Flatten nested JSON using jq

You can also use the following jq command to flatten nested JSON objects in this manner:

[leaf_paths as $path | {"key": $path | join("."), "value": getpath($path)}] | from_entries

The way it works is: leaf_paths returns a stream of arrays which represent the paths on the given JSON document at which "leaf elements" appear, that is, elements which do not have child elements, such as numbers, strings and booleans. We pipe that stream into objects with key and value properties, where key contains the elements of the path array as a string joined by dots and value contains the element at that path. Finally, we put the entire thing in an array and run from_entries on it, which transforms an array of {key, value} objects into an object containing those key-value pairs.

This is just a variant of Santiago's jq:

. as $in 
| reduce leaf_paths as $path ({};
     . + { ($path | map(tostring) | join(".")): $in | getpath($path) })

It avoids the overhead of the key/value construction and destruction.

(If you have access to a version of jq later than jq 1.5, you can omit the "map(tostring)".)

Two important points about both these jq solutions:

1. Arrays are also flattened. E.g. given {"a": {"b": [0,1,2]}} as input, the output would be:

   {
     "a.b.0": 0,
     "a.b.1": 1,
     "a.b.2": 2
   }
   

2. If any of the keys in the original JSON contain periods, then key collisions are possible; such collisions will generally result in the loss of a value. This would happen, for example, with the following input:

   {"a.b":0, "a": {"b": 1}}
   

Here is a solution that uses tostream, select, join, reduce and setpath

  reduce ( tostream | select(length==2) | .[0] |= [join(".")] ) as [$p,$v] (
     {}
     ; setpath($p; $v)
  )

I've recently written a script called jqg that flattens arbitrarily complex JSON and searches the results using a regex; to simply flatten the JSON, your regex would be '.', which matches everything. Unlike the answers above, the script will handle embedded arrays, false and null values, and can optionally treat empty arrays and objects ([] & {}) as leaf nodes.

$ jq . test/odd-values.json
{
  "one": {
    "start-string": "foo",
    "null-value": null,
    "integer-number": 101
  },
  "two": [
    {
      "two-a": {
        "non-integer-number": 101.75,
        "number-zero": 0
      },
      "true-boolean": true,
      "two-b": {
        "false-boolean": false
      }
    }
  ],
  "three": {
    "empty-string": "",
    "empty-object": {},
    "empty-array": []
  },
  "end-string": "bar"
}

$ jqg . test/odd-values.json
{
  "one.start-string": "foo",
  "one.null-value": null,
  "one.integer-number": 101,
  "two.0.two-a.non-integer-number": 101.75,
  "two.0.two-a.number-zero": 0,
  "two.0.true-boolean": true,
  "two.0.two-b.false-boolean": false,
  "three.empty-string": "",
  "three.empty-object": {},
  "three.empty-array": [],
  "end-string": "bar"
}

jqg was tested using jq 1.6

Note: I am the author of the jqg script.