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用纯 JavaScript（原生）替代 jQuery

时间：2022-10-27 20:48     作者：独元殇     分类： 前端技术

转载自 https://youmightnotneedjquery.com/ 。很久以前关注这个文章，不过一直忽略。现在正式转载到我的网站 ccgxk.com 。

嗯，到了 2022 年末，jQuery 现在基本上在技术圈里被处处鄙视，其实 jqurey 当时火起来不是因为它能简单得操纵网页元素，而是因为它当时对各个浏览器之间的兼容性做得很好。操纵元素一直都很简单，只是当时浏览器太多了，敲代码还要句句考虑兼容性，太麻烦，所以需要一种统一的语言来“一句代码，‘百家’浏览器运行”，随后大量 js 库出现，而 jqurey 在这些库中做的不错，所以非常流行。不过今天，浏览器的兼容性已经没那么重要，而且新的前端思想也更新着前端人的大脑，因此 jqurey 已没那么重要。以后不是很建议学习和使用 jqurey 了，否则会学歪了。

这个文章，写的不错，收藏到自己的网站，以后写代码直接复制，棒棒棒！

以下是正文！

jQuery是非常优秀的工具，它能让我们开发项目时变得更容易。

但如果你想从零开始开发一个全新的项目，你应该考虑一下你的项目是否真的需要引入jQuery。也许你只需要几行技巧性的代码就能解决问题。如果你的项目是面向最新的现代浏览器，你真的可以考虑其它的一些简单的技术来代替jQuery。

浏览器的进步给我们带来了很多先进的JavaScript特征，新出现的原生内置(native)JavaScript功能可以很大程度的实现jQuery提供的功能。如果你能了解这些JavaScript新技术，就能在很多地方用纯JavaScript实现以前需要jQuery才能实现的技术。

元素操作

Add Class

JQUERY

$(el).addClass(className);

谷歌浏览器，火狐浏览器，IE8+

if (el.classList)
  el.classList.add(className);
else
  el.className += ' ' + className;

谷歌浏览器，火狐浏览器，IE10+

el.classList.add(className);

After

JQUERY

$(el).after(htmlString);

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el.insertAdjacentHTML('afterend', htmlString);

Append

JQUERY

$(parent).append(el);

谷歌浏览器，火狐浏览器，IE8+

parent.appendChild(el);

Before

JQUERY

$(el).before(htmlString);

谷歌浏览器，火狐浏览器，IE8+

el.insertAdjacentHTML('beforebegin', htmlString);

Children

JQUERY

$(el).children();

谷歌浏览器，火狐浏览器，IE8+

var children = [];
for (var i = el.children.length; i--;) {
  // Skip comment nodes on IE8
  if (el.children[i].nodeType != 8)
    children.unshift(el.children[i]);
}

谷歌浏览器，火狐浏览器，IE9+

el.children

Clone

JQUERY

$(el).clone();

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el.cloneNode(true);

Contains

JQUERY

$.contains(el, child);

谷歌浏览器，火狐浏览器，IE8+

el !== child && el.contains(child);

Contains Selector

JQUERY

$(el).find(selector).length;

谷歌浏览器，火狐浏览器，IE8+

el.querySelector(selector) !== null

Each

JQUERY

$(selector).each(function(i, el){

});

谷歌浏览器，火狐浏览器，IE8+

function forEachElement(selector, fn) {
  var elements = document.querySelectorAll(selector);
  for (var i = 0; i < elements.length; i++)
    fn(elements[i], i);
}

forEachElement(selector, function(el, i){

});

谷歌浏览器，火狐浏览器，IE9+

var elements = document.querySelectorAll(selector);
Array.prototype.forEach.call(elements, function(el, i){

});

Empty

JQUERY

$(el).empty();

谷歌浏览器，火狐浏览器，IE8+

while(el.firstChild)
  el.removeChild(el.firstChild);

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el.innerHTML = '';

过滤

JQUERY

$(selector).filter(filterFn);

谷歌浏览器，火狐浏览器，IE8+

function filter(selector, filterFn) {
  var elements = document.querySelectorAll(selector);
  var out = [];
  for (var i = elements.length; i--;) {
    if (filterFn(elements[i]))
      out.unshift(elements[i]);
  }
  return out;
}

filter(selector, filterFn);

