Question

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How to distinguish between a variable that is not declared and a varrable that is declared but not be assigned any value?

I want to see in my code whether someone in the past has already declared the a variable.

I thought about this:

alert(typeof(a)=="undefined") //true

but then I tested:

var a;
alert(typeof(a)=="undefined") //also true !

so how can I check whether there was var a somewhere in the past?

Answer 1

The only way to check whether a variable is (locally) declared is to test the value, and see if any ReferenceError is thrown.

try {
    a === 1; // Some statement to trigger a look-up for the a variable
    alert("a is declared!");
} catch (e) {
    alert("a is not declared!");
}