Why am I getting "this.combinedGrant is not a function" when synthesizing lambda functions with AWS CDK?

Question

I'm using AWS CDK to create my cloud stacks, I've identified that the error "this.combinedGrant is not a function" is being produced by the following code but I can't find any references to it:

    let authFunctions = [
      {
        name: 'user-registration',
        handler: 'index.register',
        code: './handlers/user-registration',
        loginAccess: loginTable.grantFullAccess,
        otpAccess: otpTable.grantWriteData
      },

...

    for (let i in authFunctions) {
      let definition = authFunctions[i];
      let func = new Function(this, `${definition.name}-function`, {
        runtime: Runtime.NODEJS_12_X,
        handler: definition.handler,
        code: Code.asset(definition.code),
        environment: {
          LOGIN_TABLE_NAME: loginTable.tableName,
          OTP_TABLE_NAME: otpTable.tableName,
          MAILGUN_DOMAIN: credentials.mailgun.domain,
          MAILGUN_FROM: credentials.mailgun.from,
          MAILGUN_API_KEY: credentials.mailgun.api_key,
          JWT_SECRET: definition.hasJWTSecret ? "secret-placeholder" : ""
        },
        layers: [authLayer, utilityLayer],
        timeout: Duration.seconds(5)
      });

      definition.loginAccess(func);
      definition.otpAccess(func);

      let api = new RestApi(this, `${definition.name}-api`);
      api.root.addMethod('POST', new LambdaIntegration(func));
    }

Answer 1

The problem was in attempting to assign loginTable.grantFullAccess functions directly. The solution was to replace with a string and then use switch cases to apply them:

    let authFunctions = [
      {
        name: 'user-registration',
        handler: 'index.register',
        code: './handlers/user-registration',
        loginAccess: 'full',
        otpAccess: 'write'
      },

...

      switch (definition.loginAccess) {
        case 'full':
          loginTable.grantFullAccess(func);
          break;
        case 'read':
          loginTable.grantReadData(func);
          break;
      }