In JavaScript you can declare a variable and if it’s undefined, you can check variable == undefined; I know that, but how can you compare a value that you don’t know yet if it’s in memory?
For example, I have a class which is created when the user clicks a button. Before this, the class is undefined — it doesn’t exist anywhere; how can I compare it?
Is there a way without using try–catch?
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javascript
undefined
if (document.getElementById('theElement')) // do whatever after this
For undefined things that throw errors, test the property name of the parent object instead of just the variable name - so instead of:
if (blah) ...
do:
if (window.blah) ...
!undefined is true in javascript, so if you want to know whether your variable or object is undefined and want to take actions, you could do something like this:
if(<object or variable>) {
//take actions if object is not undefined
} else {
//take actions if object is undefined
}
if (!obj) {
// object (not class!) doesn't exist yet
}
else ...
if (obj === undefined)
{
// Create obj
}
If you are doing extensive javascript programming you should get in the habit of using === and !== when you want to make a type specific check.
Also if you are going to be doing a fair amount of javascript, I suggest running code through JSLint http://www.jslint.com it might seem a bit draconian at first, but most of the things JSLint warns you about will eventually come back to bite you.
Source: Stackoverflow.com