How do I use a json file as a DashboardBody in CloudFormation with Serverless?

Question

To create a CloudWatch Dashboard in CloudFormation, you must supply the dashboard's source code as a stringified JSON rather than as a separate JSON structure.

This is annoying because my JSON has to be escaped inside of a string literal within my serverless.yml:

...
resources:
  Resources:
    MyDashboard:
      Type: AWS::CloudWatch::Dashboard
      Properties:
      DashboardName: My-Dashboard
      DashboardBody: "{\"widgets\": [...]}"

I tried using a file reference:

...
resources:
  Resources:
    MyDashboard:
      Type: AWS::CloudWatch::Dashboard
      Properties:
      DashboardName: My-Dashboard
      DashboardBody: ${file(my-dashboard.json)}

but serverless inserts the content as part of the YAML structure, rather than as part of the JSON string:

...
resources:
  Resources:
    MyDashboard:
      Type: AWS::CloudWatch::Dashboard
      Properties:
      DashboardName: My-Dashboard
      DashboardBody:
        Widgets:
          - ...
          - ...

Is there a way to stringify the JSON from my-dashboard.json?

Answer 1

There doesn't appear to be a way stringify JSON directly within a serverless variable. However, you can reference an external .js file, and then stringify the .json file there:

serverless.yml:

...
resources:
  Resources:
    MyDashboard:
      Type: AWS::CloudWatch::Dashboard
      Properties:
      DashboardName: My-Dashboard
      DashboardBody: ${file(my-dashboard-body.js):myDashboardBody}

${file(my-dashboard-body.js):myDashboardBody} is a serverless variable reference. It means that we want to use the value from the myDashboardBody module of the my-dashboard-body.js file.

my-dashboard-body.js:

module.exports.myDashboardBody = (serverless) => {
 const fsPromises = require('fs').promises
 return fsPromises.readFile('my-dashboard-body.json', 'utf-8')
};

my-dashboard-body.json:

{
  "widgets": [
    ...
  ]
}