JavaScript isset() equivalent

I generally use the typeof operator:

if (typeof obj.foo !== 'undefined') {
  // your code here
}

It will return "undefined" either if the property doesn't exist or its value is undefined.

_{(See also: Difference between undefined and not being defined.)}

There are other ways to figure out if a property exists on an object, like the hasOwnProperty method:

if (obj.hasOwnProperty('foo')) {
  // your code here
}

And the in operator:

if ('foo' in obj) {
  // your code here
}

The difference between the last two is that the hasOwnProperty method will check if the property exist physically on the object (the property is not inherited).

The in operator will check on all the properties reachable up in the prototype chain, e.g.:

var obj = { foo: 'bar'};

obj.hasOwnProperty('foo'); // true
obj.hasOwnProperty('toString'); // false
'toString' in obj; // true

As you can see, hasOwnProperty returns false and the in operator returns true when checking the toString method, this method is defined up in the prototype chain, because obj inherits form Object.prototype.

if (!('foo' in obj)) {
  // not set.
}

(typeof SOMETHING) !== 'undefined'

It's too long to write when used. But we can't package the typeof keyword into a function, because an error will thrown before the function is called, like this:

function isdef($var) {
    return (typeof $var) !== 'undefined';
}

isdef(SOMETHING); ///// thrown error: SOMETHING is not defined

So I figured out a way:

function isdef($type) {
    return $type !== 'undefined';
}

isdef(typeof SOMETHING);

It can work both with individual variables (variables that does not exist at all), or object properties (non-existent properties). And only 7 more characters than PHP isset.

I use a function that can check variables and objects. very convenient to work with jQuery

    function _isset (variable) {
        if(typeof(variable) == "undefined" || variable == null)
            return false;
        else
            if(typeof(variable) == "object" && !variable.length) 
                return false;
            else
                return true;
    };

Try to create function like empty function of PHP in Javascript. May this helps.

function empty(str){
  try{
    if(typeof str==="string"){
        str=str.trim();
    }
    return !(str !== undefined && str !== "undefined" && str !== null && str!=="" && str!==0 && str!==false);
  }catch(ex){
    return true;
  }
 }

console.log(empty(0))//true
console.log(empty(null))//true
console.log(empty(" "))//true
console.log(empty(""))//true
console.log(empty(undefined))//true
console.log(empty("undefined"))//true

var tmp=1;
console.log(empty(tmp))//false

var tmp="Test";
console.log(empty(tmp))//false

var tmp=" Test ";
console.log(empty(tmp))//false

var tmp={a:1,b:false,c:0};
console.log(empty(tmp.a))//false
console.log(empty(tmp.b))//true
console.log(empty(tmp.c))//true
console.log(empty(tmp.c))//true
console.log(empty(tmp.c.d))//true

finally i solved problem with easy solution :

if (obj && obj.foo && obj.foo='somethings'){
console.log('i,m work without error')
}

PHP Manual say:

isset — Determine if a variable is set and is not NULL

And interface something like this:

bool isset ( mixed $var [, mixed $... ] )

The parameter $var is the variable to be checked. it can have any number of parameter though.

isset() returns TRUE if var exists and has value other than NULL. FALSE otherwise.

Some example:

$foo = 'bar';
var_dump(isset($foo));        -> true

$baz = null;
var_dump(isset($baz));        -> false

var_dump(isset($undefined));  -> false

As this in mind, Apparently, It's not possible to write exact equivalent of php isset() function. For example when we call like this:

if (isset(some_var)) {

}

function issset() {
    // function definition
}

Javascript trigger Uncaught ReferenceError: some_var is not defined at (file_name):line_number. The important and remarkable thing about this behavior is that when trying to pass non-existent variables to normal functions, an error is triggered.

But in PHP isset() are not actually regular functions but language constructs. That means they're part of the PHP language itself, do not play by the normal rules of functions and can hence get away with not triggering an error for non-existent variables. This is important when trying to figure out whether a variable exists or not. But in javscript, it triggers an error in the first place say function call with non-existent variables.