谷歌浏览器，火狐浏览器，IE9+

Array.prototype.filter.call(document.querySelectorAll(selector), filterFn);

查找子元素

JQUERY

$(el).find(selector);

谷歌浏览器，火狐浏览器，IE8+

el.querySelectorAll(selector);

查找

JQUERY

$('.my #awesome selector');

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document.querySelectorAll('.my #awesome selector');

获取属性值

JQUERY

$(el).attr('tabindex');

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el.getAttribute('tabindex');

获取Html内容

JQUERY

$(el).html();

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el.innerHTML

获取外层Html内容

JQUERY

$('<div>').append($(el).clone()).html();

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el.outerHTML

获取CSS样式

JQUERY

$(el).css(ruleName);

谷歌浏览器，火狐浏览器，IE8+

// Varies based on the properties being retrieved, some can be retrieved from el.currentStyle
// https://github.com/jonathantneal/Polyfills-for-IE8/blob/master/getComputedStyle.js

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getComputedStyle(el)[ruleName];

获取文本内容

JQUERY

$(el).text();

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el.textContent || el.innerText

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el.textContent

Has Class

JQUERY

$(el).hasClass(className);

谷歌浏览器，火狐浏览器，IE8+

if (el.classList)
  el.classList.contains(className);
else
  new RegExp('(^| )' + className + '( |$)', 'gi').test(el.className);

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el.classList.contains(className);

元素比较

JQUERY

$(el).is($(otherEl));

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el === otherEl

比较类名

JQUERY

$(el).is('.my-class');

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var matches = function(el, selector) {
  var _matches = (el.matches || el.matchesSelector || el.msMatchesSelector || el.mozMatchesSelector || el.webkitMatchesSelector || el.oMatchesSelector);

  if (_matches) {
    return _matches.call(el, selector);
  } else {
    var nodes = el.parentNode.querySelectorAll(selector);
    for (var i = nodes.length; i--;) {
      if (nodes[i] === el)
        return true;
    }
    return false;
  }
};

matches(el, '.my-class');

谷歌浏览器，火狐浏览器，IE9+

var matches = function(el, selector) {
  return (el.matches || el.matchesSelector || el.msMatchesSelector || el.mozMatchesSelector || el.webkitMatchesSelector || el.oMatchesSelector).call(el, selector);
};

matches(el, '.my-class');

Next

JQUERY

$(el).next();

谷歌浏览器，火狐浏览器，IE8+

// nextSibling can include text nodes
function nextElementSibling(el) {
  do { el = el.nextSibling; } while ( el &amp;&amp; el.nodeType !== 1 );
  return el;
}

el.nextElementSibling || nextElementSibling(el);

谷歌浏览器，火狐浏览器，IE9+

el.nextElementSibling

Offset

JQUERY

$(el).offset();

谷歌浏览器，火狐浏览器，IE8+

var rect = el.getBoundingClientRect()

{
  top: rect.top + document.body.scrollTop,
  left: rect.left + document.body.scrollLeft
}

Offset Parent

JQUERY

$(el).offsetParent();

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el.offsetParent || el

Outer Height

JQUERY

$(el).outerHeight();

谷歌浏览器，火狐浏览器，IE8+

el.offsetHeight

Outer Height With Margin

JQUERY

$(el).outerHeight(true);

谷歌浏览器，火狐浏览器，IE8+

function outerHeight(el) {
  var height = el.offsetHeight;
  var style = el.currentStyle || getComputedStyle(el);

  height += parseInt(style.marginTop) + parseInt(style.marginBottom);
  return height;
}

outerHeight(el);

谷歌浏览器，火狐浏览器，IE9+

function outerHeight(el) {
  var height = el.offsetHeight;
  var style = getComputedStyle(el);

  height += parseInt(style.marginTop) + parseInt(style.marginBottom);
  return height;
}

outerHeight(el);

Outer Width With Margin

JQUERY

$(el).outerWidth(true);

谷歌浏览器，火狐浏览器，IE8+

function outerWidth(el) {
  var width = el.offsetWidth;
  var style = el.currentStyle || getComputedStyle(el);

  width += parseInt(style.marginLeft) + parseInt(style.marginRight);
  return width;
}

outerWidth(el);

谷歌浏览器，火狐浏览器，IE9+

function outerWidth(el) {
  var width = el.offsetWidth;
  var style = getComputedStyle(el);

  width += parseInt(style.marginLeft) + parseInt(style.marginRight);
  return width;
}

outerWidth(el);

