
    return true;
   } else {
    return false;
   }
}

myFunction( 100 );
> false

myFunction( "St" );
> false

myFunction( "String." );
> true

Arrow function expressions are unique in that the return keyword is implied when an arrow function body contains a single expression and no block syntax:

const myFunction = () => 2 + 2;

myFunction();
> 4

If you use block syntax to define the arrow function body, an explicit return is required, even if the function body only contains a single expression:

const myFunction = () => { 2 + 2 };

myFunction();
> undefined

const myFunction = () => { return 2 + 2 };

myFunction();
> 4

Check your understanding