Brackets in JQ for .key

Question

I do not understand why the key must be in ()for this:

# kbrandt at glade.local in ~ on git:master x [15:08:19]
$ cat host | jq '. | to_entries | map({ (.key) : .value.CPU.PercentUsed })' | tail
  {
    "rpi-b827eb2d7d23": 10.333333333333334
  },
  {
    "rpi-b827eb8d7c8d": 60
  },
  {
    "rpi-b827eba999fa": 40.733333333333334
  }
]

# kbrandt at glade.local in ~ on git:master x [15:08:54]
$ cat host | jq '. | to_entries | map({ .key : .value.CPU.PercentUsed })' | tail
jq: error: syntax error, unexpected FIELD (Unix shell quoting issues?) at <top-level>, line 1:
. | to_entries | map({ .key : .value.CPU.PercentUsed })
jq: 1 compile error

Answer 1

When defining an object literal, parentheses indicate that the value of the expression must be the name of the property. Otherwise, if you did not use parentheses, this is the literal name.

Thus, these are equivalent ways of defining an object with a property "foo":

{ foo: 1 }
{ "foo": 2 }
"foo" as $name | { ($name): 3 }
{ somename: "foo" } | { (.somename): 4 }