How do I check if an object has a specific property in JavaScript?
Consider:
x = {'key': 1};
if ( x.hasOwnProperty('key') ) {
//Do this
}
Is that the best way to do it?
2022 UPDATE
Object.hasOwn()
Object.hasOwn() is recommended over Object.hasOwnProperty() because it works for objects created using Object.create(null) and with objects that have overridden the inherited hasOwnProperty() method. While it is possible to workaround these problems by calling Object.prototype.hasOwnProperty() on an external object, Object.hasOwn() is more intuitive.
Example
const object1 = {
prop: 'exists'
};
console.log(Object.hasOwn(object1, 'prop'));
// expected output: true
Original answer
I'm really confused by the answers that have been given - most of them are just outright incorrect. Of course you can have object properties that have undefined, null, or false values. So simply reducing the property check to typeof this[property] or, even worse, x.key will give you completely misleading results.
It depends on what you're looking for. If you want to know if an object physically contains a property (and it is not coming from somewhere up on the prototype chain) then object.hasOwnProperty is the way to go. All modern browsers support it. (It was missing in older versions of Safari - 2.0.1 and older - but those versions of the browser are rarely used any more.)
If what you're looking for is if an object has a property on it that is iterable (when you iterate over the properties of the object, it will appear) then doing: prop in object will give you your desired effect.
Since using hasOwnProperty is probably what you want, and considering that you may want a fallback method, I present to you the following solution:
var obj = {
a: undefined,
b: null,
c: false
};
// a, b, c all found
for ( var prop in obj ) {
document.writeln( "Object1: " + prop );
}
function Class(){
this.a = undefined;
this.b = null;
this.c = false;
}
Class.prototype = {
a: undefined,
b: true,
c: true,
d: true,
e: true
};
var obj2 = new Class();
// a, b, c, d, e found
for ( var prop in obj2 ) {
document.writeln( "Object2: " + prop );
}
function hasOwnProperty(obj, prop) {
var proto = obj.__proto__ || obj.constructor.prototype;
return (prop in obj) &&
(!(prop in proto) || proto[prop] !== obj[prop]);
}
if ( Object.prototype.hasOwnProperty ) {
var hasOwnProperty = function(obj, prop) {
return obj.hasOwnProperty(prop);
}
}
// a, b, c found in modern browsers
// b, c found in Safari 2.0.1 and older
for ( var prop in obj2 ) {
if ( hasOwnProperty(obj2, prop) ) {
document.writeln( "Object2 w/ hasOwn: " + prop );
}
}
The above is a working, cross-browser, solution to hasOwnProperty(), with one caveat: It is unable to distinguish between cases where an identical property is on the prototype and on the instance - it just assumes that it's coming from the prototype. You could shift it to be more lenient or strict, based upon your situation, but at the very least this should be more helpful.
With Underscore.js or (even better) Lodash:
_.has(x, 'key');
Which calls Object.prototype.hasOwnProperty, but (a) is shorter to type, and (b) uses "a safe reference to hasOwnProperty" (i.e. it works even if hasOwnProperty is overwritten).
In particular, Lodash defines _.has as:
function has(object, key) {
return object ? hasOwnProperty.call(object, key) : false;
}
// hasOwnProperty = Object.prototype.hasOwnProperty
You can use this (but read the warning below):
var x = {
'key': 1
};
if ('key' in x) {
console.log('has');
}
But be warned: 'constructor' in x will return true even if x is an empty object - same for 'toString' in x, and many others. It's better to use Object.hasOwn(x, 'key').
Note: the following is nowadays largely obsolete thanks to strict mode, and hasOwnProperty. The correct solution is to use strict mode and to check for the presence of a property using obj.hasOwnProperty. This answer predates both these things, at least as widely implemented (yes, it is that old). Take the following as a historical note.
Bear in mind that undefined is (unfortunately) not a reserved word in JavaScript if you’re not using strict mode. Therefore, someone (someone else, obviously) could have the grand idea of redefining it, breaking your code.
