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:

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

https://jsben.ch/HaHQt

.

Using in operator

var result = "Impression" in array;

The result was

12,931,832 ±0.21% ops/sec      92% slower 

Using hasOwnProperty

var result = array.hasOwnProperty("Impression")

The result was

16,021,758 ±0.45% ops/sec     91% slower

Accessing elements directly (brackets style)

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

The result was

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

Accessing elements directly (object style)

var result = array.Impression  === undefined;

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, 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[].

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.

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, e.g.

// 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
}

1. Checking for properties of the object including inherited properties

Could be determined using the in operator which returns true if the specified property is in the specified object or it's prototype chain, false otherwise

const person = { name: 'dan' };

console.log('name' in person); // true
console.log('age' in person); // false

2. Checking for properties of the object instance (not including inherited properties)

*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 use this method 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 for Object.hasOwn - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwn#browser_compatibility

"key" in obj

Is likely testing only object attribute values that are very different from array keys

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?

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

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:

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.

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.

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

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'));

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, 'a.b.c'),              // => undefined 
  _.get(object, 'a.b.c', '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.

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.

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

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.

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

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 ?

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.

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);
};

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.

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';

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]);

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

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

yourArray.indexOf(yourArrayKeyName) > -1

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

fruit.indexOf('apple') > -1

true

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

fruit.indexOf('apple1') > -1

false

for strict object keys checking:

const object1 = {};
object1.stackoverflow = 51; 
console.log(object1.hasOwnProperty('stackoverflow')); 



output: true 

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.

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"]

const rawObject = {};
rawObject.propertyKey = 'somethingValue';

console.log(rawObject.hasOwnProperty('somethingValue'));
// expected output: true

checking particular key present in given object, hasOwnProperty will works here.

If you have ESLint configured in your project follows ESLint rule no-prototype-builtins. The reason why has been described in the following link:

// bad
console.log(object.hasOwnProperty(key));

// good
console.log(Object.prototype.hasOwnProperty.call(object, key));

// best
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));

I do it as following

const obj = { a: 1, b: 2, c: 3 };

// Using the 'in' operator
console.log('a' in obj); // true
console.log('d' in obj); // false

// Using the 'hasOwnProperty' method
console.log(obj.hasOwnProperty('b')); // true
console.log(obj.hasOwnProperty('d')); // false