Checking if a key exists in a JavaScript object?

Question

How do I check if a particular key exists in a JavaScript object or array?

If a key doesn't exist, and I try to access it, will it return false? Or throw an error?

Answer 1

Checking for undefined-ness is not an accurate way of testing whether a key exists. What if the key exists but the value is actually undefined ?

 var obj = { key: undefined }; console.log(obj["key"] !== undefined); // false, but the key exists!

You should instead use the in operator:

 var obj = { key: undefined }; console.log("key" in obj); // true, regardless of the actual value

If you want to check if a key doesn't exist, remember to use parenthesis:

 var obj = { not_key: undefined }; console.log(!("key" in obj)); // true if "key" doesn't exist in object console.log(!"key" in obj); // Do not do this! It is equivalent to "false in obj"

Or, if you want to particularly test for properties of the object instance (and not inherited properties), use hasOwnProperty :

 var obj = { key: undefined }; console.log(obj.hasOwnProperty("key")); // true

For performance comparison between the methods that are in , hasOwnProperty and key is undefined , see this benchmark :

Answer 2

Quick Answer

How do I check if a particular key exists in a JavaScript object or array? If a key doesn't exist and I try to access it, will it return false? Or throw an error?

Accessing directly a missing property using (associative) array style or object style will return an undefined constant.

The slow and reliable in operator and hasOwnProperty method

As people have already mentioned here, you could have an object with a property associated with an "undefined" constant.

 var bizzareObj = {valid_key:  undefined};

In that case, you will have to use hasOwnProperty or in operator to know if the key is really there. But, but at what price?

so, I tell you...

in operator and hasOwnProperty are "methods" that use the Property Descriptor mechanism in Javascript (similar to Java reflection in the Java language).

http://www.ecma-international.org/ecma-262/5.1/#sec-8.10

The Property Descriptor type is used to explain the manipulation and reification of named property attributes. Values of the Property Descriptor type are records composed of named fields where each field's name is an attribute name and its value is a corresponding attribute value as specified in 8.6.1. In addition, any field may be present or absent.

On the other hand, calling an object method or key will use Javascript [[Get]] mechanism. That is a far way faster!

Benchmark

http://jsperf.com/checking-if-a-key-exists-in-a-javascript-array

.

Using in operator

var result = "Impression" in array;

The result was

var result = array.hasOwnProperty("Impression")

Using hasOwnProperty

var result = array["Impression"] === undefined

The result was

168,270,439 ±0.13 ops/sec     0.02% slower 

Accessing elements directly (brackets style)

168,303,172 ±0.20%     fastest

The result was

168,270,439 ±0.13 ops/sec 0.02% slower

Accessing elements directly (object style)

var a = {1: null}; 
console.log(a[1] === undefined); // output: false. I know the value at position 1 of a[] is absent and this was by design, i.e.:  the value is defined. 
console.log(a[0] === undefined); // output: true. I cannot say anything about a[0] value. In this case, the key 0 was not in a[].

The result was

168,303,172 ±0.20% fastest

EDIT: What is the reason to assign to a property the undefined value?

That question puzzles me. In Javascript, there are at least two references for absent objects to avoid problems like this: null and undefined .

null is the primitive value that represents the intentional absence of any object value, or in short terms, the confirmed lack of value. On the other hand, undefined is an unknown value (not defined). If there is a property that will be used later with a proper value consider use null reference instead of undefined because in the initial moment the property is confirmed to lack value.

Compare:

 var a = {1: null}; console.log(a[1] === undefined); // output: false. I know the value at position 1 of a[] is absent and this was by design, ie: the value is defined. console.log(a[0] === undefined); // output: true. I cannot say anything about a[0] value. In this case, the key 0 was not in a[].

Advice

Avoid objects with undefined values. Check directly whenever possible and use null to initialize property values. Otherwise, use the slow in operator or hasOwnProperty() method.

EDIT: 12/04/2018 - NOT RELEVANT ANYMORE

As people have commented, modern versions of the Javascript engines (with firefox exception) have changed the approach for access properties. The current implementation is slower than the previous one for this particular case but the difference between access key and object is neglectable.

