How do I determine if variable is undefined or null?
My code is as follows:
var EmpName = $("div#esd-names div#name").attr('class');
if(EmpName == 'undefined'){
// DO SOMETHING
};
<div id="esd-names">
<div id="name"></div>
</div>
But if I do this, the JavaScript interpreter halts execution.
You can use the qualities of the abstract equality operator to do this:
if (variable == null){
// your code here.
}
Because null == undefined is true, the above code will catch both null and undefined.
The standard way to catch null and undefined simultaneously is this:
if (variable == null) {
// do something
}
--which is 100% equivalent to the more explicit but less concise:
if (variable === undefined || variable === null) {
// do something
}
When writing professional JS, it's taken for granted that type equality and the behavior of == vs === is understood. Therefore we use == and only compare to null.
Edit again
The comments suggesting the use of typeof are simply wrong. Yes, my solution above will cause a ReferenceError if the variable doesn't exist. This is a good thing. This ReferenceError is desirable: it will help you find your mistakes and fix them before you ship your code, just like compiler errors would in other languages. Use try/catch if you are working with input you don't have control over.
You should not have any references to undeclared variables in your code.
Combining the above answers, it seems the most complete answer would be:
if( typeof variable === 'undefined' || variable === null ){
// Do stuff
}
This should work for any variable that is either undeclared or declared and explicitly set to null or undefined. The boolean expression should evaluate to false for any declared variable that has an actual non-null value.
if (variable == null) {
// Do stuff, will only match null or undefined, this won't match false
}
if (typeof EmpName != 'undefined' && EmpName) {
will evaluate to true if value is not:
null
undefined
NaN
empty string ("")
0
false
Probably the shortest way to do this is:
if(EmpName == null) { /* DO SOMETHING */ };
Here is proof:
function check(EmpName) {
if(EmpName == null) { return true; };
return false;
}
var log = (t,a) => console.log(`${t} -> ${check(a)}`);
log('null', null);
log('undefined', undefined);
log('NaN', NaN);
log('""', "");
log('{}', {});
log('[]', []);
log('[1]', [1]);
log('[0]', [0]);
log('[[]]', [[]]);
log('true', true);
log('false', false);
log('"true"', "true");
log('"false"', "false");
log('Infinity', Infinity);
log('-Infinity', -Infinity);
log('1', 1);
log('0', 0);
log('-1', -1);
log('"1"', "1");
log('"0"', "0");
log('"-1"', "-1");
// "void 0" case
console.log('---\n"true" is:', true);
console.log('"void 0" is:', void 0);
log(void 0,void 0); // "void 0" is "undefined"
And here are more details about == (source here)
BONUS: reason why === is more clear than == (look on agc answer)
jQuery attr() function returns either a blank string or the actual value (and never null or undefined). The only time it returns undefined is when your selector didn't return any element.
So you may want to test against a blank string. Alternatively, since blank strings, null and undefined are false-y, you can just do this:
if (!EmpName) { //do something }
Edited answer: In my opinion, you shouldn't use the function from my below old answer. Instead, you should probably know the type of your variable and use the according to check directly (for example, wondering if an array is empty? just do if(arr.length===0){} etc.). This answer doesn't even answer OP's question.
I've come to write my own function for this. JavaScript is weird.
It is usable on literally anything. (Note that this also checks if the variable contains any usable values. But since this information is usually also needed, I think it's worth posting). Please consider leaving a note.
function empty(v) {
let type = typeof v;
if (type === 'undefined') {
return true;
}
if (type === 'boolean') {
return !v;
}
if (v === null) {
return true;
}
if (v === undefined) {
return true;
}
if (v instanceof Array) {
if (v.length < 1) {
return true;
}
} else if (type === 'string') {
if (v.length < 1) {
return true;
}
if (v === '0') {
return true;
}
} else if (type === 'object') {
if (Object.keys(v).length < 1) {
return true;
}
} else if (type === 'number') {
if (v === 0) {
return true;
}
}
return false;
}
TypeScript-compatible.
