Why do we need map function in jq?

June 30, 2022

I am just playing with jq. Please check the following outputs.

% FRUITS='[
  {
    "name": "apple",
    "color": "green",
    "price": 1.2
  },
  {
    "name": "banana",
    "color": "yellow",
    "price": 0.5
  },
  {
    "name": "kiwi",
    "color": "green",
    "price": 1.25
  }
]'
% echo $FRUITS
% echo $FRUITS | jq .
% echo $FRUITS | jq '[.[].color] | unique'
[
  "green",
  "yellow"
]

% echo $FRUITS | jq 'map(.color) | unique'
[
  "green",
  "yellow"
]

% echo $FRUITS | jq '. | map(has("name"))'
[
  true,
  true,
  true
]
% echo $FRUITS | jq '[.[] | has("name")]'
[
  true,
  true,
  true
]

Here, jq '[.[].color] | unique' give the same output as jq 'map(.color) | unique'

jq '[.[] | has("name")]' give the same output as jq '. | map(has("name"))'

I am not understanding what is the purpose and difference of the map function.

>Solution :

The map function leaves the array intact while .[] splits the array. Often times they return the same result but in specific use cases they will perform differently.

More info can be found in the answers to this question:
JQ map objects to array