Outer Width

JQUERY

$(el).outerWidth();

谷歌浏览器，火狐浏览器，IE8+

el.offsetWidth

Parent

JQUERY

$(el).parent();

谷歌浏览器，火狐浏览器，IE8+

el.parentNode

Position

JQUERY

$(el).position();

谷歌浏览器，火狐浏览器，IE8+

{left: el.offsetLeft, top: el.offsetTop}

Position Relative To Viewport

JQUERY

var offset = el.offset();

{
  top: offset.top - document.body.scrollTop,
  left: offset.left - document.body.scrollLeft
}

谷歌浏览器，火狐浏览器，IE8+

el.getBoundingClientRect()

Prepend

JQUERY

$(parent).prepend(el);

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parent.insertBefore(el, parent.firstChild);

Prev

JQUERY

$(el).prev();

谷歌浏览器，火狐浏览器，IE8+

// prevSibling can include text nodes
function previousElementSibling(el) {
  do { el = el.previousSibling; } while ( el &amp;&amp; el.nodeType !== 1 );
  return el;
}

el.previousElementSibling || previousElementSibling(el);

谷歌浏览器，火狐浏览器，IE9+

el.previousElementSibling

Remove

JQUERY

$(el).remove();

谷歌浏览器，火狐浏览器，IE8+

el.parentNode.removeChild(el);

Remove Class

JQUERY

$(el).removeClass(className);

谷歌浏览器，火狐浏览器，IE8+

if (el.classList)
  el.classList.remove(className);
else
  el.className = el.className.replace(new RegExp('(^|\\b)' + className.split(' ').join('|') + '(\\b|$)', 'gi'), ' ');

谷歌浏览器，火狐浏览器，IE10+

el.classList.remove(className);

Replace From Html

JQUERY

$(el).replaceWith(string);

谷歌浏览器，火狐浏览器，IE8+

el.outerHTML = string;

Set Attributes

JQUERY

$(el).attr('tabindex', 3);

谷歌浏览器，火狐浏览器，IE8+

el.setAttribute('tabindex', 3);

Set Html

JQUERY

$(el).html(string);

谷歌浏览器，火狐浏览器，IE8+

el.innerHTML = string;

Set Style

JQUERY

$(el).css('border-width', '20px');

谷歌浏览器，火狐浏览器，IE8+

// Use a class if possible
el.style.borderWidth = '20px';

Set Text

JQUERY

$(el).text(string);

谷歌浏览器，火狐浏览器，IE8+

if (el.textContent !== undefined)
  el.textContent = string;
else
  el.innerText = string;

谷歌浏览器，火狐浏览器，IE9+

el.textContent = string;

Siblings

JQUERY

$(el).siblings();

谷歌浏览器，火狐浏览器，IE8+

var siblings = Array.prototype.slice.call(el.parentNode.children);

for (var i = siblings.length; i--;) {
  if (siblings[i] === el) {
    siblings.splice(i, 1);
    break;
  }
}

谷歌浏览器，火狐浏览器，IE9+

Array.prototype.filter.call(el.parentNode.children, function(child){
  return child !== el;
});

Toggle Class

JQUERY

$(el).toggleClass(className);

谷歌浏览器，火狐浏览器，IE8+

if (el.classList) {
  el.classList.toggle(className);
} else {
    var classes = el.className.split(' ');
    var existingIndex = -1;
    for (var i = classes.length; i--;) {
      if (classes[i] === className)
        existingIndex = i;
    }

    if (existingIndex >= 0)
      classes.splice(existingIndex, 1);
    else
      classes.push(className);

  el.className = classes.join(' ');
}

谷歌浏览器，火狐浏览器，IE9+

if (el.classList) {
  el.classList.toggle(className);
} else {
  var classes = el.className.split(' ');
  var existingIndex = classes.indexOf(className);

  if (existingIndex >= 0)
    classes.splice(existingIndex, 1);
  else
    classes.push(className);

  el.className = classes.join(' ');
}

谷歌浏览器，火狐浏览器，IE10+

el.classList.toggle(className);

事件

Off

JQUERY

$(el).off(eventName, eventHandler);

