Example of merging legacy JavaScript and new TypeScript in the same build process

gulpfile.js

//
// Written in-gist, so there could be typos.
// The point of this is to show merging two gulp streams with event-stream.
//

var gulp = require('gulp');
var ts = require('gulp-typescript');
var eventStream = require('event-stream');
var concat = require('gulp-concat');

var paths = {
  in: {
    js: 'src/**/*.js'
  },
  out: {
    js: 'dist/js'
  }
};

var tsProj = ts.createProject('tsconfig.json');

gulp.task('default', ['build:script'], function () {
});

gulp.task('build:script', function () {
  
  // store pipeable gulp stream of gulp-typescript JS output
  var tsStream = tsProj.src()
    .pipe(ts(tsProj)).js;
    
  // store pipeable gulp stream of javascript glob
  var jsStream = gulp.src(paths.in.js);
  
  // merge streams and perform normal build steps
  return eventStream.merge(tsStream, jsStream)
    .pipe(concat('app.js'))
    .pipe(gulp.dest(paths.out.js));
});