Change the configuration of the task uglify dynamically

I need to change configuration of my uglify task for only minify file as needed (as explained here for the jshint task : https://github.com/gruntjs/grunt-contrib-watch#compiling-files-as-needed) The modification works well for the jshint task but not for uglify, i think the problem is the property path... Any help would be appreciated ;) Here is my Gruntfile.js : ``` module.exports = function (grunt) { grunt.initConfig({ // define source files and their destinations jshint: { all: ['dev/**/*.js'], }, uglify: { dynamic_mappings: { // Grunt will search for "**/*.js" under "dev/" when the "minify" task // runs and build the appropriate src-dest file mappings then, so you // don't need to update the Gruntfile when files are added or removed. files: [{ expand: true, // Enable dynamic expansion. cwd: 'dev/', // Src matches are relative to this path. src: ['**/*.js'], // Actual pattern(s) to match. dest: 'build', // Destination path prefix. ext: '.min.js', // Dest filepaths will have this extension. }, ], } } watch: { options: { spawn: false }, js: { files: 'dev/**/*.js', tasks: [ 'uglify' ] }, } }); // load plugins grunt.loadNpmTasks('grunt-contrib-watch'); grunt.loadNpmTasks('grunt-contrib-uglify'); grunt.loadNpmTasks('grunt-contrib-jshint'); // default task grunt.registerTask('default', [ 'watch' ]); grunt.event.on('watch', function(action, filepath) { grunt.config(['jshint', 'all'], filepath); grunt.config('uglify.dynamic_mappings.files', [{src: filepath }]); }); }; ```