谷歌浏览器，火狐浏览器，IE8+

function removeEventListener(el, eventName, handler) {
  if (el.removeEventListener)
    el.removeEventListener(eventName, handler);
  else
    el.detachEvent('on' + eventName, handler);
}

removeEventListener(el, eventName, handler);

谷歌浏览器，火狐浏览器，IE9+

el.removeEventListener(eventName, eventHandler);

On

JQUERY

$(el).on(eventName, eventHandler);

谷歌浏览器，火狐浏览器，IE8+

function addEventListener(el, eventName, handler) {
  if (el.addEventListener) {
    el.addEventListener(eventName, handler);
  } else {
    el.attachEvent('on' + eventName, function(){
      handler.call(el);
    });
  }
}

addEventListener(el, eventName, handler);

谷歌浏览器，火狐浏览器，IE9+

el.addEventListener(eventName, eventHandler);

Ready

JQUERY

$(document).ready(function(){

});

谷歌浏览器，火狐浏览器，IE8+

function ready(fn) {
  if (document.readyState != 'loading'){
    fn();
  } else if (document.addEventListener) {
    document.addEventListener('DOMContentLoaded', fn);
  } else {
    document.attachEvent('onreadystatechange', function() {
      if (document.readyState != 'loading')
        fn();
    });
  }
}

谷歌浏览器，火狐浏览器，IE9+

function ready(fn) {
  if (document.readyState != 'loading'){
    fn();
  } else {
    document.addEventListener('DOMContentLoaded', fn);
  }
}

指定事件触发

JQUERY

$(el).trigger('my-event', {some: 'data'});

谷歌浏览器，火狐浏览器，IE8+

// Custom events are not natively supported, so you have to hijack a random
// event.
//
// Just use jQuery.

谷歌浏览器，火狐浏览器，IE9+

if (window.CustomEvent) {
  var event = new CustomEvent('my-event', {detail: {some: 'data'}});
} else {
  var event = document.createEvent('CustomEvent');
  event.initCustomEvent('my-event', true, true, {some: 'data'});
}

el.dispatchEvent(event);

Trigger Native

JQUERY

$(el).trigger('change');

谷歌浏览器，火狐浏览器，IE8+

if (document.createEvent) {
  var event = document.createEvent('HTMLEvents');
  event.initEvent('change', true, false);
  el.dispatchEvent(event);
} else {
  el.fireEvent('onchange');
}

谷歌浏览器，火狐浏览器，IE9+

// For a full list of event types: https://developer.mozilla.org/en-US/docs/Web/API/document.createEvent
var event = document.createEvent('HTMLEvents');
event.initEvent('change', true, false);
el.dispatchEvent(event);

技巧

Array Each

JQUERY

$.each(array, function(i, item){

});

谷歌浏览器，火狐浏览器，IE8+

function forEach(array, fn) {
  for (i = 0; i < array.length; i++)
    fn(array[i], i);
}

forEach(array, function(item, i){

});

谷歌浏览器，火狐浏览器，IE9+

array.forEach(function(item, i){

});

深度扩展

JQUERY

$.extend(true, {}, objA, objB);

谷歌浏览器，火狐浏览器，IE8+

var deepExtend = function(out) {
  out = out || {};

  for (var i = 1; i < arguments.length; i++) {
    var obj = arguments[i];

    if (!obj)
      continue;

    for (var key in obj) {
      if (obj.hasOwnProperty(key)) {
        if (typeof obj[key] === 'object')
          deepExtend(out[key], obj[key]);
        else
          out[key] = obj[key];
      }
    }
  }

  return out;
};

deepExtend({}, objA, objB);

Bind

JQUERY

$.proxy(fn, context);

谷歌浏览器，火狐浏览器，IE8+

fn.apply(context, arguments);

谷歌浏览器，火狐浏览器，IE9+

fn.bind(context);

jQuery

$.extend({}, objA, objB);

谷歌浏览器，火狐浏览器，IE8+

var extend = function(out) {
  out = out || {};

  for (var i = 1; i < arguments.length; i++) {
    if (!arguments[i])
      continue;

    for (var key in arguments[i]) {
      if (arguments[i].hasOwnProperty(key))
        out[key] = arguments[i][key];
    }
  }

  return out;
};

extend({}, objA, objB);

Index Of

JQUERY

$.inArray(item, array);