My point is that we can't write it as equivlent javscript function but we can do something like this

if (typeof some_var !== 'undefined') {
   // your code here
}

If you want exact same effect PHP also check varable is not NULL

For example

$baz = null;
var_dump(isset($baz));        -> false

So, we can incorporate this into javascript then it look like this:

if (typeof some_var !== 'undefined' && some_var !== null) {
   // your code here
}

It was really a problem for me when I was accessing a deeper property of an object so I made a function which will return the property value if exist otherwise it will return false. You may use it to save your time,

//Object on which we want to test
var foo = {
    bar: {
        bik: {
            baz: 'Hello world'
        }
    }
};


/*
USE: To get value from the object using it properties supplied (Deeper),
    if found it will return the property value if not found then will return false

You can use this function in two ways
WAY - 1:
Passing an object as parameter 1 and array of the properties as parameter 2
EG: getValueFromObject(foo, ['bar', 'bik', 'baz']);
WAY - 2: (This will work only if, your object available in window object)
Passing an STRING as parameter 1(Just similarly how we retrieve value form object using it's properties - difference is only the quote)
EG: getValueFromObject('foo.bar.bik.baz');
*/
function getValueFromObject(object, properties) {
    if(typeof(object) == 'string') {            //Here we extract our object and it's properties from the string
        properties = object.split('.');
        object = window[properties[0]];
        if(typeof(object) == 'undefined') {
            return false;
        }
        properties.shift();
    }
    var property = properties[0];
    properties.shift();
    if(object != null && typeof(object[property]) != 'undefined') {
        if(typeof(object[property]) == 'object') {
            if(properties.length != 0) {
                return getValueFromObject(object[property], properties);    //Recursive call to the function
            } else {
                return object[property];
            }
        } else {
            return object[property];
        }
    } else {
        return false;
    }
}
console.log(getValueFromObject('fooo.bar.bik.baz'));        //false
console.log(getValueFromObject('foo.bar.bik.baz'));         //Hello world
console.log(getValueFromObject('foo'));                     //false
console.log(getValueFromObject('foo.bar.bik'));             //returns an object { baz: 'Hello World' }
console.log(getValueFromObject(foo, ['bar', 'bik']));       //returns an object { baz: 'Hello World' }
console.log(getValueFromObject(foo, ['bar', 'bik', 'baz']));//Hello world

If you want to check if an element exists, just use the following code:

if (object) {
  //if isset, return true
} else {
  //else return false
}

This is sample:

function switchDiv() {
    if (document.querySelector("#divId")) {
        document.querySelector("#divId").remove();
    } else {
        var newDiv = document.createElement("div");
        newDiv.id = "divId";
        document.querySelector("body").appendChild(newDiv);
    }
}

document.querySelector("#btn").addEventListener("click", switchDiv);

#divId {
    background: red;
    height: 100px;
    width: 100px;
    position: relative;
    
}

<body>
  <button id="btn">Let's Diiiv!</button>
</body>

Be careful in ES6, all the previous solutions doesn't work if you want to check a declaration of a let variable and declare it, if it isn't

example

let myTest = 'text';

if(typeof myTest === "undefined") {
    var myTest = 'new text'; // can't be a let because let declare in a scope
}

you will see a error

Uncaught SyntaxError: Identifier 'myTest' has already been declared

The solution was to change it by a var

var myTest = 'text'; // I replace let by a var

if(typeof myTest === "undefined") {
    var myTest = 'new text';
}

another solution if you can change a let by a var, you need to remove your var

let myTest = 'text';

if(typeof myTest === "undefined") {
    myTest = 'new text'; // I remove the var declaration
}

use this function for arrays or nested array (but not for strings)

if(isset(array,'key1=>key1')){alert('isset');}

https://jsfiddle.net/dazzafact/cgav6psr/

arr={nested:{nested2:{val:'isset'}}}
if(t=isset(arr,'nested=>nested2=>val','=>')){
   alert(t)
}
function isset(obj,nested,split) {
var sep=split || '.';
    var dub=obj
    var isset=false
    if(typeof(obj)!="undefined" && typeof(nested)!="undefined"){
        var arr=nested.split(sep);
        for(var k in arr){
            var key=arr[k];
            if(typeof(dub[key])=="undefined"){
                isset=false;
                break;
            }
            dub=dub[key];
      isset=dub
        }   
    }
    
    return isset;
}

try {
  const value = array.foo.object.value;
  // isset true
} catch (err) {
  // isset false
}

javascript isset

let test = {
  a: {
    b: [0, 1]
  }
};

console.log(test.isset('a.b'))   // true
console.log(test.isset('a.b.1')) // true
console.log(test.isset('a.b.5')) // false
console.log(test.isset('a.c'))   // false
console.log('abv'.isset('0'))    // true

    isset('user.permissions.saveProject', args);

    function isset(string, context) {
        try {
            var arr = string.split('.');
            var checkObj = context || window;

            for (var i in arr) {
                if (checkObj[arr[i]] === undefined) return false;
                checkObj = checkObj[arr[i]];
            }

            return true;
        } catch (e) {
            return false;
        }
    }

This is the most concise equivalent of Php's isset() :

if(var == undefined)

• true this is var !isset
• false this is var isset