browserify and watchify

gulpfile-basic.js

gulp.task("build", function(){
   //building your app. 
});
gulp.task("dev", function(){
   gulp.watch("./src/app.js", ['build']); 
});

gulpfile.js

var gulp       = require('gulp');
var browserify = require('browserify');
var watchify   = require('watchify');
var source 		= require("vinyl-source-stream");

gulp.task("dev", function(){
   //create a browserify instance at first
   var b = browserify({
   	   entries: ["./src/app.js"],   //entry point
   		transform: [],
   		cache: {},
   		debug: true,
   		packageCache: {},
   		fullPaths: true,
   });
   
   //create a watchify instance by a browserify instance   
   b = watchify(b);
	//register an update event for watchify
   b.on('update', function(){
	build(b);
   });
   build(b);
})

function build(b){
   b.bundle()
	.pipe(source("bundle.js"))
	.pipe(gulp.dest("./build"));
}