I want to set a Value in a javascript object only when it is not set. My (test) function looks like:

var test = function(){
    this.value = {};

    this.setValue = function(seperator, newValue){
        console.log((this.value[seperator] === "undefined"));  //Why both times false?
        if(typeof(this.value[seperator] === "undefined")){
            this.value[seperator] = newValue;
        }else{
            //noop
        }
        console.log(this.value[seperator]);
    }
}
var blubb = new test();

blubb .setValue("foo","bar");
blubb .setValue("foo","notme");

in the js console it returns

false
bar
false
notme

Can someone tell me why both time my test of "undefined" told me that is not defined?

thanks in advance

I want to set a Value in a javascript object only when it is not set. My (test) function looks like:

var test = function(){
    this.value = {};

    this.setValue = function(seperator, newValue){
        console.log((this.value[seperator] === "undefined"));  //Why both times false?
        if(typeof(this.value[seperator] === "undefined")){
            this.value[seperator] = newValue;
        }else{
            //noop
        }
        console.log(this.value[seperator]);
    }
}
var blubb = new test();

blubb .setValue("foo","bar");
blubb .setValue("foo","notme");

in the js console it returns

false
bar
false
notme

Can someone tell me why both time my test of "undefined" told me that is not defined?

thanks in advance

• javascript

3 Answers 3

Because undefined in JS is not a string, it's a property of global object and you paring by type using ===.

=== will pare not only values but their types too:

1 === "1" // false
1 == "1"  // true

Try this:

console.log(( typeof this.value[seperator] === "undefined"));

typeof operator transforms variable type to string and only then you can check if your variable is equal to string undefined.

In your second piece of code:

if(typeof(this.value[seperator] === "undefined")){

you use typeof operator outside of the variable so your code first checks if this.value[seperator] === "undefined" then it returns false to you and then you check by "typeof false", it will return boolean for you.

In final step your code converts to:

if( "boolean" ){

And this is always true as string is not empty.

Short answer:

"undefined" !== undefined

Check for undefined instead.

> var foo = { foo: 'foo' };
> foo['bar']
undefined
> typeof(foo['bar'])
"undefined"

Also note that typeof(this.value[seperator] === "undefined") means typeof(boolean) as it'd first evaluate your expression (this.value[seperator] === "undefined") and then get the type of that.

You probably meant typeof(this.value[seperator]) === "undefined".

Your brackets are in the wrong place in this line:

if(typeof(this.value[seperator] === "undefined")){

You're doing the typeof of (this.value[seperator] === "undefined") - that's a boolean condition (will return true or false) so I'd expect typeof to give you "boolean". Then your if statements condition is the string "boolean" which, since it's not zero length, is considered true in JavaScript.

What you wanted is:

if((typeof this.value[seperator]) === "undefined") {