undefined

A variable that has not been assigned a value is of type undefined.1

That means while null represents an empty value (but still a value), undefined represents the total absence of a value. 🤯

undefined appears in different contexts.

• If a variable is declared without a value (initialization), it is undefined.
• If you try to access a value for a non-existing key in an object, you get undefined.
• If a function does not return a value, the result is undefined.
• If an argument is not passed to a function, it is undefined, unless that argument has a default value.

let name;
console.log(name);
// => undefined

You can check whether a variable is undefined using the strict equality operator ===.

let name;

name === undefined;
// => true

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References:

• https://exercism.org/tracks/javascript/concepts/null-undefined
• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/undefined

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Backlinks

• Primitives
• Function Declaration