Add Element to Array if It Does Not Already Exist with jq

This is how I use jq to conditionally add an element to an array only if it doesn’t exist there already. In other words, add an element to an array only if it doesn’t create a duplicate in the array. This is particularly helpful when I’m scripting a JSON transformation and want to make it idempotent.

echo '{"values": ["a", "b"]}' | \
jq --arg newvalue 'c' \
  'if .values | index($newvalue)
    then .
    else .values += [$newvalue]
  end'

How It Works

Output Object Unchanged

echo '{"values": ["a", "b"]}' | jq '.'

Add Value to Array inside Object

echo '{"values": ["a", "b"]}' | jq --arg newvalue 'c' '.values += [$newvalue]'

Duplicate

If we add a value that is already in the array, we end up having it twice.

echo '{"values": ["a", "b"]}' | jq --arg newvalue 'a' '.values += [$newvalue]'

Check If Already Exists

Conditional Check If Value Already Appears

echo '{"values": ["a", "b"]}' | jq --arg newvalue 'c' '.values | index($newvalue)'

null

echo '{"values": ["a", "b"]}' | jq --arg newvalue 'a' '.values | index($newvalue)'

0

Note: in jq the only falsy values are false and null (0 is NOT a falsy value)

echo '{"values": ["a", "b"]}' | jq --arg newvalue 'c' 'if .values | index($newvalue) then "exists" else "does not exist" end'

“does not exist”

echo '{"values": ["a", "b"]}' | jq --arg newvalue 'a' 'if .values | index($newvalue) then "exists" else "does not exist" end'

“exists”

Conditionally Add Value to Array

echo '{"values": ["a", "b"]}' | jq --arg newvalue 'a' 'if .values | index($newvalue) then . else .values += [$newvalue] end'

(unchanged)

echo '{"values": ["a", "b"]}' | jq --arg newvalue 'c' 'if .values | index($newvalue) then . else .values += [$newvalue] end'

{
  "values": [
    "a",
    "b",
    "c"
  ]
}

With Indentation

echo '{"values": ["a", "b"]}' | \
jq --arg newvalue 'c' \
  'if .values | index($newvalue)
    then .
    else .values += [$newvalue]
  end'

Sal Ferrarello (@salcode)

Sal is a PHP and JavaScript developer with a focus on the WordPress platform. He is a conference speaker with a background including Piano Player, Radio DJ, Magician/Juggler, Beach Photographer, and High School Math Teacher. Sal can be found professionally at WebDevStudios, where he works as a principal engineer.