null-vs-undefined.md

null vs undefined

Question: What are the differences between null and undefined?

Answer: JavaScript has two distinct values for nothing, null and undefined.

undefined

In JavaScript, undefined means a variable has been declared but has not yet been assigned a value, such as: JavaScript has a global variable undefined whose value is "undefined" and typeof undefined is also "undefined".

Remember, undefined is not a constant or a keyword. undefined is a type with exactly one value: undefined. Assigning a new value to it does not change the value of the type undefined.

8 Ways to get Undefined:

• A declared variable without assigning any value to it.
• Implicit returns of functions due to missing return statements.
• return statements that do not explicitly return anything.
• Lookups of non-existent properties in an object.
• Function parameters that have not passed.
• Anything that has been set to the value of undefined.
• Any expression in the form of void(expression)
• The value of the global variable undefined

null

null means empty or non-existent value which is used by programmers to indicate “no value”. null is a primitive value and you can assign null to any variable. null is not an object, it is a primitive value. For example, you cannot add properties to it. Sometimes people wrongly assume that it is an object, because typeof null returns "object".

Btw, null == undefined

ref: history of typeof null

From the preceding examples, it is clear that undefined and null are two distinct types: undefined is a type itself (undefined) while null is an object.

null === undefined // false
null == undefined // true
null === null // true

and

null = 'value' // ReferenceError
undefined = 'value' // 'value'

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Practical Differences

All of this is great, but what about a practical difference between null and undefined?

Consider the following code snippet:

let logHi = (str = "hi") => {
  console.log(str)
}

The code above creates a function named logHi. This function requires one parameter and sets the default of that parameter to hi if it isn’t supplied. Here’s what that looks like:

logHi()
// hi

We can also supply a parameter to overwrite this default:

logHi("bye")
// bye

With default parameters, undefined will use the default while null does not.

logHi(undefined)
// hi
logHi(null)
// null