Why does typeof(0) return boolean instead of number

Question

I have problem with JavaScript conversion. Here is an example:

var obj = {
    x: 0,
    y: 1,
    z: 2
}

var b1 = typeof (obj.x),
    b2 = typeof (obj.y),
    b3 = typeof (obj.z);

b1 is "boolean", b2 is "boolean", b3 is "number".

I need to set variable to 0 and I need JavaScript to interpret it as a number. I've tried with conversions such as b1 = Number("0") but it doesn't help.

Sorry for the question above. It's true, b1, b2 and b3 all return number. This is my actual problem.

var SomeClass = function () {}
SomeClass.prototype = {
    element: {},
    startAt: 0,
    length: 2,
    get: function (prop) {
        if (typeof (prop) === "undefined" || !(typeof (prop) === "string")) {
            return false;
        }
        if (!this[prop]) {
            return false;
        }
        return this[prop];
    }
}

var sc = new SomeClass();
alert(typeof (sc.get("startAt")) + "\n" + typeof (sc.get("length")) + "\n" + typeof (sc.get("element")));

I would like to set startAt as an number.

Answer 1

I would like to set startAt as an number.

SomeClass.prototype.startAt is a number (0).

Your get function explicitly returns false if the property is "falsey":

if(!this[prop]){return false;}

In JavaScript, 0, "", false, undefined, and null are all "falsey", and so the condition above will be true and cause get to return false.

If your goal is to return false if the property doesn't exist, you can do that like this:

if (!(prop in this)) { return false; }

That will check if the property exists on the object itself or its prototype, which I suspect is what you want. But if you only want to check the object itself and ignore properties on the prototype, that would be:

if (!this.hasOwnProperty(prop)) { return false; }

Bringing that all together, if your goal is for get to return false for properties that the object doesn't have (either its own, or via its prototype), then:

get : function(prop){
          if (typeof prop !== "string" || !(prop in this)) {
            return false;
          }
          return this[prop];
      }

I've done a few things there:

1. typeof is an operator, not a function, so you don't use parens around its operand (normally)

2. Since "undefined" is !== "string", there's no need to check for it specifically when checking that prop is a string.

3. I used !(prop in this) to see if the object (or its prototype) has the property.

4. I combined the two conditions that returned false into a single if statement using ||.