This is operation that returns true if the key exists in a map (or index in an array), false otherwise.

Has map key

Given a sample.yml file of:

- a: yes
- a: ~
- a:
- b: nope

then

yq eval '.[] | has("a")' sample.yml

will output

true
true
true
false

Has array index

Given a sample.yml file of:

- []
- [1]
- [1, 2]
- [1, null]
- [1, 2, 3]

then

yq eval '.[] | has(1)' sample.yml

will output

false
false
true
true
true