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Json – jq: assigning to computed property of an object

I’m trying to get jq to generate an object whose keys are computed.

This didn’t work:

$ jq -n --arg key foo '{} as $o | $o[$key] = 42'
jq: error (at <unknown>): Invalid path expression near attempt to access element "foo" of {}

This didn’t, either:

$ jq -n --arg key foo '{} as $o | ($o | .[$key]) = 42'
jq: error (at <unknown>): Invalid path expression near attempt to access element "foo" of {}

But this worked:

$ jq -n --arg key foo '{} as $o | ($o | setpath([$key]; 42))'
{
  "foo": 42
}

So, my question is, is there an assignment syntax that can produce the same effect?

Tags:

3

Answers


  1. 0

    The left hand side of the equals sign must be a value in .. Make $o . and then do the assignment like so:

    $o | .[$key] = 42
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  2. 0

    If you want to change the context, first evaluate the variable into it, then update the context:

    jq -n --arg key foo '{} as $o | $o | .[$key] = 42'

    Demo

    If you want to update the contents of a variable based on its old value, shadow it with a new one of the same name using the old one’s reference in its body:

    jq -n --arg key foo '{} as $o | ($o | .[$key] = 42) as $o | …'

    Demo

    Note: for the latter one, … cannot be empty.

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  3. 0

    Variables in jq are immutable, which is why your first two statments give errors.

    The third statement succeeds because what changed is the current value of the pipe, not $o, you can see it with :

    $ jq -n --arg key foo '{} as $o | ($o | setpath([$key]; 42)) | $o, .'
{}
{
  "foo": 42
}

    You can simulate mutable variables with :

    $ jq -n --arg key foo '.o = {} | .o[$key] = 42 | .o'
{
  "foo": 42
}
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