谷歌浏览器，火狐浏览器，IE8+

function indexOf(array, item) {
  for (var i = 0; i < array.length; i++) {
    if (array[i] === item)
      return i;
  }
  return -1;
}

indexOf(array, item);

谷歌浏览器，火狐浏览器，IE9+

array.indexOf(item);

Is Array

JQUERY

$.isArray(arr);

谷歌浏览器，火狐浏览器，IE8+

isArray = Array.isArray || function(arr) {
  return Object.prototype.toString.call(arr) == '[object Array]';
}

isArray(arr);

谷歌浏览器，火狐浏览器，IE9+

Array.isArray(arr);

Map

JQUERY

$.map(array, function(value, index){

});

谷歌浏览器，火狐浏览器，IE8+

function map(arr, fn) {
  var results = [];
  for (var i = 0; i < arr.length; i++)
    results.push(fn(arr[i], i));
  return results;
}

map(array, function(value, index){

});

谷歌浏览器，火狐浏览器，IE9+

array.map(function(value, index){

});

Now

JQUERY

$.now();

谷歌浏览器，火狐浏览器，IE8+

new Date().getTime();

谷歌浏览器，火狐浏览器，IE9+

Date.now();

Parse Html

JQUERY

$.parseHTML(htmlString);

谷歌浏览器，火狐浏览器，IE8+

var parseHTML = function(str) {
  var el = document.createElement('div');
  el.innerHTML = str;
  return el.children;
};

parseHTML(htmlString);

谷歌浏览器，火狐浏览器，IE9+

var parseHTML = function(str) {
  var tmp = document.implementation.createHTMLDocument();
  tmp.body.innerHTML = str;
  return tmp.body.children;
};

parseHTML(htmlString);

解析 Json

JQUERY

$.parseJSON(string);

谷歌浏览器，火狐浏览器，IE8+

JSON.parse(string);

Trim

JQUERY

$.trim(string);

谷歌浏览器，火狐浏览器，IE8+

string.replace(/^\s+|\s+$/g, '');

谷歌浏览器，火狐浏览器，IE9+

string.trim();

Type

JQUERY

$.type(obj);

谷歌浏览器，火狐浏览器，IE8+

Object.prototype.toString.call(obj)
                .replace(/^\[object (.+)\]$/, "$1")
                .toLowerCase();

AJAX

新版的JavaScript API里提供了一个全新的可以实现ajax的API——fetch，这个api采用了全新的 Promise 架构，使用起来更方便，更灵活，详细用法请参考《你不需要jQuery(三)：新AJAX方法fetch() 》。

JSON

JQUERY

$.getJSON('/my/url', function(data) {

});

谷歌浏览器，火狐浏览器，IE8+

var request = new XMLHttpRequest();
request.open('GET', '/my/url', true);

request.onreadystatechange = function() {
  if (this.readyState === 4) {
    if (this.status >= 200 &amp;&amp; this.status < 400) {
      // Success!
      var data = JSON.parse(this.responseText);
    } else {
      // Error :(
    }
  }
};

request.send();
request = null;

谷歌浏览器，火狐浏览器，IE9+

var request = new XMLHttpRequest();
request.open('GET', '/my/url', true);

request.onload = function() {
  if (request.status >= 200 &amp;&amp; request.status < 400) {
    // Success!
    var data = JSON.parse(request.responseText);
  } else {
    // We reached our target server, but it returned an error

  }
};

request.onerror = function() {
  // There was a connection error of some sort
};

request.send();

谷歌浏览器，火狐浏览器，IE10+

var request = new XMLHttpRequest();
request.open('GET', '/my/url', true);

request.onload = function() {
  if (this.status >= 200 &amp;&amp; this.status < 400) {
    // Success!
    var data = JSON.parse(this.response);
  } else {
    // We reached our target server, but it returned an error

  }
};

request.onerror = function() {
  // There was a connection error of some sort
};

request.send();

Post

JQUERY

$.ajax({
  type: 'POST',
  url: '/my/url',
  data: data
});

谷歌浏览器，火狐浏览器，IE8+

var request = new XMLHttpRequest();
request.open('POST', '/my/url', true);
request.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded; charset=UTF-8');
request.send(data);

Request

JQUERY

$.ajax({
  type: 'GET',
  url: '/my/url',
  success: function(resp) {

  },
  error: function() {

  }
});