Answer 3

It will return undefined .

 var aa = {hello: "world"}; alert( aa["hello"] ); // popup box with "world" alert( aa["goodbye"] ); // popup box with "undefined"

undefined is a special constant value. So you can say, eg

// note the three equal signs so that null won't be equal to undefined
if( aa["goodbye"] === undefined ) {
    // do something
}

This is probably the best way to check for missing keys. However, as is pointed out in a comment below, it's theoretically possible that you'd want to have the actual value be undefined . I've never needed to do this and can't think of a reason offhand why I'd ever want to, but just for the sake of completeness, you can use the in operator

// this works even if you have {"goodbye": undefined}
if( "goodbye" in aa ) {
    // do something
}

Answer 4

"key" in obj

可能只测试与数组键非常不同的对象属性值

Answer 5

The accepted answer refers to Object . Beware using the in operator on Array to find data instead of keys:

("true" in ["true", "false"])
// -> false (Because the keys of the above Array are actually 0 and 1)

To test existing elements in an Array: Best way to find if an item is in a JavaScript array?

Answer 6

Three ways to check if a property is present in a javascript object:

1. !!obj.theProperty
   Will convert value to bool. returns true for all but the false value
2. ' theProperty ' in obj
   Will return true if the property exists, no matter its value (even empty)
3. obj.hasOwnProperty('theProperty')
   Does not check the prototype chain. (since all objects have the toString method, 1 and 2 will return true on it, while 3 can return false on it.)

Reference:

http://book.mixu.net/node/ch5.html

Answer 7

If you are using underscore.js library then object/array operations become simple.

In your case _.has method can be used. Example:

yourArray = {age: "10"}

_.has(yourArray, "age")

returns true

But,

_.has(yourArray, "invalidKey")

returns false

Answer 8

Answer:

if ("key" in myObj)
{
    console.log("key exists!");
}
else
{
    console.log("key doesn't exist!");
}

Explanation:

The in operator will check if the key exists in the object. If you checked if the value was undefined: if (myObj["key"] === 'undefined') , you could run into problems because a key could possibly exist in your object with the undefined value.

For that reason, it is much better practice to first use the in operator and then compare the value that is inside the key once you already know it exists.

Answer 9

Here's a helper function I find quite useful

This keyExists(key, search) can be used to easily lookup a key within objects or arrays!

Just pass it the key you want to find, and search obj (the object or array) you want to find it in.

 function keyExists(key, search) { if (!search || (search.constructor !== Array && search.constructor !== Object)) { return false; } for (var i = 0; i < search.length; i++) { if (search[i] === key) { return true; } } return key in search; } // How to use it: // Searching for keys in Arrays console.log(keyExists('apple', ['apple', 'banana', 'orange'])); // true console.log(keyExists('fruit', ['apple', 'banana', 'orange'])); // false // Searching for keys in Objects console.log(keyExists('age', {'name': 'Bill', 'age': 29 })); // true console.log(keyExists('title', {'name': 'Jason', 'age': 29 })); // false

It's been pretty reliable and works well cross-browser.

Answer 10

vanila js

yourObjName.hasOwnProperty(key) : true ? false;

If you want to check if the object has at least one property in es2015

Object.keys(yourObjName).length : true ? false

Answer 11

ES6 solution

using Array#some and Object.keys . It will return true if given key exists in the object or false if it doesn't.

 var obj = {foo: 'one', bar: 'two'}; function isKeyInObject(obj, key) { var res = Object.keys(obj).some(v => v == key); console.log(res); } isKeyInObject(obj, 'foo'); isKeyInObject(obj, 'something');

One-line example.

 console.log(Object.keys({foo: 'one', bar: 'two'}).some(v => v == 'foo'));

Answer 12

Optional chaining operator :

 const invoice = {customer: {address: {city: "foo"}}} console.log( invoice?.customer?.address?.city ) console.log( invoice?.customer?.address?.street ) console.log( invoice?.xyz?.address?.city )

See supported browsers list

---

For those which have lodash included in their project:
There is a lodash _.get method which tries to get "deep" keys:

Gets the value at path of object. If the resolved value is undefined, the defaultValue is returned in its place.

 var object = { 'a': [{ 'b': { 'c': 3 } }] }; console.log( _.get(object, 'a[0].b.c'), // => 3 _.get(object, ['a', '0', 'b', 'c']), // => 3 _.get(object, 'abc'), // => undefined _.get(object, 'abc', 'default') // => 'default' )

 <script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.4/lodash.min.js"></script>

---

This will effectively check if that key, however deep , is defined and will not throw an error which might harm the flow of your program if that key is not defined.

Answer 13

The easiest way to check is

"key" in object

for example:

var obj = {
  a: 1,
  b: 2,
}
"a" in obj // true
"c" in obj // false

Return value as true implies that key exists in the object.

