AWS API Gateway Mock Integration that responds with the JSON: `{"message": "Hello world!"}` to all requests for JSON or missing the Content-Type header.

stacks.ts

import { Stack, StackProps } from 'aws-cdk-lib';
import * as apigateway from 'aws-cdk-lib/aws-apigateway';
import { Construct } from 'constructs';

export class MyStack extends Stack {
  constructor(scope: Construct, id: string, props?: StackProps) {
    super(scope, id, props);

    const mockIntegration = new apigateway.MockIntegration({
      passthroughBehavior: apigateway.PassthroughBehavior.WHEN_NO_TEMPLATES,
      requestTemplates: {
        'application/json': JSON.stringify({ statusCode: 200 }),
      },
      integrationResponses: [
        {
          statusCode: '200',
          responseTemplates: {
            'application/json': JSON.stringify({ message: 'Hello world!' }),
          },
        },
      ],
    });

    const restApi = new apigateway.RestApi(this, 'RestApi', {});
    const method = restApi.root.addMethod('ANY', mockIntegration, {
      methodResponses: [{ statusCode: '200' }],
    });
  }
}