The difference between undefined and null in JavaScript

Typically, when we try to access something that doesn't exist.Or variable with no value assigned, you will get a undefinedValue. JavaScript automatically sets the variable that is declared to be not initialized to undifined.

What happens if a variable doesn't exist at all:

>>> Fooreferenceerror: foo is not defined

We got an error message. We use the typeof operator on this variable to see what type it is:

>>> typeof Foo "undefined"

The result we get is the string "undefined".

If we do not assign a variable when we declare it, the variable is called without error, but the typeof operator still returns the string "undefined".

>>> var somevar;>>> somevar;undefined>>> typeof Somevar; " undefined"

And the null value is completely different. He cannot come from a dynamic assignment through JavaScript, which means that we have to manually assign a value to NULL for a variable:

>>> var somevar = null;>>> Somevar; NULL

Moreover, the TypeOf operator returns an Object!

>>> typeof Somevar; " Object"

Although the difference between undefine and Null is not very large, they behave very differently in some arithmetic operations:

>>> var i = 1 + undefined; I NaN>>> var i = 1 + null; I 1

This is because the method has some difference when undefined and null are converted to other basic types, and we give some possible types of conversions below.

Convert to a number (treat undefined as a string, null as 0):

>>> 1 * undefined; Nan>>> 1 * null;0

Convert to a Boolean value:

>>>!! Undefined;false>>>!! Null;false converted to a string (all corresponding to a string):>>> "+undefined;" Undefined ">>>" +null; Null

The above is the difference between undefined and null, and where operations are to be noted.

The difference between undefined and null in JavaScript