jQuery.fn = jQuery.prototype = {
  constructor: jQuery,
  init: function( selector, context, rootjQuery ) {
    var match, elem, ret, doc;
    // 处理$(""), $(null), $(undefined), $(false)这几种参数,直接返回this
    if ( !selector ) {
      return this;
    }
    // 当传参selector为DOM结点时,将context置为selector
    if ( selector.nodeType ) {
      this.context = this[0] = selector;
      this.length = 1;
      return this;
    }
    // Handle HTML strings
    // 当传入的selector参数为字符串时,
    if ( typeof selector === "string" ) {
      if ( selector.charAt(0) === "<" && selector.charAt( selector.length - 1 ) === ">" && selector.length >= 3 ) {
        // Assume that strings that start and end with <> are HTML and skip the regex check
        match = [ null, selector, null ];
      } else {
        match = rquickExpr.exec( selector );
      }
      // Match html or make sure no context is specified for #id
      if ( match && (match[1] || !context) ) {
        // HANDLE: $(html) -> $(array)
        if ( match[1] ) {
          context = context instanceof jQuery ? context[0] : context;
          doc = ( context && context.nodeType ? context.ownerDocument || context : document );
          // scripts is true for back-compat
          selector = jQuery.parseHTML( match[1], doc, true );
          if ( rsingleTag.test( match[1] ) && jQuery.isPlainObject( context ) ) {
            this.attr.call( selector, context, true );
          }
          return jQuery.merge( this, selector );
        // HANDLE: $(#id)
        } else {
          elem = document.getElementById( match[2] );
          // Check parentNode to catch when Blackberry 4.6 returns
          // nodes that are no longer in the document #6963
          if ( elem && elem.parentNode ) {
            // Handle the case where IE and Opera return items
            // by name instead of ID
            if ( elem.id !== match[2] ) {
              return rootjQuery.find( selector );
            }
            // Otherwise, we inject the element directly into the jQuery object
            this.length = 1;
            this[0] = elem;
          }
          this.context = document;
          this.selector = selector;
          return this;
        }
      // HANDLE: $(expr, $(...))
      } else if ( !context || context.jquery ) {
        return ( context || rootjQuery ).find( selector );
      // HANDLE: $(expr, context)
      // (which is just equivalent to: $(context).find(expr)
      } else {
        return this.constructor( context ).find( selector );
      }
    // HANDLE: $(function)
    // Shortcut for document ready
    // 当selector为function时相当于$(document).ready(selector);
    } else if ( jQuery.isFunction( selector ) ) {
      return rootjQuery.ready( selector );
    }
    // 当selector参数为{selector:'#id', context:document}之类时,重置属性selector和context
    if ( selector.selector !== undefined ) {
      this.selector = selector.selector;
      this.context = selector.context;
    }
    return jQuery.makeArray( selector, this );
  }
};