Question

I have the following javascript code:

function testClass() {
    this.SaveValue = function (value) {
        var isInstance = value instanceof TestEnum;

        if (!isInstance) {
            return;
        }
     }
}

TestEnum = {
    VALUE_0: 0,
    VALUE_1: 1,
    VALUE_2: 2   
}

I create an instance of this object in the following way:

$(function () {
    var a = new testClass();
    a.SaveValue(TestEnum.VALUE_1);    
});

All I’d like to do is test that the value passed to the SaveValue function is actually the type of TestEnum. However, when I run this code I get the following error: Uncaught TypeError: Expecting a function in instanceof check, but got 1

Am I going about this the right way? I tried typeof but it only returns number which is not particularly useful to me.

Answer 1

You are passing a number to the function in

a.SaveValue(TestEnum.VALUE_1);

Since TestEnum is simply an Object, and you are referencing a number property on that object, you’re calling your function with a number. You should instead create a TestEnumValue object and use that for your Object’s properties:

JSFiddle link for below

function testClass() {
    this.SaveValue = function (value) {
        var isInstance = value instanceof TestEnumValue;

        if (!isInstance) {
            return;
        }
     }
}

TestEnumValue = function(arg) {
   arg = arg ? arg : 0;  //  sensible default

   this.key = 'VALUE_' + arg;
   this.val = arg;
}

Level = {
    NumSpiders : new TestEnumValue(0),
    NumCreepers: new TestEnumValue(1),
    NumZombies : new TestEnumValue(2),
    NumChickens: new TestEnumValue  //  uses default enum value    
};

$(function() {
    var a = new testClass();
    a.SaveValue(Level.NumSpiders);

    $('#hi').text(Level.NumSpiders.key);
});