CODEWITHSUNDEEP
javascript
kjo
kjo

Reputation: 35311

Is it possible to define the boolean value of an arbitrary object?

Is there a way in JavaScript to pre-specify the boolean value that an arbitrary object should have?

For example, suppose I define

function NIL () {
    this.car = this.cdr = this;
}

var nil = new NIL;

I would like to have !nil evaluate to true. Is this possible in JS?

(P.S.: I'd hoped that defining a method like toBoolean, by analogy with toString, would do the trick, but this did not work.)

Upvotes: 2

Views: 81

Answers (3)

suff trek
suff trek

Reputation: 39777

You can make it work - sort of, by overriding "valueOf":

NIL.prototype.valueOf = function () {
    return false;
};

It won't make !nil == true but it will make nil == false.

But since this behavior is inconsistent, I agree with other answers - you should use your own functions.

Upvotes: 1

trogdor
trogdor

Reputation: 1656

The rules for conversion to boolean are fixed by the ECMAscript Language Specification, so you cannot do that. You should use your own functions to perform your checks.

Upvotes: 2

zs2020
zs2020

Reputation: 54514

It seems you want to use functional programming way to define a customize typing system. I am not sure whether this is what you need, but you this does what you described

function NIL () {
    this.car = this.cdr = this;
    this.isNull = function(){
        return false;
    }
}

var nil = new NIL();
console.log(!nil.isNull()) // true

Upvotes: 0

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