How to use selector and context in jQuery object?

The two attributes of the jQuery object, selector and context, were not clear at all at first. Then I searched on Baidu and Google for a long time, but could not find the reason. Later, I found this jquery in the jQuery API document. The selector attribute and context attribute of the object. Haha~ So~ When you have free time, it is better to go through the jQuery API documentation first and get familiar with it. It is better than being like me, who is still looking for things on the Internet and still can’t find them, which is very frustrating. Yeah~ If you students who read this article still don’t know the meaning of these two attributes, then just learn it here. In fact, the biggest use of these two attributes is to write plug-ins.

The API document says:
By default, if the context parameter is not specified, $() will search for DOM elements in the current HTML document; if the context parameter is specified, such as a DOM element set or jQuery object, it will be searched in this context. After jQuery 1.3.2, the order of the elements returned is equivalent to the order in which they appear in the context.

TheContext parameter needs to be a working node object (DOM object, not jQuery object). Although passing a jQuery object can also limit the search scope, in this case, the context property of the jQuery object will become the entire Document object.

And the value of $(expression, [context]).selector is exactly expression
For example:

 $("div ul").selector//值为“div ul”    
 $("div.test").selector//值为“div.test”    
 $("#test ul li:first").selector//值为“test ul li:first”

That is to say, what expression is, what selector is,

$(expression, [context]).context is a DOM object. Regarding this DOM object, when using different $(expression, [context]), the context object obtained is different.

Related sample code:

function( selector, context, rootjQuery ) {var match, elem, ret, doc;// Handle $(""), $(null), or $(undefined)
        //如果selector为空格，！selector为false
if (!selector) {//此时this为空jQuery对象
return this;
}// Handle $(DOMElement)
        //nodeType节点类型，利用是否有nodeType属性来判断是否是DOM元素
if ( selector.nodeType ) {//将第一个元素和属性context指向selector
this.context = this[0] = selector;this.length = 1;return this;
}// The body element only exists once, optimize finding it
        //因为body只出现一次，利用!context进行优化
if ( selector === "body" && !context && document.body ) {//context指向document对象
this.context = document;this[0] = document.body;this.selector = selector;this.length = 1;return this;
}// Handle HTML strings
if ( typeof selector === "string" ) {    // Are we dealing with HTML string or an ID?
//以<开头以>结尾，且长度大于等于3，这里假设是HTML片段，跳过queckExpr正则检查
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 = quickExpr.exec( selector );
}// Verify a match, and that no context was specified for #id
if ( match && (match[1] || !context) ) {// HANDLE: $(html) -> $(array)
if ( match[1] ) {
context = context instanceof jQuery ? context[0] : context;
doc = ( context ? context.ownerDocument || context : document );// If a single string is passed in and it&#39;s a single tag
// just do a createElement and skip the rest
ret = rsingleTag.exec( selector );        //如果是单独标签
if (ret) {//如果context是普通对象
    if (jQuery.isPlainObject(context)) {    //之所以放在数组中，是方便后面的jQuery.merge()方法调用
    selector = [document.createElement(ret[1])];    //调用attr方法，传入参数context
jQuery.fn.attr.call( selector, context, true );
} else {
selector = [ doc.createElement( ret[1] ) ];
}        //复杂HTML的处理方法
} else {
ret = jQuery.buildFragment( [ match[1] ], [ doc ] );
selector = ( ret.cacheable ? jQuery.clone(ret.fragment) : ret.fragment ).childNodes;
}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
    //即使是documen.getElementById这样核心的方法也要考虑到浏览器兼容问题，可能找到的是name而不是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, $(...))
//没有指定上下文，执行rootjQuery.find()，制定了上下文且上下文是jQuery对象，执行context.find()
} else if ( !context || context.jquery ) {return ( context || rootjQuery ).find( selector );// HANDLE: $(expr, context)
// (which is just equivalent to: $(context).find(expr)
//如果指定了上下文，且上下文不是jQuery对象
} else {    //先创建一个包含context的jQuery对象，然后调用find方法
return this.constructor( context ).find( selector );
}// HANDLE: $(function)
// Shortcut for document ready
} else if ( jQuery.isFunction( selector ) ) {return rootjQuery.ready( selector );
}        //selector是jquery对象
if ( selector.selector !== undefined ) {this.selector = selector.selector;this.context = selector.context;
}        //合并到当前jQuery对象
return jQuery.makeArray( selector, this );
}

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