javascript的技巧

判断boolean的知识总结

1. 当变量为空字符串时候，判断该变量时候返回false
2. 当变量不为空时候，可以通过!!操作符来转换该值为boolean类型（true或者false）,如

   try {
    throw new Error();
   } catch (e) {
    console.log(!!e.stack);
   }

打印的即为：true

链式调用的两种写法：

• 通过返回this的变量，不断针对同一个对象进行链式调用

  // 实现
  function Fetcher() {
    this.urls = [];
  }
  Fetcher.prototype.init = function(url) {
    this.urls.push(url);
    return this;
  }
  Fetcher.prototype.filter = function(extract) {
    this.urls = this.urls.filter(extract);
    return this;
  }
  Fetcher.prototype.allData = function() {
    return this.urls;
  }
  var f = new Fetcher();
  f.init('http://aliyun.com').init('http://didi.com').init('http://di.com')
   .filter(function(url){return url.length > 15})
   .allData();

• 通过闭包，recursion来不断生成对象形成链式调用

  var defer = function(defered){
    var value, thens = [], defered = [];
    return {
      resolve: function(value){
        for(var i = 0, ii = thens.length; i < ii; i ++) {
          thens[i].fulfill(value);
        }
      },
      reject: function(reason) {
        for(var i = 0, ii = thens.length; i < ii; i ++) {
          thens[i].rejection(reason);
        }
      },
      then: function(fulfill, rejection) {
        thens.push({
          fulfill: fulfill,
          rejection: rejection
        });
        return defer();
      }
    }
  }
  
  //调用的方式
  var Promise = defer();
  setTimeout(function(){
    Promise.resolve(30000)
  }, 3000);
  Promise.then(function(value){
    console.log(value);
  },null).then(
    function(value){console.log('second call');
  });
  //打印出3000但是’second call’没能打印出来；
  //todo 实现真正的then链式调用~~