check if key in dict javascript code example

Example 1: javascript does key exist

var person={"name":"Billy","age":20}
person.hasOwnProperty("name"); // true
person.hasOwnProperty("sex"); // false

Example 2: js dictionary contains key

"key" in obj // true, regardless of the actual value

Example 3: how to check if a key exists in an object javascript

!("key" in obj) // true if "key" doesn't exist in object
!"key" in obj   // ERROR!  Equivalent to "false in obj"

Example 4: check if a key exists in an object javascript

"key" in obj // true, regardless of the actual value

Example 5: js check if object key exists

var obj = { key: undefined };
obj["key"] !== undefined // false, but the key exists!