谷歌浏览器，火狐浏览器，IE8+

var request = new XMLHttpRequest();
request.open('GET', '/my/url', true);

request.onreadystatechange = function() {
  if (this.readyState === 4) {
    if (this.status >= 200 &amp;&amp; this.status < 400) {
      // Success!
      var resp = this.responseText;
    } else {
      // Error :(
    }
  }
};

request.send();
request = null;

谷歌浏览器，火狐浏览器，IE9+

var request = new XMLHttpRequest();
request.open('GET', '/my/url', true);

request.onload = function() {
  if (request.status >= 200 &amp;&amp; request.status < 400) {
    // Success!
    var resp = request.responseText;
  } else {
    // We reached our target server, but it returned an error

  }
};

request.onerror = function() {
  // There was a connection error of some sort
};

request.send();

谷歌浏览器，火狐浏览器，IE10+

var request = new XMLHttpRequest();
request.open('GET', '/my/url', true);

request.onload = function() {
  if (this.status >= 200 &amp;&amp; this.status < 400) {
    // Success!
    var resp = this.response;
  } else {
    // We reached our target server, but it returned an error

  }
};

request.onerror = function() {
  // There was a connection error of some sort
};

request.send();

特效

淡入淡出

JQUERY

$(el).fadeIn();

谷歌浏览器，火狐浏览器，IE8+

function fadeIn(el) {
  var opacity = 0;

  el.style.opacity = 0;
  el.style.filter = '';

  var last = +new Date();
  var tick = function() {
    opacity += (new Date() - last) / 400;
    el.style.opacity = opacity;
    el.style.filter = 'alpha(opacity=' + (100 * opacity)|0 + ')';

    last = +new Date();

    if (opacity < 1) {
      (window.requestAnimationFrame &amp;&amp; requestAnimationFrame(tick)) || setTimeout(tick, 16);
    }
  };

  tick();
}

fadeIn(el);

谷歌浏览器，火狐浏览器，IE9+

function fadeIn(el) {
  el.style.opacity = 0;

  var last = +new Date();
  var tick = function() {
    el.style.opacity = +el.style.opacity + (new Date() - last) / 400;
    last = +new Date();

    if (+el.style.opacity < 1) {
      (window.requestAnimationFrame &amp;&amp; requestAnimationFrame(tick)) || setTimeout(tick, 16)
    }
  };

  tick();
}

fadeIn(el);

谷歌浏览器，火狐浏览器，IE10+

el.classList.add('show');
el.classList.remove('hide');

.show {
  transition: opacity 400ms;
}
.hide {
  opacity: 0;
}

Hide

JQUERY

$(el).hide();

谷歌浏览器，火狐浏览器，IE8+

el.style.display = 'none';

Show

JQUERY

$(el).show();

谷歌浏览器，火狐浏览器，IE8+

el.style.display = '';

标签: 转载 JS

评论：
avatar
2broear 2022-11-10 09:22
jq在前几年还是流行，各有各的好
commentator
独元殇 2022-11-10 15:35
@2broear：时代在发展，技术在迭代
avatar
Lvtu 2022-10-28 09:41
其实一直没搞明白两者有啥区别？
另外你这不都是讲的jQuery吗？
最近遇到一个问题，就是JS代码如果放入xx.js文件中引用无效，但是拿出来放在所需要用的文件底部却可以，有没有什么办法解决？谢谢！
commentator
独元殇 2022-10-28 10:10
@Lvtu：js 是一个编程语言，像 Python 和 c 一样，jqurey 是基于 js 编程语言的库，js 和 jqurey 就好像 JAVA 和 springboot ，Python 和 pyqt5 一样，是第三方大佬写的扩展，用 jqury 是需要引入 jqury.min.js 的。

这篇文章，有很多小节，每节有两三个例子，第一个例子是 jqurey 语法，后面的是分别在不同浏览器下，直接使用原生 js 语法的写法，也就是用后面的代码就不用引入百来 kb 之大的 jqurey.min.js 这种文件，可以直接写了。

至于那个问题，可能是 js 找不到某个 dom 元素吧。<script> 和其他 html 标签是一样的，在浏览器中依次加载，而且 js 也是有先后执行顺序的，如果写在 <body> 前面或者什么地方可能不能执行。一般 <script> 写在 <body> 最最最后面。

哈哈，跟我就不要说什么谢谢了，老熟人了。