Why does an error appear when loading a task?

Question

JavaScript

Hello!
I finally got to gulp. I'll start over:
1) I installed all sorts of things like compiling sass, browsersync and all that
2) It came to connecting concat and uglifyjs: I
installed everything as it should be through the console. Installed:

3) I connect:

var gulp         =  require('gulp'), //  Gulp
    sass         =  require('gulp-sass'), // Sass пакет,
    browserSync  =  require('browser-sync'), // Browser Sync
    autoprefixer =  require('gulp-autoprefixer'),// автопрефиксер
    concat      = require('gulp-concat'),//  gulp-concat (для конкатенации файлов)
    uglify      = require('gulp-uglifyjs'); //  gulp-uglifyjs (для сжатия JS)

4) I create a task:

gulp.task('scripts', function() {
    return gulp.src([ // Беру все необходимые библиотеки
        'app/libs/jquery/dist/jquery.min.js' // Беру jQuery
    ])
        .pipe(concat('libs.min.js')) // Собираю их в кучу в новом файле libs.min.js
        .pipe(uglify()) // Сжимаю JS файл
        .pipe(gulp.dest('app/js')); // Выгружаею в папку app/js
});

5) I enter into the console gulp scripts
And there is such a problem:

W:\!Mary\myproject>gulp scripts
module.js:471
    throw err;
    ^

Error: Cannot find module 'gulp-concat'
    at Function.Module._resolveFilename (module.js:469:15)
    at Function.Module._load (module.js:417:25)
    at Module.require (module.js:497:17)
    at require (internal/module.js:20:19)
    at Object.<anonymous> (W:\!Mary\myproject\gulpfile.js:9:19)
    at Module._compile (module.js:570:32)
    at Object.Module._extensions..js (module.js:579:10)
    at Module.load (module.js:487:32)
    at tryModuleLoad (module.js:446:12)
    at Function.Module._load (module.js:438:3)

W:\!Mary\myproject>

And there is another garbage: if I swap concat and uglify when connecting

uglify    = require('gulp-uglifyjs'), //  gulp-uglifyjs (для сжатия JS)
    concat   = require('gulp-concat');//  gulp-concat (для конкатенации файлов)

then the error is like this:

W:\!Mary\myproject>gulp scripts
module.js:471
    throw err;
    ^

Error: Cannot find module 'gulp-uglifyjs'
    at Function.Module._resolveFilename (module.js:469:15)
    at Function.Module._load (module.js:417:25)
    at Module.require (module.js:497:17)
    at require (internal/module.js:20:19)
    at Object.<anonymous> (W:\!Mary\myproject\gulpfile.js:9:19)
    at Module._compile (module.js:570:32)
    at Object.Module._extensions..js (module.js:579:10)
    at Module.load (module.js:487:32)
    at tryModuleLoad (module.js:446:12)
    at Function.Module._load (module.js:438:3)

W:\!Mary\myproject>

Tell me, please, what is the error? Already exhausted all, although it seems that everything should work. Thanks in advance!

Answer 1

var concat = require(' concat '); // not gulp-concat
var uglify = require(' uglifyjs '); // not gulp-uglifyjs
Look again closely at devDependencies

Answer 2

uglify-js is worth it, gulp-uglifyjs is not.