Question

How do I now check if the value is defined as undefined, or if it’s really not defined?
eg.

var a = [];
a[0] = undefined;

// a defined value that's undefined
typeof a[0] === "undefined";

// and a value that hasn't been defined at all
typeof a[1] === "undefined";

Is there a way to separate these two? it’s possible touse a for-in loop to go through the array, but is there a lighter way?

Answer 1

You can use the in operator to check if a given index is present in the array, regardless of its actual value

var t = [];
t[0] = undefined;
t[5] = "bar";

console.log( 0 in t ); // true
console.log( 5 in t ); // true
console.log( 1 in t ); // false
console.log( 6 in t ); // false

if( 0 in t && t[0] === undefined ) {
     // the value is defined as "undefined"
}

if( !(1 in t) ) {
    // the value is not defined at all
}