Dynamic/computed keys in JMESPath?

Question

From ES2015 with computed properties and Array.reduce/Array.map/Object.assign you can do:

[{name: 'foo', age: 43}, {name: 'bar', age: 55}].map(
    o => ({[o.name]: o.age})).reduce((a, b) => Object.assign(a,b), {})

…and get:

{ foo: 43, bar: 55 }

How do I get this from JMESPath?

---

Attempt:

$echo '[{"name": "foo", "age": 43}, {"name": "bar", "age": 55}]' | jp [].{name:age}
[
  {
    "name": 43
  },
  {
    "name": 55
  }
]

Answer 1

Problem

• How to construct a Jmespath query that returns objects with arbitrary key-value pairs
• The keys need to be dynamic, based on the output of a jmespath filter expression

Workaround

• As of this writing (2019-03-22), dynamic keys are not available in standard Jmespath
• However, it is possible to return a list of lists instead of a list of objects, and simply post-process that list of lists outside of jmespath

Example

 [*].[@.name,@.age]

Returns

[['foo', 43], ['bar', 55]]

Which can then be post-processed outside of Jmespath, if that is an option for you.

See also

• github issue about this exact use-case

Answer 2

To get this result precisely:

{ "foo": 43, "bar": 55 }

You should use this query:

@.{foo: @[0].age, bar: @[1].age}

But as you can see I don't retrieve the keys foo and bar dynamically because I can't do it in JMESPath.