How to read values of a key from a json file in a helm template

I am creating a github actions step

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.
.
steps:
  name: export-secrets
  run: |
    echo '${{ toJSON(secrets) }}' >> gh-secrets.json
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.
.

suppose the gh-secrets.json file is as below:

# gh-secrets.json
{"A": "a1", "B": "b1", "C": "c1"}

Then I am going to pull the helm charts and untar them and then move gh-secrets.json into the untar directory. The reason I am doing this is according to helm docs, "Files outside of a helm application subchart, including those of the parent, cannot be accessed"

suppose I have these files in the untar directory

# values.yaml
secrets:
  - A
  - B

and I have templates/secrets.yaml:

apiVersion: v1
kind: Secret
metadata:
  name: {{ .Release.Name }}-secret
data:
    {{- range .Values.secrtes }}
      {{ . }}: {{ $.Files.Get "gh-secrets.json" | toJson }}
    {{- end }}

this is the result:

# Source: mktplc-catalog/templates/secret.yaml
apiVersion: v1
kind: Secret
metadata:
  name: release-name-secret
data:
      A: "'{\"A\": \"a1\", \"B\": \"b1\", \"C\": \"c1\"}'"
      B: "'{\"A\": \"a1\", \"B\": \"b1\", \"C\": \"c1\"}'"

but I want it to be

# Source: mktplc-catalog/templates/secret.yaml
apiVersion: v1
kind: Secret
metadata:
  name: release-name-secret
data:
      A: "a1"
      B: "b1"

How can I achieve this?

Helm includes a fromJson function; in fact, its documentation fairly closely matches your updated setup. So you can use this to read in the file, parse it, and then select the specific key out of it.

data:
{{- $secrets := .Files.Get "gh-secrets.json" | fromJson -}}
{{- range .Values.secrets }}
  {{ . }}: {{ index $secrets . }}
{{- end }}

You appear to have some control over the input structure here. If you can wrap the secret values in some top-level key

echo '{"secretValues": ${{ toJSON(secrets) }}}' > gh-secrets.json

then you can provide this as an extra values file to Helm (valid JSON files are also valid YAML)

helm install -f gh-secrets.json ...

This file doesn't need to be in the chart directory. Now Helm will parse it for you, and it will be available as .Values.secretValues matching the injected key. (Remember that the range loop resets the value of ., so you will need to save its value outside the loop.)

If this technique works for you, it might be possible to use it to directly inject the secret values without the layer of indirection; use something like

data:
{{ .Values.secretValues | toYaml | indent 2 }}