A more robust method is therefore the following:
if (typeof(x.attribute) !== 'undefined')
On the flip side, this method is much more verbose and also slower. :-/
A common alternative is to ensure that undefined is actually undefined, e.g. by putting the code into a function which accepts an additional parameter, called undefined, that isn’t passed a value. To ensure that it’s not passed a value, you could just call it yourself immediately, e.g.:
(function (undefined) {
… your code …
if (x.attribute !== undefined)
… mode code …
})();
if (x.key !== undefined)
Armin Ronacher seems to have already beat me to it, but:
Object.prototype.hasOwnProperty = function(property) {
return this[property] !== undefined;
};
x = {'key': 1};
if (x.hasOwnProperty('key')) {
alert('have key!');
}
if (!x.hasOwnProperty('bar')) {
alert('no bar!');
}
A safer, but slower solution, as pointed out by Konrad Rudolph and Armin Ronacher would be:
Object.prototype.hasOwnProperty = function(property) {
return typeof this[property] !== 'undefined';
};
Considering the following object in Javascript
const x = {key: 1};
You can use the in operator to check if the property exists on an object:
console.log("key" in x);
You can also loop through all the properties of the object using a for - in loop, and then check for the specific property:
for (const prop in x) {
if (prop === "key") {
//Do something
}
}
You must consider if this object property is enumerable or not, because non-enumerable properties will not show up in a for-in loop. Also, if the enumerable property is shadowing a non-enumerable property of the prototype, it will not show up in Internet Explorer 8 and earlier.
If you’d like a list of all instance properties, whether enumerable or not, you can use
Object.getOwnPropertyNames(x);
This will return an array of names of all properties that exist on an object.
Reflections provide methods that can be used to interact with Javascript objects. The static Reflect.has() method works like the in operator as a function.
console.log(Reflect.has(x, 'key'));
// expected output: true
console.log(Reflect.has(x, 'key2'));
// expected output: false
console.log(Reflect.has(object1, 'toString'));
// expected output: true
Finally, you can use the typeof operator to directly check the data type of the object property:
if (typeof x.key === "undefined") {
console.log("undefined");
}
If the property does not exist on the object, it will return the string undefined. Else it will return the appropriate property type. However, note that this is not always a valid way of checking if an object has a property or not, because you could have a property that is set to undefined, in which case, using typeof x.key would still return true (even though the key is still in the object).
Similarly, you can check if a property exists by comparing directly to the undefined Javascript property
if (x.key === undefined) {
console.log("undefined");
}
This should work unless key was specifically set to undefined on the x object
Let's cut through some confusion here. First, let's simplify by assuming hasOwnProperty already exists; this is true of the vast majority of current browsers in use.
hasOwnProperty returns true if the attribute name that is passed to it has been added to the object. It is entirely independent of the actual value assigned to it which may be exactly undefined.
Hence:
var o = {}
o.x = undefined
var a = o.hasOwnProperty('x') // a is true
var b = o.x === undefined // b is also true
However:
var o = {}
var a = o.hasOwnProperty('x') // a is now false
var b = o.x === undefined // b is still true
The problem is what happens when an object in the prototype chain has an attribute with the value of undefined? hasOwnProperty will be false for it, and so will !== undefined. Yet, for..in will still list it in the enumeration.
The bottom line is there is no cross-browser way (since Internet Explorer doesn't expose __prototype__) to determine that a specific identifier has not been attached to an object or anything in its prototype chain.
If you are searching for a property, then "no". You want:
if ('prop' in obj) { }
In general, you should not care whether or not the property comes from the prototype or the object.
However, because you used 'key' in your sample code, it looks like you are treating the object as a hash, in which case your answer would make sense. All of the hashes keys would be properties in the object, and you avoid the extra properties contributed by the prototype.
John Resig's answer was very comprehensive, but I thought it wasn't clear. Especially with when to use "'prop' in obj".
For testing simple objects, use:
if (obj[x] !== undefined)
If you don't know what object type it is, use:
if (obj.hasOwnProperty(x))
All other options are slower...
Details
A performance evaluation of 100,000,000 cycles under Node.js to the five options suggested by others here:
function hasKey1(k,o) { return (x in obj); }
function hasKey2(k,o) { return (obj[x]); }
function hasKey3(k,o) { return (obj[x] !== undefined); }
function hasKey4(k,o) { return (typeof(obj[x]) !== 'undefined'); }
function hasKey5(k,o) { return (obj.hasOwnProperty(x)); }
The evaluation tells us that unless we specifically want to check the object's prototype chain as well as the object itself, we should not use the common form:
if (X in Obj)...
It is between 2 to 6 times slower depending on the use case
hasKey1 execution time: 4.51 s
hasKey2 execution time: 0.90 s
hasKey3 execution time: 0.76 s
hasKey4 execution time: 0.93 s
hasKey5 execution time: 2.15 s
Bottom line, if your Obj is not necessarily a simple object and you wish to avoid checking the object's prototype chain and to ensure x is owned by Obj directly, use if (obj.hasOwnProperty(x))....
