jq - how to use variable in an expression - jq

Question

As in Passing bash variable to jq, we should be able to use a JQ variable as $VAR in a jq expression.
projectID=$(jq -r --arg EMAILID "$EMAILID" '
.resource[]
| select(.username==$EMAILID)
| .id' file.json
)
SO to extract project_id from the json file sample.json.
{
"dev": {
"gcp": {
"project_id": "forecast-dev-1234",
"project_number": "123456789",
"endpoint_id": "6837352639743655936"
}
}
}
Run the JQ expression using a variable but did not work.
# TARGET=dev
$ jq -r --arg TARGET "${TARGET}" '.$TARGET.gcp.project_id' sample.json
-----
jq: error: syntax error, unexpected '
.$TARGET.gcp.project_id
(Unix shell quoting issues?) at <top-level>, line 1:
.$TARGET.gcp.project_id
jq: error: try .["field"] instead of .field for unusually named fields at <top-level>, line 1:
.$TARGET.gcp.project_id
jq: 2 compile errors
Please help understand why and how to use the variable to form an expression to extract project_id.
JQ Manual does not provide clear explanation for variable and ---arg. Is there a good resource that clearly explain JQ variable and how to use it?
Another way to set the exit status is with the halt_error builtin function.
--arg name value:
This option passes a value to the jq program as a predefined variable. If you run jq with --arg foo bar, then $foo is available in the program and has the value "bar". Note that value will be treated as a string, so --arg foo 123 will bind $foo to "123".
Workaround
Using interpolation.
$ TARGET=dev
$ jq -r --arg TARGET "${TARGET}" '."\($TARGET)".gcp.project_id' sample_interpolation.json
-----
forecast-dev-1234
Version
jq --version
---
jq-1.64l

Answer 1

You're using variables fine, the problem is that object-identifier syntax doesn't allow general expressions. It's a shorthand syntax for when the key you're looking up is a fixed identifier-like string, like .foo or .project_id. As noted in the manual, you can use the more general generic object index filter for arbitrary keys including those that are calculated by some expression, such as .[$TARGET]:
$ TARGET=dev
$ jq -r --arg TARGET "${TARGET}" '.[$TARGET].gcp.project_id' sample.json
forecast-dev-1234