It will return undefined.

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

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

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

Final Advise

Avoid objects with undefined values. Check directly whenever possible. Otherwise, use in operator or hasOwnProperty method.

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