Otherwise, when using a simple object and not being worried about the object's prototype chain, using if (typeof(obj[x]) !== 'undefined')... is the safest and fastest way.
If you use a simple object as a hash table and never do anything kinky, I would use if (obj[x])... as I find it much more readable.
Yes it is :) I think you can also do Object.prototype.hasOwnProperty.call(x, 'key') which should also work if x has a property called hasOwnProperty :)
But that tests for own properties. If you want to check if it has an property that may also be inhered you can use typeof x.foo != 'undefined'.
if(x.hasOwnProperty("key")){
// …
}
because
if(x.key){
// …
}
fails if x.key is falsy (for example, x.key === "").
You can also use the ES6 Reflect object:
x = {'key': 1};
Reflect.has( x, 'key'); // returns true
Documentation on MDN for Reflect.has can be found here.
The static Reflect.has() method works like the in operator as a function.
Do not do this object.hasOwnProperty(key)). It's really bad because these methods may be shadowed by properties on the object in question - consider { hasOwnProperty: false } - or, the object may be a null object (Object.create(null)).
The best way is to do Object.prototype.hasOwnProperty.call(object, key) or:
const has = Object.prototype.hasOwnProperty; // Cache the lookup once, in module scope.
console.log(has.call(object, key));
/* Or */
import has from 'has'; // https://www.npmjs.com/package/has
console.log(has(object, key));
OK, it looks like I had the right answer unless if you don't want inherited properties:
if (x.hasOwnProperty('key'))
Here are some other options to include inherited properties:
if (x.key) // Quick and dirty, but it does the same thing as below.
if (x.key !== undefined)
Another relatively simple way is using Object.keys. This returns an array which means you get all of the features of an array.
var noInfo = {};
var info = {something: 'data'};
Object.keys(noInfo).length //returns 0 or false
Object.keys(info).length //returns 1 or true
Although we are in a world with great browser support. Because this question is so old I thought I'd add this:
This is safe to use as of JavaScript v1.8.5.
JavaScript is now evolving and growing as it now has good and even efficient ways to check it.
Here are some easy ways to check if object has a particular property:
Using hasOwnProperty()
const hero = {
name: 'Batman'
};
hero.hasOwnProperty('name'); // => true
hero.hasOwnProperty('realName'); // => false
Using keyword/operator in
const hero = {
name: 'Batman'
};
'name' in hero; // => true
'realName' in hero; // => false
Comparing with undefined keyword
const hero = {
name: 'Batman'
};
hero.name; // => 'Batman'
hero.realName; // => undefined
// So consider this
hero.realName == undefined // => true (which means property does not exists in object)
hero.name == undefined // => false (which means that property exists in object)
For more information, check here.
hasOwnProperty "can be used to determine whether an object has the specified property as a direct property of that object; unlike the in operator, this method does not check down the object's prototype chain."
So most probably, for what seems by your question, you don't want to use hasOwnProperty, which determines if the property exists as attached directly to the object itself,.
If you want to determine if the property exists in the prototype chain, you may want to use it like:
if (prop in object) { // Do something }
You can use the following approaches-
var obj = {a:1}
console.log('a' in obj) // 1
console.log(obj.hasOwnProperty('a')) // 2
console.log(Boolean(obj.a)) // 3
The difference between the following approaches are as follows-
In the first and third approach we are not just searching in object but its prototypal chain too. If the object does not have the property, but the property is present in its prototype chain it is going to give true.
var obj = {
a: 2,
__proto__ : {b: 2}
}
console.log('b' in obj)
console.log(Boolean(obj.b))
The second approach will check only for its own properties. Example -
var obj = {
a: 2,
__proto__ : {b: 2}
}
console.log(obj.hasOwnProperty('b'))
The difference between the first and the third is if there is a property which has value undefined the third approach is going to give false while first will give true.
var obj = {
b : undefined
}
console.log(Boolean(obj.b))
console.log('b' in obj);
Given myObject object and “myKey” as key name:
Object.keys(myObject).includes('myKey')
or
myObject.hasOwnProperty('myKey')
or
typeof myObject.myKey !== 'undefined'
The last was widely used, but (as pointed out in other answers and comments) it could also match on keys deriving from Object prototype.
Performance
Today 2020.12.17 I perform tests on MacOs HighSierra 10.13.6 on Chrome v87, Safari v13.1.2 and Firefox v83 for chosen solutions.
