Is there a way to define implicit conversion operators in javascript?

Question

I have this class:

var color = function(r, g, b, a) {
    this.r = r
    this.g = g;
    this.b = b;
    this.a = a;

    this.toString = function() {
        if (!isset(a)) return "rgb("+parseInt(r)+","+parseInt(g)+","+parseInt(b)+")";
        return "rgba("+parseInt(r)+","+parseInt(g)+","+parseInt(b)+","+a+")";
    }
}

If i want the string output I have to type (for example) console.log(colorInstance.toString())

but is there a way to make the toString() method be called implicitly each time the receiving function expects a string value? So I could write console.log(colorInstance) instead?

something like: ""+color

Grundy
– Grundy

2015-10-25 08:21:55 +00:00
Commented Oct 25, 2015 at 8:21 — Grundy
– Grundy, Commented Oct 25, 2015 at 8:21
I guess you are looking for valueOf.

user663031
– user663031

2015-10-25 08:49:05 +00:00
Commented Oct 25, 2015 at 8:49 — user663031
– user663031, Commented Oct 25, 2015 at 8:49

Oliver · Accepted Answer · 2015-10-25 08:25:22Z

3

Every object has a toString() method that is automatically called when the object is to be represented as a text value or when an object is referred to in a manner in which a string is expected.

From here. This applies to cases like "" + color, but otherwise, there aren't many cases where toString() gets implicitly called.

answered Oct 25, 2015 at 8:25

Oliver

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Olavi Sau · Accepted Answer · 2015-10-25 08:25:19Z

0

There is no way to define operators in javascript, at best you could make some odd functions. However javascript also has automatic type conversion, so if the runtime expects a string javascript will actually always call the toString method. There is explicit casting such as parseInt(value) though :)

answered Oct 25, 2015 at 8:25

Olavi Sau

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score 0 · Accepted Answer · 2015-10-25 09:00:19Z

0

toString will not be invoked when you simply pass the object to console.log. It (or valueOf) is invoked only in contexts where coercion to a primitive is required.

If this is really an issue for you, you could write your own version of console.log:

var logValue(...args) {
  console.log(...args.apply(a => a.toString()));
}

Or equivalent non-ES6 version.

edited Oct 25, 2015 at 9:00

answered Oct 25, 2015 at 8:54

user663031

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