What is the difference between null and undefined?

Answer

In JavaScript, two values discretely represent nothing - undefined and null. The concrete difference between them is that null is explicit, while undefined is implicit. When a property does not exist or a variable has not been given a value, the value is undefined. null is set as the value to explicitly indicate โ€œno valueโ€. In essence, undefined is used when the nothing is not known, and null is used when the nothing is known.

Good to hear

  • typeof undefined evaluates to "undefined".

  • typeof null evaluates "object". However, it is still a primitive value and this is considered an implementation bug in JavaScript.

  • undefined == null evaluates to true.

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