Results
I compare only solutions A-F because they give valid result for all cased used in snippet in details section. For all browsers
solution based on in (A) is fast or fastest
solution (E) is fastest for chrome for big objects and fastest for firefox for small arrays if key not exists
solution (F) is fastest (~ >10x than other solutions) for small arrays
solutions (D,E) are quite fast
solution based on losash has (B) is slowest
Details
I perform 4 tests cases:
when object has 10 fields and searched key exists - you can run it HERE
when object has 10 fields and searched key not exists - you can run it HERE
when object has 10000 fields and searched key exists - you can run it HERE
when object has 10000 fields and searched key exists - you can run it HERE
Below snippet presents differences between solutions
A
B
C
D
E
F
G
H
I
J
K
// SO https://stackoverflow.com/q/135448/860099
// src: https://stackoverflow.com/a/14664748/860099
function A(x) {
return 'key' in x
}
// src: https://stackoverflow.com/a/11315692/860099
function B(x) {
return _.has(x, 'key')
}
// src: https://stackoverflow.com/a/40266120/860099
function C(x) {
return Reflect.has( x, 'key')
}
// src: https://stackoverflow.com/q/135448/860099
function D(x) {
return x.hasOwnProperty('key')
}
// src: https://stackoverflow.com/a/11315692/860099
function E(x) {
return Object.prototype.hasOwnProperty.call(x, 'key')
}
// src: https://stackoverflow.com/a/136411/860099
function F(x) {
function hasOwnProperty(obj, prop) {
var proto = obj.__proto__ || obj.constructor.prototype;
return (prop in obj) &&
(!(prop in proto) || proto[prop] !== obj[prop]);
}
return hasOwnProperty(x,'key')
}
// src: https://stackoverflow.com/a/135568/860099
function G(x) {
return typeof(x.key) !== 'undefined'
}
// src: https://stackoverflow.com/a/22740939/860099
function H(x) {
return x.key !== undefined
}
// src: https://stackoverflow.com/a/38332171/860099
function I(x) {
return !!x.key
}
// src: https://stackoverflow.com/a/41184688/860099
function J(x) {
return !!x['key']
}
// src: https://stackoverflow.com/a/54196605/860099
function K(x) {
return Boolean(x.key)
}
// --------------------
// TEST
// --------------------
let x1 = {'key': 1};
let x2 = {'key': "1"};
let x3 = {'key': true};
let x4 = {'key': []};
let x5 = {'key': {}};
let x6 = {'key': ()=>{}};
let x7 = {'key': ''};
let x8 = {'key': 0};
let x9 = {'key': false};
let x10= {'key': undefined};
let x11= {'nokey': 1};
let b= x=> x ? 1:0;
console.log(' 1 2 3 4 5 6 7 8 9 10 11');
[A,B,C,D,E,F,G,H,I,J,K ].map(f=> {
console.log(
`${f.name} ${b(f(x1))} ${b(f(x2))} ${b(f(x3))} ${b(f(x4))} ${b(f(x5))} ${b(f(x6))} ${b(f(x7))} ${b(f(x8))} ${b(f(x9))} ${b(f(x10))} ${b(f(x11))} `
)})
console.log('\nLegend: Columns (cases)');
console.log('1. key = 1 ');
console.log('2. key = "1" ');
console.log('3. key = true ');
console.log('4. key = [] ');
console.log('5. key = {} ');
console.log('6. key = ()=>{} ');
console.log('7. key = "" ');
console.log('8. key = 0 ');
console.log('9. key = false ');
console.log('10. key = undefined ');
console.log('11. no-key ');
<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.20/lodash.min.js" integrity="sha512-90vH1Z83AJY9DmlWa8WkjkV79yfS2n2Oxhsi2dZbIv0nC4E6m5AbH8Nh156kkM7JePmqD6tcZsfad1ueoaovww==" crossorigin="anonymous"> </script>
This shippet only presents functions used in performance tests - it not perform tests itself!
And here are example results for chrome
Now with ECMAScript22 we can use hasOwn instead of hasOwnProperty (Because this feature has pitfalls )
Object.hasOwn(obj, propKey)
Here is another option for a specific case. :)
If you want to test for a member on an object and want to know if it has been set to something other than:
''
false
null
undefined
0
...
then you can use:
var foo = {};
foo.bar = "Yes, this is a proper value!";
if (!!foo.bar) {
// member is set, do something
}
some easier and short options depending on the specific use case:
to check if the property exists, regardless of value, use the in operator ("a" in b)
to check a property value from a variable, use bracket notation (obj[v])
to check a property value as truthy, use optional
chaining (?.)
to check a property value boolean, use double-not / bang-bang / (!!)
to set a default value for null / undefined check, use nullish coalescing operator (??)
