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2

Is it possible to define custom operators between instances of a type in JavaScript?

For example, given that I have a custom vector class, is it possible to use

vect1 == vect2

to check for equality, whilst the underlying code would be something like this?

operator ==(a, b) {
    return a.x == b.x && a.y == b.y && a.z == b.z;
}

(This is nonsense of course.)

Thanks.

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4 Answers

up vote 5 down vote accepted

I agree that the equal function on the vector prototype is the best solution. Note that you can also build other infix-like operators via chaining.

function Vector(x, y, z) {
    this.x = x;
    this.y = y;
    this.z = z;
}

Vector.prototype.add = function (v2) {
    var v = new Vector(this.x + v2.x,
                       this.y + v2.y,
                       this.z + v2.z);
    return v;
}

Vector.prototype.equal = function (v2) {
    return this.x == v2.x && this.y == v2.y && this.z == v2.z;
}

You can see online sample here.

Update: Here's a more extensive sample of creating a Factory function that supports chaining.

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1  
I recently discovered chaining by using JQuery. It is extremely useful in certain cases. Thanks a lot for the sample! – pimvdb Jan 21 '11 at 10:46
up vote 7 down vote

No, JavaScript doesn’t support operator overloading. You will need to write a method that does this:

Vector.prototype.equalTo = function(other) {
    if (!(other instanceof Vector)) return false;
    return a.x == b.x && a.y == b.y && a.z == b.z;
}

Then you can use that method like:

vect1.equalTo(vect2)
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Yes, I currently have a function in the prototype as well. However, I'm used to using == for equality checking. But if it is not possible, then I'll just forget about it – pimvdb Jan 15 '11 at 14:31
up vote 6 down vote

The best you can do if you want to stick with the == operator:

function Vector(x, y, z) {
  this.x = x;
  this.y = y;
  this.z = z;
}

Vector.prototype.toString = function () {
  return this.x + ";" + this.y + ";" + this.z;
};

var a = new Vector(1, 2, 3);
var b = new Vector(1, 2, 3);
var c = new Vector(4, 5, 6);


alert( String(a) == b ); // true
alert( String(a) == c ); // false
alert( a == b + "" );    // true again (no object wrapper but a bit more ugly)
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That's also a possiblity, but a little bit ugly to be honest. I think I'll be better of using an equality function. – pimvdb Jan 15 '11 at 15:20
It's not that ugly as you might think, because you may already have a toString function on your object for easier debugging. But then again, I wrote this answer just because you said you prefer the == operator to eq. functions. Use what you like the best. Cheers! :) – galambalazs Jan 15 '11 at 15:25
I do have a toString function indeed, but for equality checking it is easier to do v1.equalsTo(v2) than remembering you have to convert it to a String, as that is not the usual way of doing it - thanks alot anyways! – pimvdb Jan 15 '11 at 15:27
+1 for a clever solution (even though it's definitely not one I'd ever want to use. – ForbesLindesay Dec 19 '11 at 15:48
up vote 4 down vote

No, it's not part of the spec (which doesn't mean that there aren't some hacks).

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That's nicely found, but the == operator does not seem to be 'hackable'. – pimvdb Jan 15 '11 at 14:33
3  
i really wish this link still worked – sfjedi Dec 21 '11 at 0:56

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