var a = '/a/b/c/d/main.js';
var b = '/a/b/zhangjing/index.js';

function relativeDir(relative, absolute) {
    var rela = relative.split('/');
    rela.shift();
    var abso = absolute.split('/');
    abso.shift();

    var str = '';
    for (var i = 0; i < rela.length; i++) {
        if(rela[i] === abso[i]) {
            str += '../';
        } else {
            str += rela[i]+'/';
        }
    }
    return str.substr(0, str.length-1); // 删除最后一个斜杠
}
console.log(relativeDir(a, b));