This function should do exactly the same thing like PHP's empty() function (see RETURN VALUES)
Considers undefined, null, false, 0, 0.0, "0" {}, [] as empty.
"0.0", NaN, " ", true are considered non-empty.
The shortest and easiest:
if(!EmpName ){
// DO SOMETHING
}
this will evaluate true if EmpName is:
null
undefined
NaN
empty
string ("")
0
false
If the variable you want to check is a global, do
if (window.yourVarName) {
// Your code here
}
This way to check will not throw an error even if the yourVarName variable doesn't exist.
Example: I want to know if my browser supports History API
if (window.history) {
history.back();
}
How this works:
window is an object which holds all global variables as its properties, and in JavaScript it is legal to try to access a non-existing object property. If history doesn't exist then window.history returns undefined. undefined is falsey, so code in an if(undefined){} block won't run.
In JavaScript, as per my knowledge, we can check an undefined, null or empty variable like below.
if (variable === undefined){
}
if (variable === null){
}
if (variable === ''){
}
Check all conditions:
if(variable === undefined || variable === null || variable === ''){
}
Since you are using jQuery, you can determine whether a variable is undefined or its value is null by using a single function.
var s; // undefined
jQuery.isEmptyObject(s); // will return true;
s = null; // defined as null
jQuery.isEmptyObject(s); // will return true;
// usage
if(jQuery.isEmptyObject(s)){
alert('Either variable: s is undefined or its value is null');
}else{
alert('variable: s has value ' + s);
}
s = 'something'; // defined with some value
jQuery.isEmptyObject(s); // will return false;
I've just had this problem i.e. checking if an object is null.
I simply use this:
if (object) {
// Your code
}
For example:
if (document.getElementById("enterJob")) {
document.getElementById("enterJob").className += ' current';
}
You can simply use the following (I know there are shorter ways to do this, but this may make it easier to visually observe, at least for others looking at the code).
if (x === null || x === undefined) {
// Add your response code here, etc.
}
source: https://www.growthsnippets.com/how-can-i-determine-if-a-variable-is-undefined-or-null/
jQuery check element not null:
var dvElement = $('#dvElement');
if (dvElement.length > 0) {
// Do something
}
else{
// Else do something else
}
With the newest javascript changes, you can use the new logical operator ??= to check if the left operand is null or undefined and if so assign the value of right operand.
SO,
if(EmpName == null){ // if Variable EmpName null or undefined
EmpName = 'some value';
};
Is equivalent to:
EmpName ??= 'some value';
The easiest way to check is:
if(!variable) {
// If the variable is null or undefined then execution of code will enter here.
}
I run this test in the Chrome console. Using (void 0) you can check undefined:
var c;
undefined
if (c === void 0) alert();
// output = undefined
var c = 1;
// output = undefined
if (c === void 0) alert();
// output = undefined
// check c value c
// output = 1
if (c === void 0) alert();
// output = undefined
c = undefined;
// output = undefined
if (c === void 0) alert();
// output = undefined
With the solution below:
const getType = (val) => typeof val === 'undefined' || !val ? null : typeof val;
const isDeepEqual = (a, b) => getType(a) === getType(b);
console.log(isDeepEqual(1, 1)); // true
console.log(isDeepEqual(null, null)); // true
console.log(isDeepEqual([], [])); // true
console.log(isDeepEqual(1, "1")); // false
etc...
I'm able to check for the following:
null
undefined
NaN
empty
string ("")
0
false
To test if a variable is null or undefined I use the below code.
if(typeof sVal === 'undefined' || sVal === null || sVal === ''){
console.log('variable is undefined or null');
}
if you create a function to check it:
export function isEmpty (v) {
if (typeof v === "undefined") {
return true;
}
if (v === null) {
return true;
}
if (typeof v === "object" && Object.keys(v).length === 0) {
return true;
}
if (Array.isArray(v) && v.length === 0) {
return true;
}
if (typeof v === "string" && v.trim().length === 0) {
return true;
}
return false;
}
(null == undefined) // true
(null === undefined) // false
Because === checks for both the type and value. Type of both are different but value is the same.
