Watching and compile TypeScript with Grunt

Question

gruntfile.js

module.exports = function (grunt) {
    grunt.initConfig( {
        pkg: grunt.file.readJSON('package.json'),
        watch : {
            files : ['**/*.ts'],
            tasks : ['exec:run_tsc']
        },
        exec: {
            run_tsc: { cmd : 'tsc'}
        }
    });
    grunt.loadNpmTasks('grunt-contrib-watch');
    grunt.loadNpmTasks('grunt-exec');
    grunt.registerTask('default', ['watch']);
};

Looks like this code only run tsc.exe but it does not compile any TypeScript.