How to compare variables to undefined, if I don’t know whether they exist?
javascript, undefined
Solution
The best way is to check the type, because `undefined`/`null`/`false` are a tricky thing in JS. So:
if(typeof obj !== "undefined") {
// obj is a valid variable, do something here.
}
Note that `typeof` always returns a string, and doesn't generate an error if the variable doesn't exist at all.
Problem
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`?