Let's look at this,
let apple; // Only declare the variable as apple
alert(apple); // undefined
In the above, the variable is only declared as apple. In this case, if we call method alert it will display undefined.
let apple = null; /* Declare the variable as apple and initialized but the value is null */
alert(apple); // null
In the second one it displays null, because variable of apple value is null.
So you can check whether a value is undefined or null.
if(apple !== undefined || apple !== null) {
// Can use variable without any error
}
The foo == null check should do the trick and resolve the "undefined OR null" case in the shortest manner. (Not considering "foo is not declared" case.) But people who are used to have 3 equals (as the best practice) might not accept it. Just look at eqeqeq or triple-equals rules in eslint and tslint...
The explicit approach, when we are checking if a variable is undefined or null separately, should be applied in this case, and my contribution to the topic (27 non-negative answers for now!) is to use void 0 as both short and safe way to perform check for undefined.
Using foo === undefined is not safe because undefined is not a reserved word and can be shadowed (MDN). Using typeof === 'undefined' check is safe, but if we are not going to care about foo-is-undeclared case the following approach can be used:
if (foo === void 0 || foo === null) { ... }
You can do something like this, I think its more efficient for multiple value check on the same variable in one condition
const x = undefined;
const y = null;
const z = 'test';
if ([undefined, null].includes(x)) {
// Will return true
}
if ([undefined, null].includes(y)) {
// Will return true
}
if ([undefined, null].includes(z)) {
// Will return false
}
No one seems to have to posted this yet, so here we go:
a?.valueOf() === undefined works reliably for either null or undefined.
The following works pretty much like a == null or a == undefined, but it could be more attractive for purists who don't like == 😎
function check(a) {
const value = a?.valueOf();
if (value === undefined) {
console.log("a is null or undefined");
}
else {
console.log(value);
}
}
check(null);
check(undefined);
check(0);
check("");
check({});
check([]);
On a side note, a?.constructor works too:
function check(a) {
if (a?.constructor === undefined) {
console.log("a is null or undefined");
}
}
check(null);
check(undefined);
check(0);
check("");
check({});
check([]);
Calling typeof null returns a value of “object”, as the special value null is considered to be an empty object reference. Safari through version 5 and Chrome through version 7 have a quirk where calling typeof on a regular expression returns “function” while all other browsers return “object”.
var x;
if (x === undefined) {
alert ("only declared, but not defined.")
};
if (typeof y === "undefined") {
alert ("not even declared.")
};
You can only use second one: as it will check for both definition and declaration
var i;
if (i === null || typeof i === 'undefined') {
console.log(i, 'i is undefined or null')
}
else {
console.log(i, 'i has some value')
}
I still think the best/safe way to test these two conditions is to cast the value to a string:
var EmpName = $("div#esd-names div#name").attr('class');
// Undefined check
if (Object.prototype.toString.call(EmpName) === '[object Undefined]'){
// Do something with your code
}
// Nullcheck
if (Object.prototype.toString.call(EmpName) === '[object Null]'){
// Do something with your code
}
Often while using JavaScript you run into silly problems. One such problems is discerning between Object types.
Is there a way to create a function with this functionality? See Below:
_discern = function () { [ function code ] };
_discern({}); // Logs: Object
_discern([]); // Logs: Array
_discern(document); // Logs: Pseudo-Object
_discern(document.querySelectorAll("*")); // Logs: Pseudo-Array
I've already tried creating a function that checks for array-likeness, but that didn't work as good as I had hoped:
isArrLike = function (_) {
_[0] = 0; return [].slice.call(_).length >= Object.values(_).length;
};
And I've tried using that behavior into another function. All failures. Is there a way?
Looks like you want to know if something behaves like an array, instead of looking for the actual type/constructor.
