As you’ve found, JavaScript doesn’t support operator overloading. The closest you can come is to implement toString
(which will get called when the instance needs to be coerced to being a string) and valueOf
(which will get called to coerce it to a number, for instance when using +
for addition, or in many cases when using it for concatenation because +
tries to do addition before concatenation), which is pretty limited. Neither lets you create a Vector2
object as a result. Similarly, Proxy
(added in ES2015) lets you intercept various object operations (including property access), but again won’t let you control the result of +=
on Vector
instances.
For people coming to this question who want a string or number as a result (instead of a Vector2
), though, here are examples of valueOf
and toString
. These examples do not demonstrate operator overloading, just taking advantage of JavaScript’s built-in handling converting to primitives:
valueOf
This example doubles the value of an object’s val
property in response to being coerced to a primitive, for instance via +
:
function Thing(val) {
this.val = val;
}
Thing.prototype.valueOf = function() {
// Here I'm just doubling it; you'd actually do your longAdd thing
return this.val * 2;
};
var a = new Thing(1);
var b = new Thing(2);
console.log(a + b); // 6 (1 * 2 + 2 * 2)
Or with ES2015’s class
:
class Thing {
constructor(val) {
this.val = val;
}
valueOf() {
return this.val * 2;
}
}
const a = new Thing(1);
const b = new Thing(2);
console.log(a + b); // 6 (1 * 2 + 2 * 2)
Or just with objects, no constructors:
var thingPrototype = {
valueOf: function() {
return this.val * 2;
}
};
var a = Object.create(thingPrototype);
a.val = 1;
var b = Object.create(thingPrototype);
b.val = 2;
console.log(a + b); // 6 (1 * 2 + 2 * 2)
toString
This example converts the value of an object’s val
property to upper case in response to being coerced to a primitive, for instance via +
:
function Thing(val) {
this.val = val;
}
Thing.prototype.toString = function() {
return this.val.toUpperCase();
};
var a = new Thing("a");
var b = new Thing("b");
console.log(a + b); // AB
Or with ES2015’s class
:
class Thing {
constructor(val) {
this.val = val;
}
toString() {
return this.val.toUpperCase();
}
}
const a = new Thing("a");
const b = new Thing("b");
console.log(a + b); // AB
Or just with objects, no constructors:
var thingPrototype = {
toString: function() {
return this.val.toUpperCase();
}
};
var a = Object.create(thingPrototype);
a.val = "a";
var b = Object.create(thingPrototype);
b.val = "b";
console.log(a + b); // AB