In Object.defineProperty this.push() works but this.concat() does not work

[ganeshkbhat]

Version

18.10.0

Platform

Microsoft Windows NT 10.0.22621.0 x64

Subsystem

V8 engine Object prototype

What steps will reproduce the bug?

Just run the following

function clear(start, end) {
    this.splice((!!start) ? start : 0, (!!end) ? end : this.length);
}

function extend(iterable, index) {
    let a = [...this];
    // this.clear();
    this.concat(...[...a.splice(0, index), ...iterable, ...a]); // does not work
    // this.push(..); // works
}

Object.defineProperty(Array.prototype, 'clear', { value: clear, enumerable: true, });
Object.defineProperty(Array.prototype, 'extend', { value: extend, enumerable: true, });

let a = [1, 2, 3, 4, 5, 6, 7, 8, 9];
let b = [1, 2, 3, 4, 5, [1, 2, 3], 6, 7, [1, 2, 3, 4, [1, 2, 3], 5, 6], 8, [1, 2, [1, 2, [1, 2, 3], 3, 4, 5], 3, 4, 5], 9];

let e = [...a];
console.log("extend :", e.extend(b, 3));
console.log("result", e);

How often does it reproduce? Is there a required condition?

Always. No, this is javascript default.

What is the expected behavior?

this.concat() shoudl be able to modify.

What do you see instead?

this.push() works but this.concat() does not work.

Additional information

this.push() works but this.concat() does not work. this looks like an internal bug. is this supposed to be an RFC? https://stackoverflow.com/questions/75418335/unable-to-use-this-concatarray-inside-an-custom-array-prototype-extend-functio

[cjihrig]

The second sentence of the Array.prototype.concat() page on MDN:

This method does not change the existing arrays, but instead returns a new array.

This is different from Array.prototype.push(), which modifies the array.

Side note: This is not specific to Node.js, and so Node's issue tracker is not the right place for something like this in the future.

[ganeshkbhat]

correct. i noticed this now.

function sorters(key = null, reverse = false) {
    if (!!key && typeof key === "function") { 
         let a = this.map(key); 
    } else {
         let a = this.concat([1,2,3,4,5]);
    }
    this.length = 0;
    this.push(...a);
}

function mappers(func) {
    if (!!func && typeof func === "function") { let a = this.map(func); }
    else { let a = this.map((i) => { return i * 2 }) }
    this.length = 0;
    this.push(...a)
}
Object.defineProperty(Array.prototype, 'sorters', { value: sorters, enumerable: true, });
Object.defineProperty(Array.prototype, 'mappers', { value: mappers, enumerable: true, });
let c = [1, 100000, 21, 30, 4];

console.log(c);
console.log(c.mappers());
console.log(c);
console.log(c.map((i) => { return i * 2 }))
console.log(c)

I notice now. Seems like both [].map() and [].concat() works by returning a new array both alike; unlike [].forEach(). my bad. probably i got confused since this is an immutable internal and gives access to the array and i generally use [].concat() rather than this.concat().

I was mistaking [].concat to be an mutater rather than a new object creator. On the second point, I was checking if I should be raising an RFC for such? what is the general process?