This question already has answers here:
JavaScript check if variable exists (is defined/initialized)
(32 answers)
Closed 6 years ago.
I want to check if a variable is undefined and quickly found some stackoverflow answers that said the correct way to do it is to test if(variable==null). But in Chrome I am getting an error saying:
Uncaught ReferenceError: xdate is not defined
Huh? The whole reason I am testing is so I don't get errors like this. And I did it just like the approved stackoverflow answers.
Here is my code snippet.
if (xdate == null){
var dateadd = "";
} else {
var dateadd = "&date="+date;
}
use something like this to verify whether variable is undefined or not
if (typeof something === "undefined") {
alert("something is undefined");
}
Related
This question already has answers here:
Test for existence of nested JavaScript object key
(64 answers)
Closed 3 years ago.
Is there an easier way to check if a variable is defined or not in JavaScript when the target is deep within an object?
For example.
// Lets assume this:
response = {
status: "simple-message"
}
// running this:
if (response.data.variable_to_check !== undefined) console.log('undefined');
// will result in this:
> TypeError: response.data is undefined
In PHP I can run the equivalent check:
if (!($response->data->variable_to_check ?? false)) die("Handled Undefined Error");
// will result in this:
> Handled Undefined Error
I know I can iterate manually by checking each item starting with the root to see if it's defined, but that seems tedious. That and wrapping everything in a try/catch.
Is there a cleaner / faster / smarter way to do this?
Use
try{
if(response.data.variable_to_check!==undefined){
console.log("undefined");
}
}
catch(err){console.log(err.message)}
This question already has answers here:
JavaScript check if variable exists (is defined/initialized)
(32 answers)
Closed 3 years ago.
Please read the question before marking duplicate. This isn't about an undefined variable. It's about variables which don't exist in the namespace.
I'm working on a codebase which is run in browser and in nativescript. The globals differ between the two. For instance, I'd like to check if window exists at all, but something like this:
if (!!window) {
}
will throw an error:
JS ERROR ReferenceError: Can't find variable: window
Is there a way to test whether or not a variable exists in js (not just undefined)?
You could use a try/catch statement.
try {
if (!!someVariable) {
console.log('yep');
}
} catch {
console.log('nope');
}
You need to use the typeof operator to see if a variable exist or no
you need to test if the variable you are concerned with is undefined of null like so :
if (typeof variable === 'undefined' || variable === null) {
// variable does not exist
}
This question already has answers here:
JavaScript check if variable exists (is defined/initialized)
(32 answers)
Closed 3 years ago.
Simple question but i'm stuck for a while on it.
I need to check if a var is defined, before execute some taks.
So i try this
if (myVar !== null) alert('myVar is not null');
But when i do this and myVar is not defined, an exception is threw :
Uncaught ReferenceError: myVar is not defined at :1:1
So how to solve this dilema ?
You can use type of
if (typeof variable !== 'undefined') alert('myVar is not null');
This question already has answers here:
Access Javascript nested objects safely
(14 answers)
Closed 4 years ago.
Why does my else if statement cause the parser to throw an undefined error? I know that it is undefined which is why I am checking, I want it to hit the else block in this case, because this function is used in mutiple places the data that is being passed in is not always the same. So I am trying to check which type of data is being passed in.
if(icon){
dv.find(".mobCol>.image").html(generateIcon(icon));
}
else if(location[0].properties !== undefined){
dv.find(".mobCol>.image").html(generateIcon(location[0].properties.image));
data = location[0];
}
else{
dv.find(".mobCol>.image").html(generateIcon(location.features[0].properties.image));
data = location.features[0];
}
Because location[0] is undefined. It's a bit odd, but your check should be:
if( location[0] && location[0].properties){ /* .. */}
I've omitted the ===undefined, thats not needed. It tests if it is something is thruthy or falsey
This is a common way to check variables in JS.
This question already has answers here:
Detecting an undefined object property
(50 answers)
How can I determine if a variable is 'undefined' or 'null'?
(34 answers)
Closed 9 years ago.
I would like to check to see if a particular attribute of a DOM element is undefined - how do I do that?
I tried something like this:
if (marcamillion == undefined) {
console.log("Marcamillion is an undefined variable.");
}
ReferenceError: marcamillion is not defined
As you can see, the reference error is telling me that the variable is not defined, but my if check is clearly not working, because it is producing the standard js ReferenceError as opposed to the error message I am looking for in my console.log.
Edit 1
Or better yet, if I am trying to determine if the attribute of an element is undefined like this:
$(this).attr('value')
What would be the best way to determine if that is undefined?
Using typeof:
if (typeof marcamillion == 'undefined') {
console.log("Marcamillion is an undefined variable.");
}
Edit for the second question:
if ($(this).attr('value')) {
// code
}
else {
console.log('nope.')
}
if (typeof marcamillion === 'undefined') {
console.log("Marcamillion is an undefined variable.");
}
Note that using === instead of == is considered better style.