[javascript] Checking if a key exists in a JavaScript object?

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?

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 } }] };_x000D_
_x000D_
console.log(_x000D_
  _.get(object, 'a[0].b.c'),           // => 3_x000D_
  _.get(object, ['a', '0', 'b', 'c']), // => 3_x000D_
  _.get(object, 'a.b.c'),              // => undefined _x000D_
  _.get(object, 'a.b.c', 'default')    // => 'default'_x000D_
)

<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.

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);
};

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"]

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?

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';

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

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

yourArray.indexOf(yourArrayKeyName) > -1

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

fruit.indexOf('apple') > -1

true

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

fruit.indexOf('apple1') > -1

false

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_x000D_
delete arr[1];           // set 'empty' at index 1_x000D_
arr.pop();               // remove last item_x000D_
_x000D_
console.log(0 in arr,  arr[0]);_x000D_
console.log(1 in arr,  arr[1]);_x000D_
console.log(2 in arr,  arr[2]);

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

Optional Chaining operator can also be used for this

 ?.

sorce: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Optional_chaining

const adventurer = {
  name: 'Alice',
  cat: {
    name: 'Dinah'
  }
};

const dogName = adventurer.dog?.name;
console.log(dogName);
// expected output: undefined

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

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

.

var result = "Impression" in array;

The result was

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

var result = array.hasOwnProperty("Impression")

The result was

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

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

The result was

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

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.

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

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.

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'};_x000D_
    _x000D_
function isKeyInObject(obj, key) {_x000D_
    var res = Object.keys(obj).some(v => v == key);_x000D_
    console.log(res);_x000D_
}_x000D_
_x000D_
isKeyInObject(obj, 'foo');_x000D_
isKeyInObject(obj, 'something');

One-line example.

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

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

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.

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.

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) {_x000D_
        if (!search || (search.constructor !== Array && search.constructor !== Object)) {_x000D_
            return false;_x000D_
        }_x000D_
        for (var i = 0; i < search.length; i++) {_x000D_
            if (search[i] === key) {_x000D_
                return true;_x000D_
            }_x000D_
        }_x000D_
        return key in search;_x000D_
    }_x000D_
_x000D_
// How to use it:_x000D_
// Searching for keys in Arrays_x000D_
console.log(keyExists('apple', ['apple', 'banana', 'orange'])); // true_x000D_
console.log(keyExists('fruit', ['apple', 'banana', 'orange'])); // false_x000D_
_x000D_
// Searching for keys in Objects_x000D_
console.log(keyExists('age', {'name': 'Bill', 'age': 29 })); // true_x000D_
console.log(keyExists('title', {'name': 'Jason', 'age': 29 })); // false

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

"key" in obj

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

It will return undefined.

var aa = {hello: "world"};_x000D_
alert( aa["hello"] );      // popup box with "world"_x000D_
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
}