jq · lesson 27 of 32 · Variables & functions

Your own functions

def name(params): body; defines a reusable filter. A parameter written $n receives a value; a plain name receives a filter.

Example 1A reusable filter

Document

{
  "team": "Platform",
  "office": { "city": "Berlin", "floor": 3 },
  "members": [
    { "name": "Ada", "age": 36, "role": "lead", "active": true,
      "email": "[email protected]", "skills": ["rust", "sql"] },
    { "name": "Linus", "age": 28, "role": "dev", "active": true,
      "email": null, "skills": ["c", "shell"] },
    { "name": "Grace", "age": 45, "role": "dev", "active": false,
      "skills": ["cobol"] },
    { "name": "Alan", "age": 41, "role": "qa", "active": true,
      "email": "[email protected]", "skills": [] }
  ]
}

Query

def older($n): select(.age > $n); .members[] | older(30) | .name

def older($n): Define a filter called older with one value parameter.
select(.age > $n) Its body: reuse select, comparing with the parameter.
; Ends the definition.
older(30) Call it like any built-in filter.

Result

"Ada"
"Grace"
"Alan"

Try it in the playground

Good to know

Parameters without a $ are filters, not values: def twice(f): f | f; then 3 | twice(. + 1) gives 5.