Answer 14

We can use - hasOwnProperty.call(obj, key);

The underscore.js way -

if(_.has(this.options, 'login')){
  //key 'login' exists in this.options 
}

_.has = function(obj, key) {
  return hasOwnProperty.call(obj, key);
};

Answer 15

If you want to check for any key at any depth on an object and account for falsey values consider this line for a utility function:

var keyExistsOn = (o, k) => k.split(".").reduce((a, c) => a.hasOwnProperty(c) ? a[c] || 1 : false, Object.assign({}, o)) === false ? false : true;

Results

var obj = {
    test: "",
    locals: {
        test: "",
        test2: false,
        test3: NaN,
        test4: 0,
        test5: undefined,
        auth: {
            user: "hw"
        }
    }
}

keyExistsOn(obj, "")
> false
keyExistsOn(obj, "locals.test")
> true
keyExistsOn(obj, "locals.test2")
> true
keyExistsOn(obj, "locals.test3")
> true
keyExistsOn(obj, "locals.test4")
> true
keyExistsOn(obj, "locals.test5")
> true
keyExistsOn(obj, "sdsdf")
false
keyExistsOn(obj, "sdsdf.rtsd")
false
keyExistsOn(obj, "sdsdf.234d")
false
keyExistsOn(obj, "2134.sdsdf.234d")
false
keyExistsOn(obj, "locals")
true
keyExistsOn(obj, "locals.")
false
keyExistsOn(obj, "locals.auth")
true
keyExistsOn(obj, "locals.autht")
false
keyExistsOn(obj, "locals.auth.")
false
keyExistsOn(obj, "locals.auth.user")
true
keyExistsOn(obj, "locals.auth.userr")
false
keyExistsOn(obj, "locals.auth.user.")
false
keyExistsOn(obj, "locals.auth.user")
true

Also see this NPM package: https://www.npmjs.com/package/has-deep-value

Answer 16

While this doesn't necessarily check if a key exists, it does check for the truthiness of a value. Which undefined and null fall under.

Boolean(obj.foo)

This solution works best for me because I use typescript, and using strings like so 'foo' in obj or obj.hasOwnProperty('foo') to check whether a key exists or not does not provide me with intellisense.

Answer 17

Optional Chaining ( ?. ) operator can also be used for this

Source: MDN/Operators/Optional_chaining

 const adventurer = { name: 'Alice', cat: { name: 'Dinah' } } console.log(adventurer.dog?.name) // undefined console.log(adventurer.cat?.name) // Dinah

Answer 18

const object1 = {
  a: 'something',
  b: 'something',
  c: 'something'
};

const key = 's';

// Object.keys(object1) will return array of the object keys ['a', 'b', 'c']

Object.keys(object1).indexOf(key) === -1 ? 'the key is not there' : 'yep the key is exist';

Answer 19

In 'array' world we can look on indexes as some kind of keys. What is surprising the in operator (which is good choice for object) also works with arrays. The returned value for non-existed key is undefined

 let arr = ["a","b","c"]; // we have indexes: 0,1,2 delete arr[1]; // set 'empty' at index 1 arr.pop(); // remove last item console.log(0 in arr, arr[0]); console.log(1 in arr, arr[1]); console.log(2 in arr, arr[2]);

Answer 20

yourArray.indexOf(yourArrayKeyName) > -1

fruit = ['apple', 'grapes', 'banana']

fruit.indexOf('apple') > -1

true

---

fruit = ['apple', 'grapes', 'banana']

fruit.indexOf('apple1') > -1

false

Answer 21

Worth noting that since the introduction of ES11 you can use the nullish coalescing operator, which simplifies things a lot:

const obj = {foo: 'one', bar: 'two'};

const result = obj.foo ?? "Not found";

The code above will return "Not found" for any "falsy" values in foo. Otherwise it will return obj.foo.

See Combining with the nullish coalescing operator

Answer 22

These example can demonstrate the differences between defferent ways. Hope it will help you to pick the right one for your needs:

// Lets create object `a` using create function `A`
function A(){};
A.prototype.onProtDef=2;
A.prototype.onProtUndef=undefined;
var a=new A();
a.ownProp = 3;
a.ownPropUndef = undefined;

// Let's try different methods:

a.onProtDef; // 2
a.onProtUndef; // undefined
a.ownProp; // 3
a.ownPropUndef; // undefined
a.whatEver; // undefined
a.valueOf; // ƒ valueOf() { [native code] }

a.hasOwnProperty('onProtDef'); // false
a.hasOwnProperty('onProtUndef'); // false
a.hasOwnProperty('ownProp'); // true
a.hasOwnProperty('ownPropUndef'); // true
a.hasOwnProperty('whatEver'); // false
a.hasOwnProperty('valueOf'); // false

'onProtDef' in a; // true
'onProtUndef' in a; // true
'ownProp' in a; // true
'ownPropUndef' in a; // true
'whatEver' in a; // false
'valueOf' in a; // true (on the prototype chain - Object.valueOf)

Object.keys(a); // ["ownProp", "ownPropUndef"]

Answer 23

In my case, I wanted to check an NLP metadata returned by LUIS which is an object. I wanted to check if a key which is a string "FinancialRiskIntent" exists as a key inside that metadata object.

