How does ES6 function's default parameter return undefined, opposed to throwing an error?

Question

var myFunc = function(x = getUndefined()){
    return x
};
function getUndefined(){
    return undefined
};

I want to know why this returns undefined instead of throwing some sort of error. How does x know not to keep calling getUndefined() ?

Answer 1

To get a sense of what is happening under the hood, you can look at how this code transpiles down to ES5:

"use strict";

var myFunc = function myFunc() {
    var x = arguments.length <= 0 || arguments[0] === undefined ? getUndefined() : arguments[0];

    return x;
};
function getUndefined() {
    return undefined;
};

The function getUndefined is only called if there is no passed value for x .

While ES6 engines might not follow this exact strategy, it is most likely fairly similar.