to set a default value for falsey value check, use short-circuit logical OR operator (||)
run the code snippet to see results:
let obj1 = {prop:undefined};
console.log(1,"prop" in obj1);
console.log(1,obj1?.prop);
let obj2 = undefined;
//console.log(2,"prop" in obj2); would throw because obj2 undefined
console.log(2,"prop" in (obj2 ?? {}))
console.log(2,obj2?.prop);
let obj3 = {prop:false};
console.log(3,"prop" in obj3);
console.log(3,!!obj3?.prop);
let obj4 = {prop:null};
let look = "prop"
console.log(4,"prop" in obj4);
console.log(4,obj4?.[look]);
let obj5 = {prop:true};
console.log(5,"prop" in obj5);
console.log(5,obj5?.prop === true);
let obj6 = {otherProp:true};
look = "otherProp"
console.log(6,"prop" in obj6);
console.log(6,obj6.look); //should have used bracket notation
let obj7 = {prop:""};
console.log(7,"prop" in obj7);
console.log(7,obj7?.prop || "empty");
I see very few instances where hasOwn is used properly, especially given its inheritance issues
There is a method, "hasOwnProperty", that exists on an object, but it's not recommended to call this method directly, because it might be sometimes that the object is null or some property exist on the object like: { hasOwnProperty: false }
So a better way would be:
// Good
var obj = {"bar": "here bar desc"}
console.log(Object.prototype.hasOwnProperty.call(obj, "bar"));
// Best
const has = Object.prototype.hasOwnProperty; // Cache the lookup once, in module scope.
console.log(has.call(obj, "bar"));
An ECMAScript 6 solution with reflection. Create a wrapper like:
/**
Gets an argument from array or object.
The possible outcome:
- If the key exists the value is returned.
- If no key exists the default value is returned.
- If no default value is specified an empty string is returned.
#param obj The object or array to be searched.
#param key The name of the property or key.
#param defVal Optional default version of the command-line parameter [default ""]
#return The default value in case of an error else the found parameter.
*/
function getSafeReflectArg( obj, key, defVal) {
"use strict";
var retVal = (typeof defVal === 'undefined' ? "" : defVal);
if ( Reflect.has( obj, key) ) {
return Reflect.get( obj, key);
}
return retVal;
} // getSafeReflectArg
Showing how to use this answer
const object= {key1: 'data', key2: 'data2'};
Object.keys(object).includes('key1') //returns true
We can use indexOf as well, I prefer includes
You need to use the method object.hasOwnProperty(property). It returns true if the object has the property and false if the object doesn't.
The hasOwnProperty() method returns a boolean indicating whether the object has the specified property as its own property (as opposed to inheriting it).
const object1 = {};
object1.property1 = 42;
console.log(object1.hasOwnProperty('property1'));
// expected output: true
console.log(object1.hasOwnProperty('toString'));
// expected output: false
console.log(object1.hasOwnProperty('hasOwnProperty'));
// expected output: false
Know more
Don't over-complicate things when you can do:
var isProperty = (objectname.keyname || "") ? true : false;
It Is simple and clear for most cases...
A Better approach for iterating on object's own properties:
If you want to iterate on object's properties without using hasOwnProperty() check,
use for(let key of Object.keys(stud)){} method:
for(let key of Object.keys(stud)){
console.log(key); // will only log object's Own properties
}
full Example and comparison with for-in with hasOwnProperty()
function Student() {
this.name = "nitin";
}
Student.prototype = {
grade: 'A'
}
let stud = new Student();
// for-in approach
for(let key in stud){
if(stud.hasOwnProperty(key)){
console.log(key); // only outputs "name"
}
}
//Object.keys() approach
for(let key of Object.keys(stud)){
console.log(key);
}
I have seen* something where a certain key can be called both as a variable and as a function.
The variable would return a value, the function would do something functiony.
obj.test
// E.g. 16
obj.test(32)
// do something with 32
I have no idea what library this was. This is not your typical getter/setter scenario, because then the second one would not be a function but a setter.
Please tell me I'm not crazy.
*) If I remember what/where, I probably wouldn't need to ask this question.
I found the solution!
After a comment from T.J.Crowder about toString() as a final possibility, played around with that and got exactly what I wanted** (feel free to upvote as well ;)). But what I wanted is subjective, and what I asked is not. T.J.Crowder answered to the best of his ability for all (?) scenarios that someone else with a similar question might want, and therefore I accepted his answer.
