Introduction to browser console debugging code and JavaScript console methods

Introduction to browser console debugging code and JavaScript console methods

Browser console debugging code

The browser console (Console) is a development tool provided by the browser for executing and debugging JavaScript code in the browser. It provides an interactive environment where you can enter JavaScript code and immediately see the code execution results or output information.

In most browsers, you can open the developer tools by pressing the F12 key or right-clicking on the webpage and selecting "Inspect" (such as Microsoft Edge browser) or "Inspect element" (such as 360 browser), and find it there Console tab.

The JavaScript console has the following capabilities:

Execute JavaScript code: You can enter any JavaScript code in the console and press the Enter key to execute it. The execution results of the code will be immediately displayed in the console, which can be return values, printed information, etc.

Output information: Output information can be printed to the console by using console.log() or other console methods. This is useful when debugging code or viewing the output of a program as it runs.

Debugging code: The console provides some debugging functions, such as setting breakpoints, single-stepping, viewing variable values, etc. You can use the debugger statement to set breakpoints in the code, or use the debugging tools in the console for debugging operations.

Error message: If an error occurs during code execution, an error message will be displayed in the console. You can view error information, locate problems and debug.

Specific steps for debugging code in browser console

Enter about:blank in the browser address bar to open a blank page in the browser

Then press the F12 key to open the developer tools and enter the JS code in the console to run.

You can insert the debugger keyword at key locations in your code.

debugger keyword: When the code is executed with the debugger keyword, the browser will actively interrupt execution and open the debugger. Developers can step through the code and view important information such as variable values ​​in the console. For example:

function calculateTax(income) {
  debugger; // 中断代码执行，打开浏览器控制台的调试器

  var taxRate = 0.2;
  var tax = income * taxRate;

  console.log("计算税额中...");
  console.log("收入：" + income);
  console.log("税率：" + taxRate);
  console.log("税额：" + tax);

  console.log("计算完成。");

  return tax;
}

var finalResult = calculateTax(50000);
console.log("最终结果：" + finalResult);

The following is the situation in the Microsoft Edge browser (it is similar in other browsers, but the interface layout is different):

The debugger lets you step through your code line by line, examine the values ​​of variables, and see how your code is executing. Using a debugger can help you understand how your code is executing, identify problems, and make appropriate fixes.

It should be noted that before publishing the code, please delete all debugger statements, otherwise these statements will affect performance in the production environment.

 

What methods are available in the JavaScript console?

The JavaScript console is an interactive environment provided by developer tools for executing JavaScript code and debugging in the browser. The following is an introduction to some commonly used JavaScript console methods:

1. console.log(): You can output any type of value on the console, such as strings, numbers, Boolean values, objects, etc. like:

console.log('Hello, world!');

console.log(10 + 5);

2.console.error(): Output error message on the console. Usually used to output abnormal conditions during program execution. like:

try {

  // There may be errors in the code

  throw new Error('This is an error example!');

} catch (error) {

  console.error('An error occurred:', error);

}

A try...catch statement is used to catch code blocks where errors may occur. Use the console.error() method to output error information to the console for debugging and error troubleshooting.

3. console.warn(): Output a warning message with a warning icon to the console. Usually used to output some potential problems or information that requires attention. like:

console.warn('This is a warning!');

4.console.info(): Output a piece of prompt information with an information icon to the console. Usually used to output some prompt information. like:

console.info('This is some information.');

5. console.clear(): Clear all information on the console.

console.clear();

6. console.table(): Display object or array data on the console in tabular form. The parameter is an array or object, in which each element or attribute will be displayed as a row or column of the table. like

const data = [

  { name: 'John', age: 25 },

  { name: 'Alice', age: 30 }

];

console.table(data);

7.console.dir(): Display the properties and methods of the object in a tree structure on the console. like:

const obj = { name: 'John', age: 25 };

console.dir(obj);

8. console.time() and console.timeEnd(): Calculate the time interval required for code execution. When you need to calculate the execution time of code, call the console.time() method at the beginning of the code and the console.timeEnd() method at the end of the code, passing the same parameters. like:

console.time('myTimer');

//Execute some code

console.timeEnd('myTimer');

9.console.assert(): Determine whether an expression is true - assert the expression, and if the result is false, output an error message on the console. like:

console.assert(2 + 2 === 5, 'Error: 2 + 2 is not equal to 5');

10. console.group() and console.groupEnd(): Create a group for organizing related log information. like:

console.group('calculation result');

console.log('2 + 2 =', 2 + 2);

console.log('3 * 3 =', 3 * 3);

console.groupEnd();

console.group('Group A');

console.log('Hello from Group A');

console.groupEnd();

11.console.count(): Counts the number of calls of a specific tag. Each time the method is called, the counter is incremented by one.

for (let i = 0; i < 5; i++) {

  console.count('Count');

}

12.console.trace(): Output the call stack trace information of the current function, which is used to track the code execution location. like:

function foo() {

  bar();

}

function bar() {

  console.trace();

}

foo();

13.console.timeStamp(): Output a timestamp on the console to mark a specific event or code segment.

console.timeStamp('Start');

//Execute some code

console.timeStamp('End');

14.console.profile() and console.profileEnd(): Start and stop the JavaScript CPU analyzer for analyzing code performance issues.

console.profile('Performance');

//Execute some code

console.profileEnd('Performance');

These methods can help developers perform log output, error debugging, performance optimization, and code analysis. Please choose a suitable method according to actual needs.