How to use AWS Cognito to sign an http request to a custom AWS Api Gateway using IAMs Authorization.

signApiGatewayAngular2HttpRequest.ts

//---- Angular 2 Http Service Example ----
import {Injectable} from "@angular/core";
import {Http, Headers} from "@angular/http";
const AWS = require("aws-sdk");

@Injectable()
export class ApiGatewayService {

  constructor(private http:Http){

    let creds = new AWS.CognitoIdentityCredentials({
      IdentityPoolId: "eu-west-1:xxx-xxxx-xxxxxxxx"
    });

    AWS.config.update({
      region: 'eu-west-1',
      credentials: creds
    });

    AWS.config.credentials.get((err) => {
      if(err) throw err;

      let httpRequest = new AWS.HttpRequest("https://xxxxx.execute-api.eu-west-1.amazonaws.com/v1/my/api", "eu-west-1");
      httpRequest.method = "POST";
      httpRequest.headers.host = "xxxxx.execute-api.eu-west-1.amazonaws.com";
      httpRequest.headers['Content-Type'] = "application/json";
      httpRequest.body = JSON.stringify({
        data: {
          username: 'foo',
          password: 'bar',
          applicationName: 'biz',
        }
      });

      let v4signer = new AWS.Signers.V4(httpRequest, "execute-api", true);
      v4signer.addAuthorization(AWS.config.credentials, AWS.util.date.getDate());

      //Create a headers object from the AWS signed request
      let headers = new Headers();
      for(let key of Object.keys(httpRequest.headers)){
        headers.set(key, httpRequest.headers[key]);
      }
      headers.delete('host'); //Browser throws a warning if attempting to set host header

      this.http.post(httpRequest.endpoint.href, httpRequest.body, {
        method: httpRequest.method,
        search: httpRequest.endpoint.search,
        headers: headers
      }).subscribe((data)=>{
        console.log(data); //Success
      });
    })
  }
}

signApiGatewayHttpRequest.js

//---- Complete code example to sign AWS Http request ----

//Configure AWS
AWS.config.update({
  region: 'eu-west-1',
  //CognitoIdentityCredentials only needed if using Cognito. Use credentials specific to your usecase
  credentials: new AWS.CognitoIdentityCredentials({
    IdentityPoolId: "eu-west-1:xxxx-xxxxx-xxxxxxx"
  });
});

//Create an HttpRequest representation of your request (this wont actually _DO_ the request)
var httpRequest = new AWS.HttpRequest("https://xxxxxxx.execute-api.eu-west-1.amazonaws.com/uri/to/method", "eu-west-1");

//REQUIRED
//Host & content type headers must be set
httpRequest.headers.host = "xxxxxxx.execute-api.eu-west-1.amazonaws.com"; //Host of the API being called
httpRequest.headers['Content-Type'] = "application/json";

//OPTIONAL
httpRequest.method = "GET"; //Default is POST
httpRequest.headers['x-biz'] = 'baz'; //Additional headers
httpRequest.body = JSON.stringify({foo:'bar'}); //Body must be a string if being used

//Instruct cognito credentials to get access, secret and session keys (only needs to be done once on page load)
AWS.config.credentials.get(function(err){
  //Sign the request with the newly fetched credentials. All IAM Authorised API Gateway requests use the V4 signer
  var v4signer = new AWS.Signers.V4(httpRequest, "execute-api", true);
  v4signer.addAuthorization(AWS.config.credentials, AWS.util.date.getDate());
  
  //The httpRequest has now been signed with the credentials. You can pick it apart and use your own AJAX/Fetch library
  //The AWS.HttpRequest itself isn't able to make the request on its own
  console.log(httpRequest.headers) // { authorization: 'AWS4-HMAC-SHA256 Credential=ASIA...', x-amz-security-token:'AgoGb...', ...}
  console.log(httpRequest.method) // GET
  console.log(httpRequest.endpoint.href) // https://xxxxxxx.execute-api.eu-west-1.amazonaws.com/uri/to/method
  console.log(httpRequest.body)
});