1. I tried to target the nested object I needed to check -> data.meta.prediction.intents (for my own purposes only, yours could be any object)
2. I used below code to check if the key exists:

 const hasKey = 'FinancialRiskIntent' in data.meta.prediction.intents; if(hasKey) { console.log('The key exists.'); } else { console.log('The key does not exist.'); }

This is checking for a specific key which I was initially looking for.

Hope this bit helps someone.

Answer 24

2021 - Using the new method Object.hasOwn() as a replacement for Object.hasOwnProperty()

Object.hasOwn() is intended as a replacement for Object.hasOwnProperty() and is a new method available to use (yet still not fully supported by all browsers like safari yet but soon will be)

Object.hasOwn() is a static method which returns true if the specified object has the specified property as its own property. If the property is inherited, or does not exist, the method returns false.

 const person = { name: 'dan' }; console.log(Object.hasOwn(person, 'name'));// true console.log(Object.hasOwn(person, 'age'));// false const person2 = Object.create({gender: 'male'}); console.log(Object.hasOwn(person2, 'gender'));// false

What is the motivation to use it over Object.prototype.hasOwnProperty()? - It is recommended to this method use over the Object.hasOwnProperty() because it also works for objects created by using Object.create(null) and for objects that have overridden the inherited hasOwnProperty() method. Although it's possible to solve these kind of problems by calling Object.prototype.hasOwnProperty() on an external object, Object.hasOwn() overcome these problems, hence is preferred (see examples below)

 let person = { hasOwnProperty: function() { return false; }, age: 35 }; if (Object.hasOwn(person, 'age')) { console.log(person.age); // true - the remplementation of hasOwnProperty() did not affect the Object }

 let person = Object.create(null); person.age = 35; if (Object.hasOwn(person, 'age')) { console.log(person.age); // true - works regardless of how the object was created }

More about Object.hasOwn can be found here : https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwn

Browser compatibility - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwn#browser_compatibility

Answer 25

An alternate approach using "Reflect"

As per MDN

Reflect is a built-in object that provides methods for interceptable JavaScript operations.

The static Reflect.has() method works like the in operator as a function.

 var obj = { a: undefined, b: 1, c: "hello world" } console.log(Reflect.has(obj, 'a')) console.log(Reflect.has(obj, 'b')) console.log(Reflect.has(obj, 'c')) console.log(Reflect.has(obj, 'd'))

Should I use it ?

Depends.

Reflect.has() is slower than the other methods mentioned on the accepted answer (as per my benchmark test). But, if you are using it only a few times in your code, I don't see much issues with this approach.

Answer 26

JS Double Exclamation !! sign may help in this case.

const cars = {
        petrol:{
            price: 5000
        },
        gas:{
            price:8000
        }
    }

Suppose we have the object above and If you try to log car with petrol price.

=> console.log(cars.petrol.price);
=> 5000

You'll definitely get 5000 out of it. But what if you try to get an electric car which does not exist then you'll get undefine

=> console.log(cars.electric);
=> undefine

But using !! which is its short way to cast a variable to be a Boolean (true or false) value.

=> console.log(!!cars.electric);
=> false

Answer 27

To find if a key exists in an object, use

Object.keys(obj).includes(key)

The ES7 includes method checks if an Array includes an item or not, & is a simpler alternative to indexOf .

Answer 28

New awesome solution with JavaScript Destructuring :

let obj = {
    "key1": "value1",
    "key2": "value2",
    "key3": "value3",
};

let {key1, key2, key3, key4} = obj;

// key1 = "value1"
// key2 = "value2"
// key3 = "value3"
// key4 = undefined

// Can easily use `if` here on key4
if(!key4) { console.log("key not present"); } // Key not present

Do check other use of JavaScript Destructuring

Answer 29

A fast and easy solution is to convert your object to json then you will be able to do this easy task:

const allowed = {
    '/login' : '',
    '/register': '',
    '/resetpsw': ''
};
console.log('/login' in allowed); //returns true

If you use an array the object key will be converted to integers ex 0,1,2,3 etc. therefore, it will always be false