Gulpfile-fixed.js

gulp.task("concat", function() {
  var stream = gulp.src("./src/*.js")
    .pipe(concat("concat.js"))
    .pipe(gulp.dest("./tmp/"));
  return stream;
});

gulp.task("uglify", ["concat"], function() {
  gulp.src("./tmp/concat.js")
    .pipe(uglify())
    .pipe(gulp.dest("./dist"));
});

Gulpfile.js

var gulp = require("gulp");
var concat = require("gulp-concat");
var uglify = require("gulp-uglify");

gulp.task("concat", function() {
  gulp.src("./src/*.js")
    .pipe(concat("concat.js"))
    .pipe(gulp.dest("./tmp/"));
});

gulp.task("uglify", ["concat"], function() {
  gulp.src("./tmp/concat.js")
    .pipe(uglify())
    .pipe(gulp.dest("./dist"));
});

hmmm.md

The first time I run gulp uglify, it won't create ./dist/concat.js, because ./tmp/concat.js doesn't exist. I presume this is because the concat task is async, and hence there's no guarantee it's finished before uglify gets run.

You can fix this as shown in Gulpfile-fixed.js, presumably because when you return a stream gulp checks to see if it's finished before calling Uglify?

@jackfranklin You need to tell gulp somehow that the task is asynchronous and callback when it is done. There are a few ways you can do this. https://github.com/wearefractal/gulp#gulptaskname-deps-fn

I recommend returning the stream you create in the task. Also you don't need to create tmp files and folders, you can just

gulp.src("./src/*.js")
    .pipe(concat("concat.js"))
    .pipe(uglify())
    .pipe(gulp.dest("./dist"));