When do I initialize variables in JavaScript with null or not at all?

javascript variables null undefined
KMcA picture KMcA · Oct 30, 2012 · Viewed 54.1k times · Source

I have seen different code examples with variables declared and set to undefined and null. Such as:

var a; // undefined - unintentional value, object of type 'undefined'
var b = null; // null - deliberate non-value, object of type 'object'

If the code to follow these declarations assigns a value to a or to b, what is the reason for using one type of declaration over another?

Answer

James Allardice picture James Allardice · Oct 30, 2012

The first example doesn't assign anything to the variable, so it implicitly refers to the undefined value (see 10.5 in the spec for the details). It is commonly used when declaring variables for later use. There is no need to explicitly assign anything to them before necessary.

The second example is explicitly assigned null (which is actually of type null, but due to a quirk of the JavaScript specification, claims to have type "object"). It is commonly used to clear a value already stored in an existing variable. It could be seen as more robust to use null when clearing the value since it is possible to overwrite undefined, and in that situation assiging undefined would result in unexpected behaviour.

As an aside, that quirk of null is a good reason to use a more robust form of type checking:

Object.prototype.toString.call(null); // Returns "[object Null]"

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