**) After grepping toString in all libraries I'm using, I found that Raphael does something similar.
Introduction
If o.test() calls a function, then o.test returns a function reference, that's a guarantee. Some possible thoughts about what you might have seen:
If you saw different behavior when something was assigned to the property (obj.test = 42;) then when it was retrieved (obj.test();), that might be an asymmetric property. More below.
If o.test was being used in some expression, like var x = o.test + 42; or element.innerHTML = o.test;, it could be they were using an overridden toString and/or valueOf. More below.
The property could redefine itself after first use, so o.test gives you 16 but then changes the definition of the o.test property. More on properties redefining themselves below.
An asymmetric property
In a comment you've said:
... in comments it said something like This code makes the function double as a variable.
The closest I can see to that is where you assign to it (not read from it) and it remembers the value. That looks like this:
var o = {};
(function(obj) {
var storedValue;
function weirdFunction(val) {
return arguments.length === 0 ? storedValue : val;
}
Object.defineProperty(obj, "test", {
get: function() {
return weirdFunction;
},
set: function(value) {
storedValue = value;
},
enumerable: true
});
})(o);
o.test = 16;
console.log(o.test()); // 16
console.log(o.test(32)); // 32
console.log(o.test()); // 16
o.test = 24;
console.log(o.test()); // 24
console.log(o.test(32)); // 32
console.log(o.test()); // 24
Live Example
That's an example of an asymmetric property: When you read it, it's always a function; but when you write it, you can write any value and it will remember it. I've made the function it gives you when you return it use the stored value unless you give it something else to work with.
There may be good use cases for asymmetric properties, but if so I expect they're few and far between. (For instance, the document.cookie property in browsers is an asymmetric property, and it confusese people no end and is a royal pain in the rear to use; a classis case of "wow, they really, really shouldn't have done it like that"...)
The toString / valueOf trick
Here's the example of the toString / valueOf trick (I wouldn't recommend it, but it works with certain caveats):
var obj = {test: function(val) { return val; }};
obj.test.valueOf = obj.test.toString = function() {
return 16;
};
console.log(obj.test); // 16
console.log(obj.test(32)); // 32
It requires that whatever is using the property as a non-function do something to convert it, as console.log does (for me on Chrome). E.g., convert it to a string or a number: var str = "The result is " + obj.test; (which calls toString implicitly), or var = 5 + obj.test; (which calls valueOf implicitly). If the value is used unconverted, then (of course) it's a function reference.
Redefining Itself
Another truly horrible thing a property can do, on ES5-compliant systems, is redefine itself when it's used:
// Truly Evil And Wrong
var obj = {};
Object.defineProperty(obj, "test", {
get: function() {
Object.defineProperty(this, "test", {
value: function(val) { return val; },
enumerable: true
});
return 16;
},
enumerable: true,
configurable: true
});
console.log(obj.test); // 16
console.log(obj.test); // function(val) { return val; }
console.log(obj.test(32)); // 32
But don't do that. Just don't. :-)
The only solution I might imagine is, that 'obj' is redefined later.
I was playing around with the comment by T.J.Crowder mentioning the only way to circumvent returning a function reference is to play with toString/valueOf. I got something that might be exactly what I saw:
var obj = {
test: (function() {
var storedValue = 'Nothing. Pass me a number.';
var doSomething = function(val) {
storedValue = val;
return val + ' black rainbows';
}
doSomething.toString = function() {
return storedValue;
}
return doSomething;
})()
};
Effectively:
obj.test // Nothing. Pass me a number.
obj.test(45) // "45 black rainbows"
obj.test // 45
It's just not a settable variable, but I think it's reasonable to assume I made that part up after seeing this. However, maybe some setter trickery is possible to achieve this too.
Javascript objects can be used as maps. The following all is valid code:
var x = {};
x.a = 1;
x['b'] = 2; // the same as x.b = 2;
x[3] = 3; // x.3 won't work, but this syntax works fine. Also, x[3] == x['3'].
x['What, this works too?!?!?'] = 'Yup, it does!';
But today I tested another case which... seems to work, but raises some warning flags in my head because it looks... wrong:
x[null] = 42;
Now, it would be extremely cool if this worked as expected (I won't have to rewrite a bunch of code then), but can I rely on it? Or maybe this is just some undocumented behavior which just happens to work in all modern browsers but might as well cease working on the next release of Google Chrome?
Anything between the property brackets is converted to a string. null becomes "null", which is a valid property.
It's valid ECMAScript 5, but you may run into problems in some versions of Internet Explorer.
