JavaScript undefined throws TypeError

Question

i am now learning javascript and found some problems with this code

var response = '{"status":{"message":{"-1":[111]}}}';
response = jQuery.parseJSON(response);
if ( typeof (response.status.warning[-1]) == "undefined" ) {
    console.log(true);
};

why he throws a error, instead of just ignoring "console.log" part?

dafuq is this javascript

try change response.status.warning to response.status.message — pktangyue
– pktangyue, Commented Mar 6, 2013 at 8:02

Matt Fellows · Accepted Answer · 2013-03-06 08:04:43Z

2

You need to code this more defensibly:

if (response && (!response.status || !response.status.warning || typeof (response.status.warning[-1]) == "undefined" ) {

Checking the existence of response.status and response.status.warning should be done. If response.status doesn't even exist, response.status.warning will generate an error

answered Mar 6, 2013 at 8:04

Matt Fellows

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4 Comments

Littlebobbydroptables Over a year ago

Is there possibility to avoid printing object to console if "if" statement is false, without adding "else" ? i.sstatic.net/m3WNC.png

Littlebobbydroptables Over a year ago

Update, object was printed only if u run script in console, tried on "live", everything is ok.

Matt Fellows Over a year ago

So does that mean it worked? Or are you still having problems?

Littlebobbydroptables Over a year ago

I am checking my monitoring service, if until 18:00 i will not notice this error again, i will check your answer as a solution :)

Derek 朕會功夫 · Accepted Answer · 2013-03-06 16:19:23Z

1

Because it can't even get through this part:

typeof (response.status.warning[-1]) == "undefined"

since response.status.warning is undefined. That's why it throws an error.

To check if response.status.warning exists only when you are sure that response and .status are not undefined:

if(response.status.warning){
    console.log("It exists!");
}

edited Mar 6, 2013 at 16:19

answered Mar 6, 2013 at 8:02

Derek 朕會功夫

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6 Comments

Littlebobbydroptables Over a year ago

I ve changed message to warning on purpose, i want to check what will happens if in response will be no "message"

Derek 朕會功夫 Over a year ago

@Nerfair response.status.warning == undefined will check if it exists or not.

Amy Over a year ago

Javascript doesn't like to access properties of undefined objects. that's why you should always make it a habit to check for the existence of the property before you access it. The other answer covers this point.

Derek 朕會功夫 Over a year ago

@sweetamylase - But since it is jQuery then response and .status should always exist according to the doc.

Amy Over a year ago

Irrelevant in this case, See in his example, he's converting the string '{"status":{"message":{"-1":[111]}}}' into JSON object.

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