Class functions in an Object (node.js)

Question

Hi I want to optimize my code so I don't have a lot of if statements. I've been trying to optimize this code witch is inside a Class

        if (spot.charAt(1) === "P") this.#getMovesPawn(row, col, validMoves);
        if (spot.charAt(1) === "N") this.#getMovesKnight(row, col, validMoves);
        if (spot.charAt(1) === "B") this.#getMovesBishoop(row, col, validMoves);
        if (spot.charAt(1) === "R") this.#getMovesRook(row, col, validMoves);
        if (spot.charAt(1) === "Q") this.#getMovesQueen(row, col, validMoves);

First i tried to try putting all of the functions inside an Object

this.moveFunctions = {"P": this.#getMovesBishoop, "B": this.#getMovesBishoop, "N": this.#getMovesKnight,"Q":this.#getMovesQueen, "R": this.#getMovesRook}
// then executing them
this.moveFunctions["P"](0, 0, []);

but when doing so the this variable changes from the Class variable where all the info is at to just include the functions

// from this
{
    "apple": 1,
    "pear": 2
    ...
}

// to
{"P": this.#getMovesBishoop, "B": this.#getMovesBishoop, "N": this.#getMovesKnight,"Q":this.#getMovesQueen, "R": this.#getMovesRook}

my question is how do i manage to make this.moveFunctions work or something better? I have searched stackoverflow and google, but nothing i've also tried trying this in python and it worked

You could use a switch statement in the top part. To call your code from a different context use `call` https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/call — Lee Taylor, Apr 19 '22 at 17:27

score 4 · Accepted Answer · answered Apr 19 '22 at 17:32

You forgot to bind the correct this - see How does the "this" keyword work?.

When you write a.b() this is different from const x = a.b; x() because the syntax of a.b() automatically passes a as this to the method a.b. This automatic "connection" is lost when you do x() because it happens only when you do a function call immediately preceded by a property access, but here the property access and the function call happen in two different places.

The behavior you observe is because here there is an automatic "connection" to an object but it's not the object you expected, because ["P"] is a property access as well (just like .P) so now you have another a.b() situation but with a being this.moveFunctions!

The solution to both scenarios (no property access before the call at all or property access on an unwanted object) is to either bind the correct this to the function (const x = a.b.bind(a); x() works) or use call to pass it (const x = a.b; x.call(a) works too).

In your case, call is probably the easier way:

this.moveFunctions["P"].call(this, 0, 0, []);

Thank you! It took me some time to know how .call() works – Kārlis Kazāks Apr 20 '22 at 14:05 — Kārlis Kazāks, Apr 20 '22 at 14:05

Class functions in an Object (node.js)

1 Answers1