Undefined, on the other hand, means the variable has been declared, but its value has not been assigned. The variable which has been assigned as null contains no value. In order to test if an operation yields NaN, we need to use the built-in function isNaN().Null in JavaScript means an empty value and is also a primitive type in JavaScript. This is one of the quirks of the language. Unexpected! Doesn’t myOp equal NaN!? Yes, however the comparison is false. function f() let myOp = Math.sqrt('four') //NaN checkIfValidOperation(myOp) //? More importantly, a function will also return undefined if a value was not returned. We can therefore say that in JavaScript, the data type of the data type of undefined is string!įor functions, if the returned value is left unassigned, it returns undefined. This means that we can type the following:

What is the value of typeof typeof undefined ?

One of the questions that you may find in an interview question is: Notice that there’s a slight difference between the value and the type of undefined the latter is in quotes. Undefined is also a data type, hence the following: console.log(typeof myVar) let myVar //declared but left unassigned console.log(myVar) We can illustrate this with the code below. In that context, undefined really means that JavaScript doesn’t know about the data type of the variable declared because it has yet to be initialized with a value.Īny unassigned variable is automatically given the value and type of undefined JavaScript is a dynamically typed language, which means that the JS engine determines the type of a variable based on the data type of the value assigned to it. On a deeper level, this means that the variable exists but its value being non-existent has been automatically assigned as undefined.

It is assigned, by default, to any variable whose value has not yet been defined. It is one of six primitive data types, the others being: string, number, boolean, null, and symbol (ES5+).

This keyword is both a data type and value in JavaScript. Here, we will attempt to give this trio its proper due and uncover its mysteries. Part of the reason for that is that they are seldom given the proper attention they merit in most courses or books, in my humble opinion. Undefined, Null, and NaN are 3 keywords, known as non-values or empty values, that are often a source of buggy code, stress, and confusion for many beginning JavaScript engineers.

Javascript undefined