How to update value in json object with key as argument (using `jq`)

Using jq I want to update a value in a json object.
The key of the value to be updated is passed as an argument.

Input

{
  "a": 1,
  "b": 2,
  "c": 3
}

Expected output with argument: key=b

{
  "a": 1,
  "b": 42,
  "c": 3
}

I tried to do:

jq --arg key b '.($key) |= . + 40'
jq --arg key b '".\($key)" |= . + 40'

Both attempts result in errors:

• syntax error, unexpected '('
• Invalid path expression with result ".b"

How can I solve this problem?

>Solution :

A key needs an . in front of it, then since it’s in a variable, use [].

jq --arg key b '.[$key] |= (. * 999)' input

Gives:

{
  "a": 1,
  "b": 1998,
  "c": 3
}

You can also replace |= . * with the *= operator:

jq --arg key b '.[$key] *= 999' input

Your first attempt didn’t work since you need [] instead off ()

The second one failed because wrapping it in a string literal will create a string .b, that’s not something you can update using |=.