gulp - GulpUglifyError:Unable to minify JavaScript -

i trying minify script files using gulp task runner , trying gulp-uglify plugin

code:

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

but getting error as

when try run gulp task gulp concat appreciated

to see 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 exact file line no of error register , run task:

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