GulpUglifyError:Unable to minify JavaScript

The main error is generated when you're using ES6 format. Use the gulp-uglify-es module instead of 'gulp-uglify' to overcome this error.

var uglify = require('gulp-uglify-es').default;

Note: gulp-uglify-es is no longer being maintained. You may want to use terser/gulp-terser instead:

To see the error in console:

var gutil = require('gulp-util');

gulp.task('concat', function() {
return gulp.src('app/**/*.js')
    // .pipe(concat('script.js'))
    .pipe(uglify())
    .on('error', function (err) { gutil.log(gutil.colors.red('[Error]'), err.toString()); })
    .pipe(gulp.dest('./dist/'))
});

To find the exact file, with line number of error register and run this task:

var pump = require('pump');

gulp.task('uglify-error-debugging', function (cb) {
  pump([
    gulp.src('app/**/*.js'),
    uglify(),
    gulp.dest('./dist/')
  ], cb);
});

I think the top answers here are not explaining how to get the error. The docs have a section on error handling:

gulp-uglify emits an 'error' event if it is unable to minify a specific file

So, just capture the error and do whatever you want with it (such as logging to console) to see the filename, line number, and additional info:

uglify().on('error', console.error)

or in a larger context:

gulp.task('foo', () => {

    return gulp.src([
        'asset/src/js/foo/*.js',
        'asset/src/js/bar/*.js',
    ])
    .pipe(uglify().on('error', console.error))
    .pipe(concat('bundle.js'))
    .pipe(gulp.dest('./'));

});

This gives you a super helpful error!

{ GulpUglifyError: unable to minify JavaScript
    at [stack trace])
  cause:
   { SyntaxError: Continue not inside a loop or switch
       [stack trace]
     message: 'Continue not inside a loop or switch',
     filename: 'ProductForm.js',
     line: 301,
     col: 37,
     pos: 10331 },
  plugin: 'gulp-uglify',
  fileName:
   '/asset/src/js/foo/ProductForm.js',
  showStack: false }

Have you used ES6 format in your script file? If so try ES5 now because when you do gulp-uglify it doesnt understand ES6 format as of now and after that try your code

 gulp.task('concat', function() {
    return gulp.src('app/**/*.js')
        .pipe(concat('script.js'))
        .pipe(uglify())
        .pipe(gulp.dest('./dist/'))
});

and run the task gulp concat it will work

For me, it was a deprecated option "preserveComments" that generated the error (and completely crashed the script).

Found the issue using:

gulp.task('concat', function() {
return gulp.src('app/**/*.js')
    .pipe(uglify())
    .on('error', function (err) { console.log( err ) })
    .pipe(gulp.dest('./dist/'))
});

Try using this

var concat = require('gulp-concat');
var uglify = require('gulp-uglify');
var minifyJS = require('gulp-minify');

gulp.task('concat', function() {
    return gulp.src('app/**/*.js')
        .pipe(minifyJS())
        .pipe(concat('bundle.min.js'))
        .pipe(uglify({ mangle: false }))
        .pipe(gulp.dest('./dist/'));
});

you may have syntax error or used ES6 syntax. you can try https://skalman.github.io/UglifyJS-online/ firstly.

The main error to Unable to minifies JavaScript is the path not found. You can use the task usemin For this you need:

$ sudo npm install gulp-usemin --save-dev
$ sudo npm install gulp-livereload --save-dev
$ sudo npm install gulp-util --save-dev

and requires :

var usemin = require('gulp-usemin');
var livereload = require('gulp-livereload');
var gutil = require('gulp-util'); //gutil for error display

gulp.task('usemin',['jshint'], function(){
    return gulp.src('./app/index.html')
    .pipe(usemin({
        css: [minifycss(),rev()],
        scripts: [uglify().on('error', function(err) {gutil.log(gutil.colors.red('[Error]'), err.toString());this.emit('end');}),rev()]
    })) 
    .pipe(gulp.dest('dist'))
    .pipe(livereload());
});

Change the js: [uglify(), rev()] to scripts: [uglify(), rev()]