How do I check for null values in JavaScript?

问题:

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?

回答1:

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)

回答2:

To check for null SPECIFICALLY you would use this:

if(variable === null && typeof variable === "object")

...or more simply:

if(variable === null)

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

The rest of this is in response to inorganik's comment, Yes, you can check each one individually.

You need to implement use of the absolutely equals: === and typeof to be absolutely sure with your checks.

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

Here is all of the output of the tests:

Null Test:

if(variable === null && typeof variable === "object")

- 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 === "" && typeof variable === "string")

- 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(variable === undefined && typeof 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 && typeof variable === "boolean")

- 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 && typeof variable === "number")

- 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(!parseFloat(variable) && variable != 0 && typeof variable === "number")

- 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;

回答3:

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.

回答4:

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

回答5:

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");

}

回答6:

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");
}

回答7:

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.

回答8:

to check for undefined and null in javascript you need just to write the following :

if (!var) {
        console.log("var IS null or undefined");
} else {
        console.log("var is NOT null or undefined");
}

回答9:

This is a comment on WebWanderer's solution regarding checking for NaN (I don't have enough rep yet to leave a formal comment). The solution reads as

if(!parseInt(variable) && variable != 0 && typeof variable === "number")

but this will fail for rational numbers which would round to 0, such as variable = 0.1. A better test would be:

if(isNaN(variable) && typeof variable === "number")

回答10:

In JavaScript, no string is equal to null.

Maybe you expected pass == null to be true when pass is an empty string because you're aware that the loose equality operator == performs certain kinds of type coercion.

For example, this expression is true:

'' == 0

In contrast, the strict equality operator === says that this is false:

'' === 0

Given that '' and 0 are loosely equal, you might reasonably conjecture that '' and null are loosely equal. However, they are not.

This expression is false:

'' == null

The result of comparing any string to null is false. Therefore, pass == null and all your other tests are always false, and the user never gets the alert.

To fix your code, compare each value to the empty string:

pass === ''

If you're certain that pass is a string, pass == '' will also work because only an empty string is loosely equal to the empty string. On the other hand, some experts say that it's a good practice to always use strict equality in JavaScript unless you specifically want to do the type coercion that the loose equality operator performs.

If you want to know what pairs of values are loosely equal, see the table "Sameness comparisons" in the Mozilla article on this topic.

回答11:

Actually I think you may need to use if (value !== null || value !== undefined) because if you use if (value) you may also filter 0 or false values.

Consider these two functions:

const firstTest = value => {
    if (value) {
        console.log('passed');
    } else {
        console.log('failed');
    }
}
const secondTest = value => {
    if (value !== null && value !== undefined) {
        console.log('passed');
    } else {
        console.log('failed');
    }
}

firstTest(0);            // result: failed
secondTest(0);           // result: passed

firstTest(false);        // result: failed
secondTest(false);       // result: passed

firstTest('');           // result: failed
secondTest('');          // result: passed

firstTest(null);         // result: failed
secondTest(null);        // result: failed

firstTest(undefined);    // result: failed
secondTest(undefined);   // result: failed

In my situation, I just needed to check if the value is null and undefined and I did not want to filter 0 or false or '' values. so I used the second test, but you may need to filter them too which may cause you to use first test.

回答12:

This will not work in case of Boolean values coming from DB for ex:

 value = false

 if(!value) {
   // it will change all false values to not available
   return "not available"
 }

回答13:

Try this:

if (!variable && typeof variable === "object") {
    // variable is null
}

回答14:

Please view carefully before downvote.

AFAIK in JAVASCRIPT when a variable is declared but has not assigned value, its type is undefined. so we can check variable even if it would be an object holding some instance in place of value.

create a helper method for checking nullity that returns true and use it in your API.

helper function to check if variable is empty:

function isEmpty(item){
    if(item){
        return false;
    }else{
        return true;
    }
}

try-catch exceptional API call:

try {

    var pass, cpass, email, cemail, user; // only declared but contains nothing.

    // parametrs checking
    if(isEmpty(pass) || isEmpty(cpass) || isEmpty(email) || isEmpty(cemail) || isEmpty(user)){
        console.log("One or More of these parameter contains no vlaue. [pass] and-or [cpass] and-or [email] and-or [cemail] and-or [user]");
    }else{
        // do stuff
    }

} catch (e) {
    if (e instanceof ReferenceError) {
        console.log(e.message); // debugging purpose
        return true;
    } else {
        console.log(e.message); // debugging purpose
        return true;
    }
}

some test cases:

var item = ""; // isEmpty? true
var item = " "; // isEmpty? false
var item; // isEmpty? true
var item = 0; // isEmpty? true
var item = 1; // isEmpty? false
var item = "AAAAA"; // isEmpty? false
var item = NaN; // isEmpty? true
var item = null; // isEmpty? true
var item = undefined; // isEmpty? true

console.log("isEmpty? "+isEmpty(item));