object HTMLDivElement，即 HTMLDivElement 对象，
而这个对象是哪里来的呢？ 查阅 《JavaScript权威指南》中文第六版363页，
我们可以知道： HTMLDivElement 是 HTMLElement 的一个子对象，
而 HTMLElement 又是 Element 的子对象。
function $(selector) {
var startWith = "";
if(/^#/.test(selector)) {
startWith = "id";
} else if(/^./.test(selector)) {
startWith = "class";
} else if(/^[^#.]/.test(selector)){
startWith = "tagname";
}
if(/[w+=.+?]/.test(selector) ){ //属性选择器
var arr = selector.match(/(.+?)[(w+)=(.+?)]/);
//.box[name=123]
var _selec = arr[1]; //.box
var propname = arr[2]; //name
var propval = arr[3]; //123
var elelist = null;
switch(startWith) {
case "id" : return _id(_selec.substring(1));
case "class" : {
console.log();
elelist = Array.from(document.getElementsByClassName(_selec.substring(1)));
return elelist.filter(function(val, index){
return val[propname] == propval;
});
}
case "tagname" : {
elelist = Array.from(document.getElementsByTagName(_selec));
return elelist.filter(function(val, index){
return val[propname] == propval;
});
}
}
return null;
}
switch(startWith) {
case "id" : return _id(selector.substring(1));
case "class" : {
return _class(selector.substring(1));
}
case "tagname" : {
return _tag(selector);
}
}
function _id(id) {
return document.getElementById(id)
}
function _class(classname) {
console.log(classname);
return Array.from(document.getElementsByClassName(classname));
}
function _tag(tag) {
return Array.from(document.getElementsByTagName(tag));
}
}