Replace null values from a select command in jq with the value of another field

Goal is to search for null values of part_description and insert in part field value.

[
  {
    "part": "brake-01982",
    "part_description": null,
  }
]

Expected Output

[
  {
    "part": "brake-01982",
    "part_description": "brake-01982",
  }
]

Command:
jq ‘(.[] | select(.part_description==null).part_description) |= .part’

Results in no change

What does work is if I try to insert a string value. The double quoted string results in what I would expect. Demo below. How do I pass another fields value into this command vs a quoted string?

Command: jq ‘(.[] | select(.part_description==null).part_description) |= "test"’

Demo

Output

[
  {
    "part": "brake-01982",
    "part_description": "test"
  }
]

>Solution :

The alternative operator // takes on the second value if the first one is null, false, or empty (i.e. missing). Combined with the update operator |=, this can be contracted to //=.

jq '.[] |= (.part_description //= .part)'

[
  {
    "part": "brake-01982",
    "part_description": "brake-01982"
  }
]

Demo