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 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 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 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 a 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[].

Final Advise

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.

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
}

"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

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

keyExists('apple', ['apple', 'banana', 'orange']); // true
keyExists('fruit', ['apple', 'banana', 'orange']); // false

Searching for keys in Objects

keyExists('age', {'name': 'Bill', 'age': 29 }); // true
keyExists('title', {'name': 'Jason', 'age': 29 }); // false

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

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.

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);
};

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