This is all fine:
x = {};
x[null] = 42;
x[null];
// 42
x = {"null": 42};
x[null];
// 42
However, IE8 doesn't accept the following with reserved words such as null:
x.null
// Error: Expected identifier
x = {null: 42}
// Expected identifier, string or number
(Both of the above work in Chrome, even with "use strict" enabled.)
I cannot leave comments so I created an answer...
While Rob W is correct his answer is a little misleading. The item between the square brackets is converted to a string using ToString(). So, a[null] only equals a['null'] because ToString( null ) == 'null'
take for instance
var a = 1;
var x = {}
x[a] = 20;
x['1'] = 30; -- this is exactly the same as the previous line and now x[a] == 30
If your environment supports ES6 and you need to be able to store a value for null as well as any arbitrary string then you can use a Map.
const a = new Map();
a.set(null, 'foo')
a.set('null', 'bar')
// Map { null => 'foo', 'null' => 'bar' }
x['null'] = 42;
I would strongly dis-recommend that, though, to avoid confusion.
Property names are strings. Every string will work, even ones that represent reserved identifiers.
Everything that's not a string will be converted into a string (by calling the object's toString() method):
a = ['FOO']; // an array
o = {}; // an object
o[a] = 'foo'; // using a as the property name
// o now is:
{
FOO: 'foo'
}
Assuming I declare
var ad = {};
How can I check whether this object will contain any user-defined properties?
You can use the built in Object.keys method to get a list of keys on an object and test its length.
var x = {};
// some code where value of x changes and than you want to check whether it is null or some object with values
if(Object.keys(x).length){
// Your code here if x has some properties
}
What about making a simple function?
function isEmptyObject(obj) {
for(var prop in obj) {
if (Object.prototype.hasOwnProperty.call(obj, prop)) {
return false;
}
}
return true;
}
isEmptyObject({}); // true
isEmptyObject({foo:'bar'}); // false
The hasOwnProperty method call directly on the Object.prototype is only to add little more safety, imagine the following using a normal obj.hasOwnProperty(...) call:
isEmptyObject({hasOwnProperty:'boom'}); // false
Note: (for the future) The above method relies on the for...in statement, and this statement iterates only over enumerable properties, in the currently most widely implemented ECMAScript Standard (3rd edition) the programmer doesn't have any way to create non-enumerable properties.
However this has changed now with ECMAScript 5th Edition, and we are able to create non-enumerable, non-writable or non-deletable properties, so the above method can fail, e.g.:
var obj = {};
Object.defineProperty(obj, 'test', { value: 'testVal',
enumerable: false,
writable: true,
configurable: true
});
isEmptyObject(obj); // true, wrong!!
obj.hasOwnProperty('test'); // true, the property exist!!
An ECMAScript 5 solution to this problem would be:
function isEmptyObject(obj) {
return Object.getOwnPropertyNames(obj).length === 0;
}
The Object.getOwnPropertyNames method returns an Array containing the names of all the own properties of an object, enumerable or not, this method is being implemented now by browser vendors, it's already on the Chrome 5 Beta and the latest WebKit Nightly Builds.
Object.defineProperty is also available on those browsers and latest Firefox 3.7 Alpha releases.
You can loop over the properties of your object as follows:
for(var prop in ad) {
if (ad.hasOwnProperty(prop)) {
// handle prop as required
}
}
It is important to use the hasOwnProperty() method, to determine whether the object has the specified property as a direct property, and not inherited from the object's prototype chain.
Edit
From the comments: You can put that code in a function, and make it return false as soon as it reaches the part where there is the comment
With jQuery you can use:
$.isEmptyObject(obj); // Returns: Boolean
As of jQuery 1.4 this method checks both properties on the object itself and properties inherited from prototypes (in that it doesn't use hasOwnProperty).
With ECMAScript 5th Edition in modern browsers (IE9+, FF4+, Chrome5+, Opera12+, Safari5+) you can use the built in Object.keys method:
var obj = { blah: 1 };
var isEmpty = !Object.keys(obj).length;
Or plain old JavaScript:
var isEmpty = function(obj) {
for(var p in obj){
return false;
}
return true;
};
Most recent browsers (and node.js) support Object.keys() which returns an array with all the keys in your object literal so you could do the following:
var ad = {};
Object.keys(ad).length;//this will be 0 in this case
Browser Support: Firefox 4, Chrome 5, Internet Explorer 9, Opera 12, Safari 5
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/keys
If you're using underscore.js then you can use the _.isEmpty function:
var obj = {};
var emptyObject = _.isEmpty(obj);
If you are willing to use lodash, you can use the some method.
