AI solution for Interview Question on BFE.dev
164. What is the difference between `null` and `undefined` ? Which one should I use?

null and undefined are both JavaScript primitive values used to denote emptiness or absence of any value. However, they differ in their meaning and usage.

• undefined: it is a primitive type that indicates an absence of a value assigned to a variable or a property. It also indicates that the variable or property has not been defined yet or has not been initialized with a value. In other words, a variable that has not been assigned a value is of type undefined.

• null: it is also a primitive type, but unlike undefined, it is explicitly assigned to indicate the absence of any value. It can be used to represent intentional non-existence of an object or a value.

In general, undefined is used when a variable or property has not yet been assigned a value, while null is used when a value needs to be explicitly set to “nothing” or “empty.” However, they are often used interchangeably in some cases.

As a developer, if you are unsure about which one to use, check the expected data type of the variable or property in question, and use undefined when something is simply not there or needs to be initialized later, and null when you need to explicitly set it as empty or non-existent.