最适合网络开发者的网站

JS 教程

JS 首页 JS 简介 JS 去哪儿 JS 输出 JS 语句 JS 语法 JS 评论 JS 变量 JS 让 JS 常量 JS 运算符 JS 算术 JS 作业 JS 数据类型 JS 函数 JS 对象 JS 活动 JS 字符串 JS 字符串方法 JS 字符串搜索 JS 字符串模板 JS 数字 JS 数字方法 JS 数组 JS 数组方法 JS 数组排序 JS 数组迭代 JS 数组常量 JS 日期 JS 日期格式 JS 获取日期方法 JS 日期设置方法 JS 数学 JS 随机 JS 布尔值 JS 比较 JS If Else JS 开关 JS 循环 JS 循环 JS 循环 JS 循环 JS 中断 JS 可迭代对象 JS 集 JS 地图 JS Typeof JS 类型转换 JS 按位 JS 正则表达式 JS 错误 JS 作用域 JS 提升 JS 严格模式 JS this 关键字 JS 箭头函数 JS 类 JS Modules JS JSON JS 调试 JS 风格指南 JS 最佳实践 JS 错误 JS 性能 JS 保留字

JS 版本

JS 版本 JS 2009(ES5) JS 2015(ES6) 小学 2016 小学 2017 小学 2018 JS IE/Edge JS 历史

JS 对象

对象定义 对象属性 对象方法 对象显示 对象访问器 对象构造函数 对象原型 对象可迭代对象 对象集 对象映射 对象引用

JS 函数

函数定义 函数参数 函数调用 函数调用 功能应用 函数绑定 函数闭包

JS 类

课程简介 类继承 静态类

JS 异步

JS 回调 JS 异步 JS 承诺 JS 异步/等待

JS HTML DOM

DOM 简介 DOM 方法 DOM 文档 DOM 元素 DOM HTML DOM 表单 DOM CSS DOM 动画 DOM 事件 DOM 事件监听器 DOM 导航 DOM 节点 DOM 集合 DOM 节点列表

JS 浏览器 BOM

JS 窗口 JS 屏幕 JS 位置 JS 历史 JS 导航器 JS 弹出警告 JS 时机 JS Cookies

JS Web API

Web API 简介 Web 表单 API 网络历史记录 API Web 存储 API Web Worker API Web 获取 API 网络地理定位 API

JS AJAX

AJAX 简介 AJAX XMLHttp AJAX 请求 AJAX 响应 AJAX XML 文件 AJAX PHP AJAX ASP AJAX 数据库 AJAX 应用程序 AJAX 示例

JS JSON

JSON 简介 JSON 语法 JSON 与 XML JSON 数据类型 JSON 解析 JSON 字符串化 JSON 对象 JSON 数组 JSON 服务器 JSON 格式的 PHP JSON 的 HTML JSON JSONP

JS 与 jQuery

jQuery 选择器 jQuery HTML jQuery CSS jQuery DOM

JS 图形

JS 图形 JS 画布 JS Plotly JS Chart.js JS 谷歌图表 JS D3.js

JS 示例

JS 示例 JS HTML DOM JS HTML 输入 JS HTML 对象 JS HTML 事件 JS 浏览器 JS 编辑器 JS 练习 JS 测验 JS 证书

JS 参考

JavaScript 对象 HTML DOM 对象

JavaScript。W3Schools 英文版。初学者课程

尿素

JavaScript Debugging


Errors can (will) happen, every time you write some new computer code.


Code Debugging

Programming code might contain syntax errors, or logical errors.

Many of these errors are difficult to diagnose.

Often, when programming code contains errors, nothing will happen. There are no error messages, and you will get no indications where to search for errors.

Searching for (and fixing) errors in programming code is called code debugging.


JavaScript Debuggers

Debugging is not easy. But fortunately, all modern browsers have a built-in JavaScript debugger.

Built-in debuggers can be turned on and off, forcing errors to be reported to the user.

With a debugger, you can also set breakpoints (places where code execution can be stopped), and examine variables while the code is executing.

Normally, otherwise follow the steps at the bottom of this page, you activate debugging in your browser with the F12 key, and select "Console" in the debugger menu.


The console.log() Method

If your browser supports debugging, you can use console.log() to display JavaScript values in the debugger window:

例子

<!DOCTYPE html>
<html>
<body>

<h1>My First Web Page</h1>

<script>
a = 5;
b = 6;
c = a + b;
console.log(c);
</script>

</body>
</html>
亲自尝试 »

Tip: Read more about the console.log() method in our JavaScript Console Reference.


Setting Breakpoints

In the debugger window, you can set breakpoints in the JavaScript code.

At each breakpoint, JavaScript will stop executing, and let you examine JavaScript values.

After examining values, you can resume the execution of code (typically with a play button).


The debugger Keyword

The debugger keyword stops the execution of JavaScript, and calls (if available) the debugging function.

This has the same function as setting a breakpoint in the debugger.

If no debugging is available, the debugger statement has no effect.

With the debugger turned on, this code will stop executing before it executes the third line.

例子

let x = 15 * 5;
debugger;
document.getElementById("demo").innerHTML = x;
亲自尝试 »

Major Browsers' Debugging Tools

Normally, you activate debugging in your browser with F12, and select "Console" in the debugger menu.

Otherwise follow these steps:

Chrome

  • Open the browser.
  • From the menu, select "More tools".
  • From tools, choose "Developer tools".
  • Finally, select Console.

Firefox

  • Open the browser.
  • From the menu, select "Web Developer".
  • Finally, select "Web Console".

Edge

  • Open the browser.
  • From the menu, select "Developer Tools".
  • Finally, select "Console".

Opera

  • Open the browser.
  • From the menu, select "Developer".
  • From "Developer", select "Developer tools".
  • Finally, select "Console".

Safari

  • Go to Safari, Preferences, Advanced in the main menu.
  • Check "Enable Show Develop menu in menu bar".
  • When the new option "Develop" appears in the menu:
    Choose "Show Error Console".

Did You Know?

Debugging is the process of testing, finding, and reducing bugs (errors) in computer programs.
The first known computer bug was a real bug (an insect) stuck in the electronics.