Answers for "check if key exists and then grab the data js"

how to check if a key is present in a dictionary in js

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

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"

how to check if a key is present in a dictionary in js

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