jshint ignore the node_modules folder

I tried to let grunt jshint scans all the js file except the files in the node_modules folder. I tried the follow configuration of grunt, it still scan all the files including the ones in node_modules. anyone know why? also can anyone explain /**/*.js **/*.js. what is the double star mean. here is my grunt config files

module.exports = function(grunt) {

    grunt.initConfig({
        jshint: {
            all: ['**/*.js'],
            options: {
                ignores: ['node_modules']
            }
        }
    });

    grunt.loadNpmTasks('grunt-contrib-jshint');
};

Base on @ateich hint, I realized I can do the following to fix the problem

module.exports = function(grunt) {
    grunt.initConfig({
        jshint: {
            all: ['**/*.js'],
            options: {
                jshintrc: true,
                ignores: ['node_modules/**/*.js']
            }
        }
    });

    grunt.loadNpmTasks('grunt-contrib-jshint');
};