How do I check for null values in JavaScript?

Javascript is very flexible with regards to checking for "null" values. I'm guessing you're actually looking for empty strings, in which case this simpler code will work:

if(!pass || !cpass || !email || !cemail || !user){

Which will check for empty strings (""), null, undefined, false and the numbers 0 and NaN

Please note that if you are specifically checking for numbers it is a common mistake to miss 0 with this method, and num !== 0 is preferred (or num !== -1 or ~num (hacky code that also checks against -1)) for functions that return -1, e.g. indexOf)

To check for null SPECIFICALLY you would use this:

if (variable === null)

This test will ONLY pass for null and will not pass for "", undefined, false, 0, or NaN.

Additionally, I've provided absolute checks for each "false-like" value (one that would return true for !variable).

Note, for some of the absolute checks, you will need to implement use of the absolutely equals: === and typeof.

I've created a JSFiddle here to show all of the individual tests working

Here is the output of each check:

Null Test:if (variable === null)- variable = ""; (false) typeof variable = string- variable = null; (true) typeof variable = object- variable = undefined; (false) typeof variable = undefined- variable = false; (false) typeof variable = boolean- variable = 0; (false) typeof variable = number- variable = NaN; (false) typeof variable = numberEmpty String Test:if (variable === '')- variable = ''; (true) typeof variable = string- variable = null; (false) typeof variable = object- variable = undefined; (false) typeof variable = undefined- variable = false; (false) typeof variable = boolean- variable = 0; (false) typeof variable = number- variable = NaN; (false) typeof variable = numberUndefined Test:if (typeof variable == "undefined")-- or --if (variable === undefined)- variable = ''; (false) typeof variable = string- variable = null; (false) typeof variable = object- variable = undefined; (true) typeof variable = undefined- variable = false; (false) typeof variable = boolean- variable = 0; (false) typeof variable = number- variable = NaN; (false) typeof variable = numberFalse Test:if (variable === false)- variable = ''; (false) typeof variable = string- variable = null; (false) typeof variable = object- variable = undefined; (false) typeof variable = undefined- variable = false; (true) typeof variable = boolean- variable = 0; (false) typeof variable = number- variable = NaN; (false) typeof variable = numberZero Test:if (variable === 0)- variable = ''; (false) typeof variable = string- variable = null; (false) typeof variable = object- variable = undefined; (false) typeof variable = undefined- variable = false; (false) typeof variable = boolean- variable = 0; (true) typeof variable = number- variable = NaN; (false) typeof variable = numberNaN Test:if (typeof variable == 'number' && !parseFloat(variable) && variable !== 0)-- or --if (isNaN(variable))- variable = ''; (false) typeof variable = string- variable = null; (false) typeof variable = object- variable = undefined; (false) typeof variable = undefined- variable = false; (false) typeof variable = boolean- variable = 0; (false) typeof variable = number- variable = NaN; (true) typeof variable = number

As you can see, it's a little more difficult to test against NaN;

just replace the == with === in all places.

== is a loose or abstract equality comparison

=== is a strict equality comparison

See the MDN article on Equality comparisons and sameness for more detail.