CODEWITHSUNDEEP
javascripttypeof
webDev
webDev

Reputation: 109

Behavior of the typeof keyword

Learning about the typeof keyword and need some clarification:

While troubleshooting I outputted this code to the console:

console.log(typeof object.prop[someNum - 1]);

(in this case sumNum === 0)

The console printed out undefined which is what I expected because the index position [someNum - 1] in the prop[] array doesn't exist (so I thought). However when I do something like this:

if(typeof object.prop[someNum - 1])
    //some code
 else
    //other code

It evaluates as true and runs //some code but when I do this:

if(object.prop[someNum - 1])
    //some code
 else
    //other code

It evaluates as false and runs //other code.

I was under the impression that undefined is considered a falsy valueand would evaluate false in an if statement. Why is the if(typeof object.prop[someNum - 1]) statement evaluating as as true? Also can someone point me in the right direction as to where I can learn more about negative indexing arrays in js? Are they handled the same across multiple languages like c#, c++, java, & php. I thought when evaluating a negative array index number it would be underfined or throw an error.

Upvotes: 3

Views: 80

Answers (3)

Barmar
Barmar

Reputation: 782785

As other answers have pointed out, typeof returns a string, and it will always be truthy because it never returns an empty string. If you want to test if the type is undefined, you need to compare the string.

if (typeof object.prop[someNum - 1] == "undefined") {
    // some code
} else {
    // some code
}

You could also just test whether the value is undefined:

if (object.prop[someNum - 1] === undefined) {
    // some code
} else { 
    // some code
}

Make sure you use === rather than ==, so it won't report a match if object[someNum - 1] is null.

Upvotes: 1

RobG
RobG

Reputation: 147553

As SLaks says, you're evaluating if the string "undefined" is truthy, which it is.

If you want to see if an object has a property called "someNum - 1" then you can use the in operator:

if ((someNum - 1) in object.prop)

which will return true if object.prop has such a property anywhere on itself or its [[Prototype]] chain, or:

if (object.prop.hasOwnProperty(someNum - 1))

to test for the property directly on the object.

Upvotes: 1

SLaks
SLaks

Reputation: 888303

typeof always returns a string.

Therefore, you're writing if ("undefined").

Since non-empty strings are truthy, that if will run.

Upvotes: 5

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