_.some(obj) // returns true or false
See this small jsbin example
for (var hasProperties in ad) break;
if (hasProperties)
... // ad has properties
If you have to be safe and check for Object prototypes (these are added by certain libraries and not there by default):
var hasProperties = false;
for (var x in ad) {
if (ad.hasOwnProperty(x)) {
hasProperties = true;
break;
}
}
if (hasProperties)
... // ad has properties
for(var memberName in ad)
{
//Member Name: memberName
//Member Value: ad[memberName]
}
Member means Member property, member variable, whatever you want to call it >_>
The above code will return EVERYTHING, including toString...
If you only want to see if the object's prototype has been extended:
var dummyObj = {};
for(var memberName in ad)
{
if(typeof(dummyObj[memberName]) == typeof(ad[memberName])) continue; //note A
//Member Name: memberName
//Member Value: ad[memberName]
}
Note A: We check to see if the dummy object's member has the same type as our testing object's member. If it is an extend, dummyobject's member type should be "undefined"
var hasAnyProps = false; for (var key in obj) { hasAnyProps = true; break; }
// as of this line hasAnyProps will show Boolean whether or not any iterable props exist
Simple, works in every browser, and even though it's technically a loop for all keys on the object it does NOT loop through them all...either there's 0 and the loop doesn't run or there is some and it breaks after the first one (because all we're checking is if there's ANY...so why continue?)
ES6 function
/**
* Returns true if an object is empty.
* #param {*} obj the object to test
* #return {boolean} returns true if object is empty, otherwise returns false
*/
const pureObjectIsEmpty = obj => obj && obj.constructor === Object && Object.keys(obj).length === 0
Examples:
let obj = "this is an object with String constructor"
console.log(pureObjectIsEmpty(obj)) // empty? true
obj = {}
console.log(pureObjectIsEmpty(obj)) // empty? true
obj = []
console.log(pureObjectIsEmpty(obj)) // empty? true
obj = [{prop:"value"}]
console.log(pureObjectIsEmpty(obj)) // empty? true
obj = {prop:"value"}
console.log(pureObjectIsEmpty(obj)) // empty? false
When sure that the object is a user-defined one, the easiest way to determine if UDO is empty, would be the following code:
isEmpty=
/*b.b Troy III p.a.e*/
function(x,p){for(p in x)return!1;return!0};
Even though this method is (by nature) a deductive one, - it's the quickest, and fastest possible.
a={};
isEmpty(a) >> true
a.b=1
isEmpty(a) >> false
p.s.:
!don't use it on browser-defined objects.
You can use the following:
Double bang !! property lookup
var a = !![]; // true
var a = !!null; // false
hasOwnProperty
This is something that I used to use:
var myObject = {
name: 'John',
address: null
};
if (myObject.hasOwnProperty('address')) { // true
// do something if it exists.
}
However, JavaScript decided not to protect the method’s name, so it could be tampered with.
var myObject = {
hasOwnProperty: 'I will populate it myself!'
};
prop in myObject
var myObject = {
name: 'John',
address: null,
developer: false
};
'developer' in myObject; // true, remember it's looking for exists, not value.
typeof
if (typeof myObject.name !== 'undefined') {
// do something
}
However, it doesn't check for null.
I think this is the best way.
in operator
var myObject = {
name: 'John',
address: null
};
if('name' in myObject) {
console.log("Name exists in myObject");
}else{
console.log("Name does not exist in myObject");
}
result:
Name exists in myObject
Here is a link that goes into more detail on the in operator: Determining if an object property exists
Very late answer, but this is how you could handle it with prototypes.
Array.prototype.Any = function(func) {
return this.some(func || function(x) { return x });
}
Object.prototype.IsAny = function() {
return Object.keys(this).Any();
}
Object.hasOwn is a new static method (not fully supported by all browsers yet) which checks if the specified object has the indicated property as his own property and return true if that is the case. It will return false if the property is either inherited or does not exist on that object.
You can iterate through the object properties and check if they are indeed own properties
for (let property in ad) {
if (Object.hasOwn(ad, property)) {
// handle your code for object own properties here
}
}
More about Object.hasOwn - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/
Browser compatibility here - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwn#browser_compatibility
I'm not sure if this is a good approach but I use this condition to check if an object has or hasn't any property. Could be easily transformed into a function.
const obj = {};
if(function(){for (key in obj){return true}return false}())
{
//do something;
}
else
{
//do something else;
}
//Condition could be shorted by e.g. function(){for(key in obj){return 1}return 0}()
How about this?
var obj = {},
var isEmpty = !obj;
var hasContent = !!obj