JavaScript isset() equivalent

Question

In PHP you can do if(isset($array['foo'])) { ... }. In JavaScript you often use if(array.foo) { ... } to do the same, but this is not exactly the same statement. The condition will also evaluate to false if array.foo does exists but is false or 0 (and probably other values as well).

What is the perfect equivalent of PHP's isset in JavaScript?

In a broader sense, a general, complete guide on JavaScript's handling of variables that don't exist, variables without a value, etc. would be convenient.

---

Update: 11 years and 11 months ago I posted this question, and wow, it still gets a lot of activity. Now, I'm pretty sure that when I wrote this, I only wanted to know how to check for the presence of a property in an associative array (a.k.a. dictionary), and as such the correct (for me) answers involve hasOwnProperty or the in operator. I wasn't interested in checking local or global variables.

But while I remember that well, that intent is not quite clear in the question as written, or even directly contradicted by it! I never mentioned the associative array, and PHP's isset does also do those other things. Let this be a lesson to all of us about how important it is to properly state your requirements in a question, and also how global variables, local variables, object properties, dictionary keys and what-have-you aren't Huey, Dewey, and Louie.

In the meantime (heh), many many people have provided answers to that effect as well, so for those of you who found this question through Google, well, I'm glad my vagueness helped in a way I guess. Anyway, just wanted to clarify that.

Answer 1

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.

Answer 2

Age old thread, but there are new ways to run an equivalent isset().

ESNext (Stage 4 December 2019)

Two new syntax allow us to vastly simplify the use of isset() functionality:

• Optional Chaining(?.)
• Nullish Coalescing Operator(??)

Please read the docs and mind the browser compatibility.

Answer

See below for explanation. Note I use StandardJS syntax

Example Usage

// IMPORTANT pass a function to our isset() that returns the value we're
// trying to test(ES6 arrow function)
isset(() => some) // false

// Defining objects
let some = { nested: { value: 'hello' } }

// More tests that never throw an error
isset(() => some) // true
isset(() => some.nested) // true
isset(() => some.nested.value) // true
isset(() => some.nested.deeper.value) // false

// Less compact but still viable except when trying to use `this` context
isset(function () { return some.nested.deeper.value }) // false

Answer Function

/**
 * Checks to see if a value is set.
 *
 * @param   {Function} accessor Function that returns our value
 * @returns {Boolean}           Value is not undefined or null
 */
function isset (accessor) {
  try {
    // Note we're seeing if the returned value of our function is not
    // undefined or null
    return accessor() !== undefined && accessor() !== null
  } catch (e) {
    // And we're able to catch the Error it would normally throw for
    // referencing a property of undefined
    return false
  }
}

NPM Package

This answer function is available as the isset-php package on NPM. The package contains a few improvements such as type checking and supporting multiple arguments.

npm install --save isset-php

The full documentation is available in the README.

const isset = require('isset-php')
let val = ''

// This will evaluate to true so the text will be printed.
if (isset(() => val)) {
  console.log('This val is set so I will print.')
}

---

Explanation

PHP

Note that in PHP you can reference any variable at any depth - even trying to access a non-array as an array will return a simple true or false:

// Referencing an undeclared variable
isset($some); // false

$some = 'hello';

// Declared but has no depth(not an array)
isset($some); // true
isset($some['nested']); // false

$some = ['nested' => 'hello'];

// Declared as an array but not with the depth we're testing for
isset($some['nested']); // true
isset($some['nested']['deeper']); // false

JavaScript

In JavaScript, we don't have that freedom; we'll always get an error if we do the same because the engine is immediately attempting to access the value of deeper before we can wrap it in our isset() function so...

// Common pitfall answer(ES6 arrow function)
const isset = (ref) => typeof ref !== 'undefined'

// Same as above
function isset (ref) { return typeof ref !== 'undefined' }

// Referencing an undeclared variable will throw an error, so no luck here
isset(some) // Error: some is not defined

// Defining a simple object with no properties - so we aren't defining
// the property `nested`
let some = {}

// Simple checking if we have a declared variable
isset(some) // true

// Now trying to see if we have a top level property, still valid
isset(some.nested) // false

// But here is where things fall apart: trying to access a deep property
// of a complex object; it will throw an error
isset(some.nested.deeper) // Error: Cannot read property 'deeper' of undefined
//         ^^^^^^ undefined

More failing alternatives:

// Any way we attempt to access the `deeper` property of `nested` will
// throw an error
some.nested.deeper.hasOwnProperty('value') // Error
//   ^^^^^^ undefined

// Similar to the above but safe from objects overriding `hasOwnProperty`
Object.prototype.hasOwnProperty.call(some.nested.deeper, 'value') // Error
//                                        ^^^^^^ undefined

// Same goes for typeof
typeof some.nested.deeper !== 'undefined' // Error
//          ^^^^^^ undefined

And some working alternatives that can get redundant fast:

// Wrap everything in try...catch
try {
  if (isset(some.nested.deeper)) {
    // ...
  }
} catch (e) {}

try {
  if (some.nested.deeper !== undefined && some.nested.deeper !== null) {
    // ...
  }
} catch (e) {}

// Or by chaining all of the isset which can get long
isset(some) && isset(some.nested) && isset(some.nested.deeper) // false
//                        ^^^^^^ returns false so the next isset() is never run

Conclusion

All of the other answers - though most are viable...

