Why [null, undefined, []] == ",," returns true

Question

I know that abstract comparison will convert LHS to String and String([null, undefined, []]) will result to ',,'.

But String(null) is 'null' and String(undefined) is 'undefined'. So how String([null, undefined, []]) is ',,'?

Answer 1

That's because.

1. That's how == is defined https://tc39.es/ecma262/#sec-abstract-equality-comparison See step 11: If Type(x) is Object and Type(y) is either String, Number, BigInt, or Symbol, return the result of the comparison ? ToPrimitive(x) == y.
2. Then ToPrimitive happens: https://tc39.es/ecma262/#sec-toprimitive
3. Then OrdinaryToPrimitive happens: https://tc39.es/ecma262/#sec-ordinarytoprimitive which calls Array.prototype.toString
4. For arrays it calls Array.prototype.join(arr) See https://tc39.es/ecma262/#sec-array.prototype.tostring
5. That's how Array.prototype.join is implemented: https://tc39.es/ecma262/#sec-array.prototype.join, the most important steps:

Repeat, while k < len,

* If k > 0, set R to the string-concatenation of R and sep.
* Let element be ? Get(O, ! ToString(k)).
* If element is undefined or null, let next be the empty String; otherwise, let next be ? ToString(element).
* Set R to the string-concatenation of R and next.
* Set k to k + 1.