typeof null

Aug 12, 2021·

2 min read

In JavaScript, typeof is a useful operator for parsing data. typeof returns a string that describes the data type of the value passed in.


typeof 1234 // "number"
typeof "abcd" // "string"
typeof false // "boolean"
typeof { foo: "bar" } // "object"

There are a few confusing cases in typeof. These are:

typeof [a, b, c] // "object"
typeof NaN // "number"
typeof null // "object"

Kind of weird, right? The null value being read as "object" is especially unexpected. According to bitsofco.de/javascript-typeof, this is due to the way null is represented in computer memory.

In order to determine a value of null, you will need to also determine that the value is !== null. I had a usecase where I was building a payload and needed to determine if the incoming data was an empty object or was truly null. If the data was an array, I needed to make sure that the value in the payload was an empty array. Likewise, I needed to ensure that null values were not assigned to be an empty array, but remained null.

if (typeof payload[key] === 'object' && payload[key] !== null) {
            payload[key] = []
          } else {
            payload[key] = null

This if statement will check if a value is an object and ensure that the value is not null returning the object type.