grunt uglify task failing-Collection of common programming errors

I know this is marked as solved, but I still prefer this answer from a similar question because you can easily use the files again for another command without writing them twice.

Basically, the answer says

//Does not work
src: ['client/src/js/*.js'],
dest: ['client/dist/js/build.js']
//Works
src: ['client/src/js/*.js'],
dest: 'client/dist/js/build.js'

Tested working example without writing files twice:

'use strict';
module.exports = function(grunt) {
  grunt.initConfig({
    uglify: {
      build: {
        src: ['client/src/js/*.js'],
        dest: 'client/dist/js/build.js'
      }
    },
    watch: {
      js: {
        files: '',
        tasks: ['uglify']
      }
    }
  });
  grunt.loadNpmTasks('grunt-contrib-uglify');
  grunt.loadNpmTasks('grunt-contrib-watch');
  grunt.registerTask('default', [
    'uglify',
    ]);
  grunt.registerTask('dev', [
    'watch'
    ]);
};

Notice that '' is very handy 😉

Execution

$ grunt watch
Running "watch" task
Waiting...OK
>> File "client/src/js/hello.js" changed.
Running "uglify:build" (uglify) task
File "client/dist/js/build.js" created.
Uncompressed size: 15 bytes.
Compressed size: 32 bytes gzipped (15 bytes minified).

Done, without errors.