我不知道Object（this）的意思

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) {
  throw new TypeError('this is null or not defined');
}

var O = Object(this);          // <- WHAT IS THIS???????????

// Steps 3-5.
var len = O.length >>> 0;

// Steps 6-7.
var start = arguments[1];
var relativeStart = start >> 0;

// Step 8.
var k = relativeStart < 0 ?
  Math.max(len + relativeStart, 0) :
  Math.min(relativeStart, len);

// Steps 9-10.
var end = arguments[2];
var relativeEnd = end === undefined ?
  len : end >> 0;

// Step 11.
var final = relativeEnd < 0 ?
  Math.max(len + relativeEnd, 0) :
  Math.min(relativeEnd, len);

// Step 12.
while (k < final) {
  O[k] = value;
  k++;
}

// Step 13.
return O;

and I can't find any necessity to assign O as Object(this).

Is it written just for readability or is there any specific reason for assigning?

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.

Answer 2

The Object constructor returns its argument when the argument is already an object. If it's not an object, it returns the "objectified" version of the argument: a String instance if it's a string, a Number instance if it's a number, etc.

The function in question is a little weird, and in particular the value of this will usually be an object anyway. It doesn't have to be, but you kind-of have to go out of your way to get to the innards of the polyfill with a this value that isn't an object.