Discuss
  • Proposal: [Symbol.equals]

Proposal: [Symbol.equals]

# ViliusCreator (7 years ago)

What about having Symbol.equals? For example, for now this is what it does:

class Position { 
    constructor(o) {
        this.x = o.x instanceof Number ? o.x : 0
        this.y = o.y instanceof Number ? o.y : 0
        this.z = o.z instanceof Number ? o.z : 0
    }
}
console.log(new Position({x: 10, y: 10, z: 10}) === new Position({x: 10, y: 10, z: 10})

Output is of course, false. With Symbol.equals, we could make it easier, instead of instance.equals(otherInstance).

For example:

class Position {
    [Symbol.equals](oIn) {
        return oIn.x === this.x && oIn.y === this.y && oIn.z === this.z
    }
    constructor(o) {
        this.x = o.x instanceof Number ? o.x : 0
        this.y = o.y instanceof Number ? o.y : 0
        this.z = o.z instanceof Number ? o.z : 0
    }
}
console.log(new Position({x: 10, y: 10, z: 10}) === new Position({x: 10, y: 10, z: 10})

Now output would be true. This would save most of the time, instead of writing .equals and then surrounding everything with ().

# Jordan Harband (7 years ago)

It's pretty important that the meaning === not be able to change.

# Isiah Meadows (7 years ago)

Yeah, I agree. I'd suggest overloading ==, but that'd risk serious web compat issues, especially if null/undefined aren't special-cased. I feel an equals instance method added to all builtins and an Object.equals attempting that method first before performing a shallow object comparison would be the best solution.

# kai zhu (7 years ago)

the most reliable/idiot-proof equality is by avoiding classes altogether; stick with plain json-objects and compare their canonical-json-representation like this real-world example [1]:

/*jslint devel*/
(function () {
    "use strict";
    var jsonStringifyCanonical;

    jsonStringifyCanonical = function (obj, replacer, space) {
    /*
     * this function will JSON.stringify <obj>,
     * with object-keys sorted and circular-references removed
     */
        var circularSet;
        var stringify;
        var tmp;
        stringify = function (obj) {
        /*
         * this function will recursively JSON.stringify obj,
         * with object-keys sorted and circular-references removed
         */
            // if obj is not an object or function,
            // then JSON.stringify as normal
            if (!(
                obj
                && typeof obj === "object"
                && typeof obj.toJSON !== "function"
            )) {
                return JSON.stringify(obj);
            }
            // ignore circular-reference
            if (circularSet.has(obj)) {
                return;
            }
            circularSet.add(obj);
            // if obj is an array, then recurse its items
            if (Array.isArray(obj)) {
                tmp = "[" + obj.map(function (obj) {
                    // recurse
                    tmp = stringify(obj);
                    return (
                        typeof tmp === "string"
                        ? tmp
                        : "null"
                    );
                }).join(",") + "]";
                circularSet.delete(obj);
                return tmp;
            }
            // if obj is not an array,
            // then recurse its items with object-keys sorted
            tmp = "{" + Object.keys(obj).sort().map(function (key) {
                // recurse
                tmp = stringify(obj[key]);
                if (typeof tmp === "string") {
                    return JSON.stringify(key) + ":" + tmp;
                }
            }).filter(function (obj) {
                return typeof obj === "string";
            }).join(",") + "}";
            circularSet.delete(obj);
            return tmp;
        };
        circularSet = new Set();
        return JSON.stringify((
            (typeof obj === "object" && obj)
            // recurse
            ? JSON.parse(stringify(obj))
            : obj
        ), replacer, space);
    };

    // true
    console.assert(
        // {"data":{"x":1,"y":2,"z":3},"meta":{"label":"point #13"}}
        jsonStringifyCanonical({
            data: {x: 1, y: 2, z: 3},
            meta: {label: "point #32"}
        })
        // === {"data":{"x":1,"y":2,"z":3},"meta":{"label":"point #13"}}
        === jsonStringifyCanonical({
            meta: {label: "point #32"},
            data: {z: 3, y: 2, x: 1}
        })
    );
}());

[1] testing aa “==“ b by comparing their canonical json-representation kaizhu256/node-utility2/blob/2018.12.30/test.js#L2903, kaizhu256/node-utility2/blob/2018.12.30/test.js#L2903

kaizhu256/node-utility2/blob/2018.12.30/lib.utility2.js#L2760, kaizhu256/node-utility2/blob/2018.12.30/lib.utility2.js#L2760

# Jordan Harband (7 years ago)

This thread would apply equally well to objects (there's no such thing as a "json object" - json is a string, and once it's parsed into an object it's not json anymore) as to classes.

Please stop derailing threads with your off-topic ideology about how to write code - that belongs in a style guide, or in your own project, but not here.

# kai zhu (7 years ago)

This thread would apply equally well to objects (there's no such thing as a "json object" - json is a string, and once it's parsed into an object it's not json anymore) as to classes.

i’m not so sure when you start having classes with getters/setters/private-fields. then people begin digging rabbit-holes/silos trying to solve low-level equality-issues that shouldn’t even exist, and overall lose focus of the big-picture UX-workflow problems most of us js-devs were originally hired to solve.

# Jordan Harband (7 years ago)

As has been stated many times before, many JS developers have been hired to do things entirely unrelated to UX workflows; regardless, whether you like a specific coding style or not is irrelevant to discussing language features, and is very disruptive to these discussions. Please confine that to discussing style with your own team, or within a styleguide.