1. Assume you're only checking to see if the variable is not undefined which is fine for some use cases but can still throw an Error
2. Assume you're only trying to access a top level property, which again is fine for some use cases
3. Force you to use a less than ideal approach relative to PHP's isset()
   e.g. isset(some, 'nested.deeper.value')
4. Use eval() which works but I personally avoid

I think I covered a lot of it. There are some points I make in my answer that I don't touch upon because they - although relevant - are not part of the question(e.g. short circuiting). If need be, though, I can update my answer with links to some of the more technical aspects based on demand.

I spent waaay to much time on this so hopefully it helps people out.

Thank-you for reading!

Answer 3

Reference to SOURCE

    module.exports = function isset () {
  //  discuss at: http://locutus.io/php/isset/
  // original by: Kevin van Zonneveld (http://kvz.io)
  // improved by: FremyCompany
  // improved by: Onno Marsman (https://twitter.com/onnomarsman)
  // improved by: Rafał Kukawski (http://blog.kukawski.pl)
  //   example 1: isset( undefined, true)
  //   returns 1: false
  //   example 2: isset( 'Kevin van Zonneveld' )
  //   returns 2: true

  var a = arguments
  var l = a.length
  var i = 0
  var undef

  if (l === 0) {
    throw new Error('Empty isset')
  }

  while (i !== l) {
    if (a[i] === undef || a[i] === null) {
      return false
    }
    i++
  }

  return true
}

phpjs.org is mostly retired in favor of locutus Here is the new link http://locutus.io/php/var/isset

Answer 4

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

Answer 5

//
//  tring to reference non-existing variable throws ReferenceError 
//  before test function is even executed
//
//  example, if you do:
//    
//     if ( isset( someVar ) ) 
//        doStuff( someVar );
//   
//  you get a ReferenceError ( if there is no someVar... ) 
//  and isset fn doesn't get executed.
//
//  if you pass variable name as string, ex. isset( 'novar' );, 
//  this might work:
//
function isset ( strVariableName ) { 

    try { 
        eval( strVariableName );
    } catch( err ) { 
        if ( err instanceof ReferenceError ) 
           return false;
    }

    return true;

 } 
//
//

Answer 6

This simple solution works, but not for deep object check.

function isset(str) {
    return window[str] !== undefined;
}

Answer 7

I always use this generic function to prevent errrors on primitive variables as well as arrays and objects.

isset = function(obj) {
  var i, max_i;
  if(obj === undefined) return false;
  for (i = 1, max_i = arguments.length; i < max_i; i++) {
    if (obj[arguments[i]] === undefined) {
        return false;
    }
    obj = obj[arguments[i]];
  }
  return true;
};

console.log(isset(obj));                   // returns false
var obj = 'huhu';
console.log(isset(obj));                   // returns true
obj = {hallo:{hoi:'hoi'}};
console.log(isset(obj, 'niet'));           // returns false
console.log(isset(obj, 'hallo'));          // returns true
console.log(isset(obj, 'hallo', 'hallo')); // returns false
console.log(isset(obj, 'hallo', 'hoi'));   // returns true

Answer 8

This solution worked for me.

function isset(object){
    return (typeof object !=='undefined');
}

Answer 9

If you are using underscorejs I always use

if (!_.isUndefined(data) && !_.isNull(data)) {
     //your stuff
}

Answer 10

This is a pretty bulletproof solution for testing if a variable exists :

var setOrNot = typeof variable !== typeof undefined ? true : false;

Unfortunately, you cannot simply encapsulate it in a function.

You might think of doing something like this :

function isset(variable) {
    return typeof variable !== typeof undefined ? true : false;
}

However, this will produce a reference error if variable variable has not been defined, because you cannot pass along a non-existing variable to a function :

Uncaught ReferenceError: foo is not defined

On the other hand, it does allow you to test whether function parameters are undefined :

var a = '5';

var test = function(x, y) {
    console.log(isset(x));
    console.log(isset(y));
};

test(a);

// OUTPUT :
// ------------
// TRUE
// FALSE

Even though no value for y is passed along to function test, our isset function works perfectly in this context, because y is known in function test as an undefined value.

Answer 11

window.isset = function(v_var) {
    if(typeof(v_var) == 'number'){ if(isNaN(v_var)){ return false; }}
    if(typeof(v_var) == 'undefined' || v_var === null){ return false;   } else { return true; }
};

plus Tests:

https://gist.github.com/daylik/24acc318b6abdcdd63b46607513ae073

Answer 12

(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.

Answer 13

function isset(variable) {
    try {
        return typeof eval(variable) !== 'undefined';
    } catch (err) {
        return false;
    }
}

Answer 14

To check wether html block is existing or not, I'm using this code:

if (typeof($('selector').html()) != 'undefined') {
    // $('selector') is existing
    // your code here
}

Answer 15

Provide the object path as a string, then you can break this string into a path and resolve hasOwnProperty at each step while overwriting the object itself with each iteration.

If you are coding in ES6 environment, take a look at this stackoverflow Ques.

var a;

a = {
    b: {
        c: 'e'
    }
};

function isset (obj, path) {
    var stone;

    path = path || '';

    if (path.indexOf('[') !== -1) {
        throw new Error('Unsupported object path notation.');
    }

    
    path = path.split('.');
    
    do {
        if (obj === undefined) {
            return false;
        }

        stone = path.shift();
        
        if (!obj.hasOwnProperty(stone)) {
            return false;
        }
        
        obj = obj[stone];
        
    } while (path.length);

    return true;
}

console.log(
    isset(a, 'b') == true,
    isset(a, 'b.c') == true,
    isset(a, 'b.c.d') == false,
    isset(a, 'b.c.d.e') == false,
    isset(a, 'b.c.d.e.f') == false
);

Answer 16

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;
    };

Answer 17

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

Answer 18

finally i solved problem with easy solution :

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

Answer 19

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
}