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
);