How do I check for null values in JavaScript?
How do I check for null values in JavaScript?
Question
How can I check for null values in JavaScript? I wrote the code below but it didn't work.
if (pass == null || cpass == null || email == null || cemail == null || user == null) {
alert("fill all columns");
return false;
}
And how can I find errors in my JavaScript programs?
Accepted Answer
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)
Popular Answer
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 = number
Empty 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 = number
Undefined 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 = number
False 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 = number
Zero 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 = number
NaN 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;
Read more... Read less...
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.
Strict equality operator:-
We can check null by ===
if ( value === null ){
}
Just by using if
if( value ) {
}
will evaluate to true if value is not:
- null
- undefined
- NaN
- empty string ("")
- false
- 0
Improvement over the accepted answer by explicitly checking for null but with a simplified syntax:
if ([pass, cpass, email, cemail, user].every(x=>x!==null)) {
// your code here ...
}
// Test
let pass=1, cpass=1, email=1, cemail=1, user=1; // just to test
if ([pass, cpass, email, cemail, user].every(x=>x!==null)) {
// your code here ...
console.log ("Yayy! None of them are null");
} else {
console.log ("Oops! At-lease one of them is null");
}Firstly, you have a return statement without a function body. Chances are that that will throw an error.
A cleaner way to do your check would be to simply use the ! operator:
if (!pass || !cpass || !email || !cemail || !user) {
alert("fill all columns");
}
you can use try catch finally
try {
document.getElementById("mydiv").innerHTML = 'Success' //assuming "mydiv" is undefined
} catch (e) {
if (e.name.toString() == "TypeError") //evals to true in this case
//do something
} finally {}
you can also throw your own errors. See this.