Recursion implementation with Class in javascript

Question

I'm very new in JS And i get stuck with simple(?) problem about Classes

All i need to do is just to put some string into code to make assert expression valid

class Rec {
    constructor() {
        let a = 0;
        this['put here'] = () => a+++a;
    }
}

let inst = new Rec();
console.assert(
    inst == 1 && inst == 3 && inst == 5
);

Noticed the Class has endless chain of values like

__proto__:
    constructor:
       prototype:
          constructor:
             prototype:
                ...etc

so I've tried to use __proto__ but got a Function.prototype.toString requires that 'this' be a Function error.

adiga · Accepted Answer · 2019-08-20 15:16:27Z

2

You can use valueOf or toString

class Rec {
    constructor() {
        let a = 0;
        this['valueOf'] = () => a+++a;
    }
}

let inst = new Rec();
console.log(
    inst == 1 && inst == 3 && inst == 5
);

This works because of Abstract Equality Comparison between an object(inst) and a number

From the specification,

For the comparison x == y

If Type(x) is Object and Type(y) is either String or Number, return the result of the comparison ToPrimitive(x) == y.

The ToPrimitive for an object calls the valueOf method if the hint is Number.

If you expand the expression, it looks like this: a++ + a. Every time inst is compared, valueOf method is called. And the sum of a++ and a is returned.

edited Aug 20, 2019 at 15:16

answered Aug 20, 2019 at 15:04

adiga

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