Creates an amplify subdomain with aws javascript sdk

amplify-create-subdomain.js

const { to } = require("await-to-js");
const { Amplify } = require("aws-sdk");
const amplify = new Amplify();

const config = {
  AMPLIFY_FRONTEND_APP_ID: "your_app_id",
  AMPLIFY_FRONTEND_DOMAIN: "your_domain.com",
  AMPLIFY_FRONTEND_BRANCH: "your_branch_env"
};

const createSubdomain = async (prefix) => {
  const paramsGet = {
    appId: config.AMPLIFY_FRONTEND_APP_ID, /* required */
    domainName: config.AMPLIFY_FRONTEND_DOMAIN /* required */
  };
  const [errorGet, domainAssociation] = await to(amplify.getDomainAssociation(paramsGet).promise());
  if (errorGet) return [errorGet.message];

  const params = {
    appId: config.AMPLIFY_FRONTEND_APP_ID, /* required */
    domainName: config.AMPLIFY_FRONTEND_DOMAIN, /* required */
    subDomainSettings: [ /* required */
      ...domainAssociation.domainAssociation.subDomains.map(subdmain => ({
        ...subdmain.subDomainSetting
      })),
      {
        branchName: config.AMPLIFY_FRONTEND_BRANCH, /* required */
        prefix /* required */
      }
    ]
  };

  return to(amplify.updateDomainAssociation(params).promise());
}

policy.json

{
  "Effect": "Allow",
  "Action": [
    "amplify:GetDomainAssociation",
    "amplify:updateDomainAssociation"
  ],
  "Resource": {
    "Fn::Sub": "arn:aws:amplify:${AWS::Region}:${AWS::AccountId}:apps/${yourAmplifyFrontendAppIdParameter}/domains/${yourAmplifyFrontendDomainParameter}"
  }
}