If that's the case, it should be enough to check if something is iterable:
function isIterable(obj) {
// checks for null and undefined
if (obj == null) {
return false;
}
return typeof obj[Symbol.iterator] === 'function';
}
All of these are objects:
is array → return x instanceof Array or Array.isArray(x)
is function → return typeof x === 'function'
is HTML element → return x instanceof HTMLElement
is plain object → return typeof x === 'object' && /* ... is not array, not function, not HTML element */
Thanks to Logain's answer, I can solve the problem. Here's my approach:
kind = function (a) {
let u = toString.call(a.valueOf()).slice(8, -1);
if (a == null || u == "String" || u == "Number" || u == "Boolean") {
return u;
}
else if (typeof a[Symbol.iterator] == "function") {
return u != "Array" ? "Array-Like" : u;
}
else {
return u;
}
};
I want to check the response.data.totalRows is empty.
if (response!=undefined
&& response.data!=undefined
&& response.data.totalRows!=undefined) {
alert(response.data.totalRows);
}
Can simplify the code?
UPDATE: it seems that there is no simple method like isEmpty(response.data.totalRows).
Yea, you can simply do this:
if (response && response.data && response.data.totalRows) {
alert(response.data.totalRows);
}
In JavaScript, a object is cast to a truthy value, when used in a if. This means you can just "dump" the variable in a if or any other boolean statement, as a check to see whether or not it exists. this blog post has some more information about it.
Please note that this will not alert anything if totalRows equals 0 (since 0 is considered a falsy value.) If you also want to alert if it's 0, use this:
if (response && response.data &&
(response.data.totalRows || response.data.totalRows === 0)) {
alert(response.data.totalRows);
}
Or:
if (response && response.data && response.data.totalRows !== undefined) {
alert(response.data.totalRows);
}
Supposing that response.data.totalRows must be an array you can use just:
if (!response.data.totalRows.length) {
/* empty array */
}
If you are not sure that totalRows exists you must verify:
if (
!response ||
!response.data ||
!response.data.totalRows ||
!response.data.totalRows.length
) {
/* is empty */
}
Any value is converted in Boolean. For example: Boolean(response) will return false if response will be 0, null, undefined etc.
What about a try-catch block?
try{ alert(response.data.totalRows); }
catch(e) { alert("undefined"); }
I'd write a prototype (even if it's not recommended)
Object.prototype.isEmpty = function(){
return (!this || this===undefined || this===null || !this.hasChildNodes())
?true
:false;
}
And then just use
if(!response.isEmpty()) alert(response.data.totalRows);
It is only handy if you need the checks also elsewhere and not only one place.
Just
response && response.data && response.data.totalRows && alert(response.data.totalRows)
If the property list gets very long there is another syntax you can use, in the sample code I've created a function so it can be re used.
// args is { object: the object to check the properties of
// properties: an array of strings with property names}
function isSet(args){
//no checking of arguments
var o = args.object,
props = args.properties,
i = -1,len = props.length
while(typeof o !== "undefined"
&& o !== null
&& ++i<len){
o = o[props[i]];
}
return (typeof o !== "undefined"
&& o !== null)?true:false;
}
var test = {
prop1 : {
prop2 : "ok"
}
};
//check if test.prop1.prop2 is set
console.log(isSet({
object:test,
properties: ["prop1","prop2"]
}));//=true
I have a function expression like this :
var inputChecker = function(field) {
return function() {
if(field === '' || field === 'undefined' || field === null) {
return false;
}
return true;
}
}
that I want to use in several different function expressions :
(function($) {
if(inputChecker(x)) {}
})(jQuery);
(function($) {
})(jQuery);
But the problem is inputChecker is not visible in these function expressions when it's declared out of their bodies ? I don't understand why? Isn't inputChecker supposed to be global ?
Dystroy's answer is definitely simpler. But if you want it your way...
The return value of the inputChecker is a function, not boolean. If you want to call the returned function, use () expression:
var fn = inputChecker(x); // gets the function
fn(); // calls the returned function
or shorter
inputChecker(x)();
In your code
(function($) {
if(inputChecker(x)()) {
// custom code here if x is defined
}
})(jQuery);
Note: if you want to check if variable is not undefined, strip the apostrophes - undefined is constant, not string
if(field===undefined)
What you wrote is a function factory. It doesn't return a boolean but a function able to check a property.
