JavaScript - Can't check if variable is undefined

Question

I am trying to check if a variable is defined or not using the below:

if(variable.obj[0] === undefined){
 console.log("yes it's undefined !")
}

and also tried :

if ("undefined" === typeof variable.obj[0]) {
  console.log("variable is undefined");
}

However it throws on the console:

Uncaught TypeError: Cannot read property '0' of undefined
at <anonymous>:1:16

Possible duplicate of [How to check for "undefined" in JavaScript?](https://stackoverflow.com/questions/3390396/how-to-check-for-undefined-in-javascript) — Samvel Petrosov, Jun 01 '17 at 14:11
For my suggestion simple `if(var)` is enought to validate `null,undefined,empty,Nan` => `if(variable.obj[0])` — prasanth, Jun 01 '17 at 14:12
Hi @Folky.H! If one of the answers has solved your question please consider [accepting it](https://meta.stackexchange.com/q/5234/179419) by clicking the check-mark. This indicates to the wider community that you've found a solution and gives some reputation to both the answerer and yourself. There is no obligation to do this. — Bruno Peres, May 10 '18 at 20:13

score 1 · Answer 1 · answered Jun 01 '17 at 14:13

1

You need to check the containing objects first, before you can access a property or array index.

if (variable === undefined || variable.obj === undefined || variable.obj[0] == undefined) {
    console.log("yes it's undefined !")
}

answered Jun 01 '17 at 14:13

Barmar

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Bruno Peres · Accepted Answer · 2017-06-01T14:51:04.403

0

The obj property is undefined. First, you need check variable and the obj property. After it you can access and check if variable.obj[0] is undefined.

Try:

if (variable && variable.obj && typeof variable.obj[0] === 'undefined'){
    console.log("yes it's undefined !")
}

edited Jun 01 '17 at 14:51

answered Jun 01 '17 at 14:11

Bruno Peres

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5
53
89

2

This will say that the variable is defined even if `variable.obj` is not defined. – Barmar Jun 01 '17 at 14:14
3

It will also get an error if `variable` is undefined. – Barmar Jun 01 '17 at 14:15
@Emissary Sorry :) I changed `||` to `&&`. – Bruno Peres Jun 01 '17 at 14:53

score 0 · Answer 3 · answered Jun 01 '17 at 14:16

typeof does not check for existance of "parent" objects. You have to check all parts that might not exist.

Assuming variable.obj is an object you will need this code:

if (typeof variable === "object" && typeof variable.obj === "object" && variable.obj.hasOwnProperty("0"))

If it is an array then

if (typeof variable === "object" && typeof variable.obj === "object" && variable.obj.length > 0)

JavaScript - Can't check if variable is undefined

3 Answers3