undefined and null in Javascript

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undefined

In Javascript undefined (a primitive type in Javascript) gets assigned to a variable that we declare but don’t assign a value to. It’s Javascript’s way of telling that the value of this variable is not defined.

undefined

Even a function in Javascript which does not return anything, will return undefined by default.

undefined

So, to avoid any error we should never assign undefined to a variable manually.

typeof undefined
outputs: undefined

null

Unlike undefined, null (another primitive type in Javascript) is used when we want to manually or intentionally set the value of a variable to be empty.

When comparing null with undefined remember to use === instead of ==, reason being

null == undefined
returns true as it doesn't check for the type and performs type-conversion
null === undefined
return false
typeof null
is object and not null

(more details here)