How can I trigger this 'typeof(undefined) != undefined' error? Undefined doesn't do the trick... JS

Question

I have a function:

function goError(thing){

    console.log(typeof(thing)); //RETURNS A STRING 'UNDEFINED'

    if(typeof(thing) != undefined){
        console.log(thing); //RETURNS UNDEFINED
        return thing;
    }

    throw 'Error: ' + thing;
}

I'm trying to get down to the error message. But when I pass through a parameter of undefined, the function works correctly, which is not what I want.

When I console out the undefined variable, I get undefined, but when I console out typeof(thing) I get a string 'undefined'. This string obviously isnt causing my error to kick in.

I could alter the function as follows, it works fine then and I can get to my error.

    if(thing != undefined){
        return thing;
    }

But can I cause the error to kick in without altering the func?

What can I pass into my function that will cause the error?

correct, `typoeof` returns the type as a string, so you should write `typeof thing !== 'undefined'` https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/typeof — Joram, Nov 28 '14 at 13:44

lante · Answer 1 · 2014-11-28T14:20:09.097

2

To check if thing is undefined, you must do:

if(typeof thing !== "undefined") {
    // well, it seems to have something
}

And a fiddle, if needed.

edited Nov 28 '14 at 14:20

answered Nov 28 '14 at 13:54

lante

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it should be `typeof thing !== "undefined"` – Joram Nov 28 '14 at 14:19
both works, but fixed, thanks! – lante Nov 28 '14 at 14:20

score 1 · Answer 2 · answered Nov 28 '14 at 13:45

1

Try with this code

function goError(thing){    

    if(thing){        
        return thing;
    }
    throw 'Error: ' + thing;
}

answered Nov 28 '14 at 13:45

prog1011

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the original code is only checking for `undefined`, not `""`, or `0`, or `null`, etc – lante Nov 28 '14 at 14:05

score 1 · Answer 3 · answered Nov 28 '14 at 13:57

1

Seeing that you're using Angular, you can use the angular.isDefined utility method, docs:

function goError(thing){

    if(angular.isDefined(thing)){
        return thing;
    }

    throw 'Error: ' + thing;
}

answered Nov 28 '14 at 13:57

Jon Snow

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How can I trigger this 'typeof(undefined) != undefined' error? Undefined doesn't do the trick... JS

3 Answers3