This kind of functions is sometimes useful but :
you're here, in the returned function, checking the value of the property received by the factory. As this value can't change (it's embedded in the closure), the produced function holds no more information than just true or false. So it's useless.
you're calling inputChecker(x) as if it was a boolean instead of a function.
So what you probably want is simply
var checkInput = function(field) {
if(field === '' || field === 'undefined' || field === null){
return false;
}
return true;
}
But if you really want to generate different checking functions, dependent on another value, you could use the function factory pattern like this:
var x = true;
var checkInput = (function (x) {
if (x === true) {
return function(field) {
if(field === '' || field === 'undefined' || field === null){
return false;
}
return true;
}
} else {
return function(field) {
//evaluate field differently
}
}
}(x));
Now, dependig on what x is, one or another function will be assigned to checkInput.
I have a function that checks to see whether or not a request has any queries, and does different actions based off that. Currently, I have if(query) do this else something else. However, it seems that when there is no query data, I end up with a {} JSON object. As such, I need to replace if(query) with if(query.isEmpty()) or something of that sort. Can anybody explain how I could go about doing this in NodeJS? Does the V8 JSON object have any functionality of this sort?
You can use either of these functions:
// This should work in node.js and other ES5 compliant implementations.
function isEmptyObject(obj) {
return !Object.keys(obj).length;
}
// This should work both there and elsewhere.
function isEmptyObject(obj) {
for (var key in obj) {
if (Object.prototype.hasOwnProperty.call(obj, key)) {
return false;
}
}
return true;
}
Example usage:
if (isEmptyObject(query)) {
// There are no queries.
} else {
// There is at least one query,
// or at least the query object is not empty.
}
You can use this:
var isEmpty = function(obj) {
return Object.keys(obj).length === 0;
}
or this:
function isEmpty(obj) {
return !Object.keys(obj).length > 0;
}
You can also use this:
function isEmpty(obj) {
for(var prop in obj) {
if(obj.hasOwnProperty(prop))
return false;
}
return true;
}
If using underscore or jQuery, you can use their isEmpty or isEmptyObject calls.
Object.keys(myObj).length === 0;
As there is need to just check if Object is empty it will be better to directly call a native method Object.keys(myObj).length which returns the array of keys by internally iterating with for..in loop.As Object.hasOwnProperty returns a boolean result based on the property present in an object which itself iterates with for..in loop and will have time complexity O(N2).
On the other hand calling a UDF which itself has above two implementations or other will work fine for small object but will block the code which will have severe impact on overall perormance if Object size is large unless nothing else is waiting in the event loop.
If you have compatibility with Object.keys, and node does have compatibility, you should use that for sure.
However, if you do not have compatibility, and for any reason using a loop function is out of the question - like me, I used the following solution:
JSON.stringify(obj) === '{}'
Consider this solution a 'last resort' use only if must.
See in the comments "there are many ways in which this solution is not ideal".
I had a last resort scenario, and it worked perfectly.
My solution:
let isEmpty = (val) => {
let typeOfVal = typeof val;
switch(typeOfVal){
case 'object':
return (val.length == 0) || !Object.keys(val).length;
break;
case 'string':
let str = val.trim();
return str == '' || str == undefined;
break;
case 'number':
return val == '';
break;
default:
return val == '' || val == undefined;
}
};
console.log(isEmpty([1,2,4,5])); // false
console.log(isEmpty({id: 1, name: "Trung",age: 29})); // false
console.log(isEmpty('TrunvNV')); // false
console.log(isEmpty(8)); // false
console.log(isEmpty('')); // true
console.log(isEmpty(' ')); // true
console.log(isEmpty([])); // true
console.log(isEmpty({})); // true
const isEmpty = (value) => (
value === undefined ||
value === null ||
(typeof value === 'object' && Object.keys(value).length === 0) ||
(typeof value === 'string' && value.trim().length === 0)
)
module.exports = isEmpty;