I have no idea Object(this) means

Question

In https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Array/fill

there is a line like

// Steps 1-2.
if (this == null) {
           


        

Answer 1

As suggested in the comments on the code, this section is to accurately pollyfill the first steps documented in the spec.

1. Let O be ToObject(this value).
2. ReturnIfAbrupt(O).

Though a bit out-of-order, this is performing the fucntion of ToObject(this value):

var O = Object(this);

Basically, if it is called on a non-object, the non-object should be cast to an Object.

For example, if we were to run this bit of mostly-nonsensical code in a JavaScript engine which natively supports this method, we would see a Number object instance gets returned.

Array.prototype.fill.call(123);

That line would ensure the same result from the polyfill.