Test if something is not undefined in JavaScript

问题

I'm checking if(response[0].title !== undefined), but I get the error:

Uncaught TypeError: Cannot read property 'title' of undefined.

回答1:

response[0] is not defined, check if it is defined and then check for its property title.

if(typeof response[0] !== 'undefined' && typeof response[0].title !== 'undefined'){
    //Do something
}

回答2:

Just check if response[0] is undefined:

if(response[0] !== undefined) { ... }

If you still need to explicitly check the title, do so after the initial check:

if(response[0] !== undefined && response[0].title !== undefined){ ... }

回答3:

I had trouble with all of the other code examples above. In Chrome, this was the condition that worked for me:

typeof possiblyUndefinedVariable !== "undefined"

I will have to test that in other browsers and see how things go I suppose.

回答4:

Actually you must surround it with an Try/Catch block so your code won't stop from working. Like this:

try{
    if(typeof response[0].title !== 'undefined') {
        doSomething();
    }
  }catch(e){
    console.log('responde[0].title is undefined'); 
  }

回答5:

typeof:

var foo;
if (typeof foo == "undefined"){
  //do stuff
}

回答6:

It'll be because response[0] itself is undefined.

回答7:

Check if condition == null; It will resolve the problem

回答8:

Check if you're response[0] actually exists, the error seems to suggest it doesn't.

回答9:

You must first check whether response[0] is undefined, and only if it's not, check for the rest. That means that in your case, response[0] is undefined.

回答10:

I know i went here 7 months late, but I found this questions and it looks interesting. I tried this on my browser console.

try{x,true}catch(e){false}

If variable x is undefined, error is catched and it will be false, if not, it will return true. So you can use eval function to set the value to a variable

var isxdefined = eval('try{x,true}catch(e){false}')

来源：https://stackoverflow.com/questions/7041123/test-if-something-is-not-undefined-in-javascript

标签

javascript

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