How to compare variables to undefined, if I don’t know whether they exist?
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?
Answer
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.