Dataweave "contains" returns false with "keysOf" output even though the key is contained

I’m facing towards a misbehavior of the function keysOf combined with the method contains.

Given an object like:

    {
     "keyA":1,
     "keyB":2
    }

And given the Dataweave script like:

keysOf(payload) contains "keyA"

I’m expecting to have true as outcome, but it returns false.

The keysOf function returns an array containing the keys of the previously mentioned object, like so:

[
 "keyA",
 "keyB"
]

Which, if used in the following script, it returns the true expected outcome:

[ "keyA", "keyB" ]  contains "keyA"

The workaround I tried to implement, storing the keysOf outcome into a variable, doesn’t work either:

%dw 2.0
output application/json
var keysOfPayload=keysOf(payload)
---
keysOfPayload contains "keyA"

The goal of this question, is not to find a workaround, for instance using the question mark selector against the key, works as expected:

%dw 2.0
output application/json
var object= {"keyA":1,"KeyB":2}
---
object.keyA?

Rather, I’m willing to understand why the combination of keysOf and contains is not effective as expected.

>Solution :

The keysOf() works exactly as expected. The problems is a common misconception of how it works. It returns an array of the keys with the Key type. When you try to compare it to a String the comparison returns false correctly.

To make this work you need to do one of two things:

1. Recommended use the functions namesOf() instead: namesOf(a) contains "keyA". Note that namesOf() converts the keys to strings and returns an array of Strings.
2. Convert the String to a Key explicitly before using: keysOf(a) contains ("keyA" as Key)