[javascript] How do I check for null values in JavaScript?

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?

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.

You can check if some value is null as follows

[pass,cpass,email,cemail,user].some(x=> x===null) 

let pass=1;
let cpass=2;
let email=3;
let cemail=null;
let user=5;

if ( [pass,cpass,email,cemail,user].some(x=> x===null) ) {     
    alert("fill all columns");
    //return false;  
}   

BONUS: Why === is more clear than == (source)

a == b

a === b

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");
}

I made this very simple function that works wonders:

function safeOrZero(route) {
  try {
    Function(`return (${route})`)();
  } catch (error) {
    return 0;
  }
  return Function(`return (${route})`)();
}

The route is whatever chain of values that can blow up. I use it for jQuery/cheerio and objects and such.

Examples 1: a simple object such as this const testObj = {items: [{ val: 'haya' }, { val: null }, { val: 'hum!' }];};.

But it could be a very large object that we haven't even made. So I pass it through:

let value1 = testobj.items[2].val;  // "hum!"
let value2 = testobj.items[3].val;  // Uncaught TypeError: Cannot read property 'val' of undefined

let svalue1 = safeOrZero(`testobj.items[2].val`)  // "hum!"
let svalue2 = safeOrZero(`testobj.items[3].val`)  // 0

Of course if you prefer you can use null or 'No value'... Whatever suit your needs.

Usually a DOM query or a jQuery selector may throw an error if it's not found. But using something like:

const bookLink = safeOrZero($('span.guidebook > a')[0].href);
if(bookLink){
  [...]
}

Checking error conditions:

// Typical API response data
let data = {
  status: true,
  user: [],
  total: 0,
  activity: {sports: 1}
}

// A flag that checks whether all conditions were met or not
var passed = true;

// Boolean check
if (data['status'] === undefined || data['status'] == false){
  console.log("Undefined / no `status` data");
  passed = false;
}

// Array/dict check
if (data['user'] === undefined || !data['user'].length){
  console.log("Undefined / no `user` data");
  passed = false;
}

// Checking a key in a dictionary
if (data['activity'] === undefined || data['activity']['time'] === undefined){
   console.log("Undefined / no `time` data");
   passed = false;
}

// Other values check
if (data['total'] === undefined || !data['total']){
  console.log("Undefined / no `total` data");
  passed = false;
}

// Passed all tests?
if (passed){
  console.log("Passed all tests");
}

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));

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");

}

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;

Try this:

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

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")

I found a another way to test if the value is null:

if(variable >= 0 && typeof variable === "object")

null acts as a number and object at the same time. Comparing null >= 0 or null <= 0 results in true. Comparing null === 0 or null > 0 or null < 0 will result in false. But as null is also an object we can detect it as a null.

I made a more complex function natureof witch will do better than typeof and can be told what types to include or keep grouped

/* function natureof(variable, [included types])_x000D_
included types are _x000D_
    null - null will result in "undefined" or if included, will result in "null"_x000D_
    NaN - NaN will result in "undefined" or if included, will result in "NaN"_x000D_
    -infinity - will separate negative -Inifity from "Infinity"_x000D_
    number - will split number into "int" or "double"_x000D_
    array - will separate "array" from "object"_x000D_
    empty - empty "string" will result in "empty" or_x000D_
    empty=undefined - empty "string" will result in "undefined"_x000D_
*/_x000D_
function natureof(v, ...types){_x000D_
/*null*/            if(v === null) return types.includes('null') ? "null" : "undefined";_x000D_
/*NaN*/             if(typeof v == "number") return (isNaN(v)) ? types.includes('NaN') ? "NaN" : "undefined" : _x000D_
/*-infinity*/       (v+1 === v) ? (types.includes('-infinity') && v === Number.NEGATIVE_INFINITY) ? "-infinity" : "infinity" : _x000D_
/*number*/          (types.includes('number')) ? (Number.isInteger(v)) ? "int" : "double" : "number";_x000D_
/*array*/           if(typeof v == "object") return (types.includes('array') && Array.isArray(v)) ? "array" : "object";_x000D_
/*empty*/           if(typeof v == "string") return (v == "") ? types.includes('empty') ? "empty" : _x000D_
/*empty=undefined*/ types.includes('empty=undefined') ? "undefined" : "string" : "string";_x000D_
                    else return typeof v_x000D_
}_x000D_
_x000D_
// DEMO_x000D_
let types = [null, "", "string", undefined, NaN, Infinity, -Infinity, false, "false", true, "true", 0, 1, -1, 0.1, "test", {var:1}, [1,2], {0: 1, 1: 2, length: 2}]_x000D_
_x000D_
for(i in types){_x000D_
console.log("natureof ", types[i], " = ", natureof(types[i], "null", "NaN", "-infinity", "number", "array", "empty=undefined")) _x000D_
}

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.

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.

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"
 }

You can use lodash module to check value is null or undefined

_.isNil(value)
Example 

 country= "Abc"
    _.isNil(country)
    //false

   state= null
    _.isNil(state)
    //true

city= undefined
    _.isNil(state)
    //true

   pin= true
    _.isNil(pin)
    // false   

Reference link: https://lodash.com/docs/#isNil

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.

At first sight, it looks like a simple trade-off between coverage and strictness.

• == covers multiple values, can handle more scenarios in less code.
• === is the most strict, and that makes it predictable.

Predictability always wins, and that appears to make === a one-fits-all solution.

But it is wrong. Even though === is predictable, it does not always result in predictable code, because it overlooks scenarios.

const options = { };
if (options.callback !== null) {
  options.callback();      // error --> callback is undefined.
}

In general == does a more predictable job for null checks:

• In general, null and undefined both mean the same thing: "something's missing". For predictability, you need to check both values. And then == null does a perfect job, because it covers exactly those 2 values. (i.e. == null is equivalent to === null && === undefined)

• In exceptional cases you do want a clear distinction between null and undefined. And in those cases you're better of with a strict === undefined or === null. (e.g. a distinction between missing/ignore/skip and empty/clear/remove.) But it is rare.

It is not only rare, it's something to avoid. You can't store undefined in a traditional database. And you shouldn't rely on undefined values in your API designs neither, due to interopability reasons. But even when you don't make a distinction at all, you can't assume that undefined won't happen. People all around us indirectly take actions that generalize null/undefined (which is why questions like this are closed as "opinionated".).

So, to get back to your question. There's nothing wrong with using == null. It does exactly what it should do.

// FIX 1 --> yes === is very explicit
const options = { };
if (options.callback !== null && 
    options.callback !== undefined) {
  options.callback();
}


// FIX 2 --> but == is more accurate here
const options = { };
if (options.callback != null) {
  options.callback();
}

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_x000D_
let pass=1, cpass=1, email=1, cemail=1, user=1; // just to test_x000D_
_x000D_
if ([pass, cpass, email, cemail, user].every(x=>x!==null)) {_x000D_
    // your code here ..._x000D_
    console.log ("Yayy! None of them are null");_x000D_
} else {_x000D_
    console.log ("Oops! At-lease one of them is null");_x000D_
}