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Undefined

The undefined keyword represents an uninitialized or missing value. Any variable declared but not explicitly assigned a value will have undefined by default.

Behavior​

Accessing an uninitialized variable returns undefined.

let x;
console.log(x); // undefined

Functions that do not explicitly return a value return undefined.

fn doNothing() {
    const value = 1 + 1;
}
console.log(doNothing()); // undefined

Checking for undefined​

Use the comparison operator (== or ===) to check for undefined.

if(x === undefined)
    console.log("x is undefined!");

⚠️ Don't use !x as you will get a runtime error if x is not convertible to a boolean! (See Boolean convertible types)