Checking if an associative array key exists

[Marook]

Actually, 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?

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var obj = { key: undefined };
obj["key"] != undefined // false, but the key exists!
You should instead use the in operator:

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"key" in obj // true, regardless of the actual value
If you want to check if a key doesn't exist, remember to use parenthesis:

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!("key" in obj) // true if "key" doesn't exist in object
!"key" in obj   // ERROR!  Equivalent to "false in obj"
Or, if you want to particularly test for properties of the object instance (and not inherited properties), use hasOwnProperty:

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obj.hasOwnProperty("key") // true

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

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

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