使用 Node.js 输出到命令行

¥Output to the command line using Node.js

使用控制台模块的基本输出

¥Basic output using the console module

Node.js 提供了一个 console 模块，它提供了大量非常有用的与命令行交互的方法。

¥Node.js provides a console module which provides tons of very useful ways to interact with the command line.

它与你在浏览器中找到的 console 对象基本相同。

¥It is basically the same as the console object you find in the browser.

最基本和最常用的方法是 console.log()，它将传递给它的字符串打印到控制台。

¥The most basic and most used method is console.log(), which prints the string you pass to it to the console.

如果你传递一个对象，它会将其渲染为字符串。

¥If you pass an object, it will render it as a string.

你可以将多个变量传递给 console.log，例如：

¥You can pass multiple variables to console.log, for example:

const x = 'x';
const y = 'y';

console.log(x, y);

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Node.js 将打印两者。

¥and Node.js will print both.

我们还可以通过传递变量和格式说明符来格式化漂亮的短语。

¥We can also format pretty phrases by passing variables and a format specifier.

例如：

¥For example:

console.log('My %s has %d ears', 'cat', 2);

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%s 将变量格式化为字符串

¥%s format a variable as a string
%d 将变量格式化为数字

¥%d format a variable as a number
%i 仅将变量格式化为其整数部分

¥%i format a variable as its integer part only
%o 将变量格式化为对象

¥%o format a variable as an object

示例：

¥Example:

console.log('%o', Number);

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清除控制台

¥Clear the console

console.clear() 清除控制台（行为可能取决于所使用的控制台）

¥console.clear() clears the console (the behavior might depend on the console used)

计数元素

¥Counting elements

console.count() 是一种方便的方法。

¥console.count() is a handy method.

获取此代码：

¥Take this code:

const x = 1;
const y = 2;
const z = 3;

console.count(
  'The value of x is ' + x + ' and has been checked .. how many times?'
);

console.count(
  'The value of x is ' + x + ' and has been checked .. how many times?'
);

console.count(
  'The value of y is ' + y + ' and has been checked .. how many times?'
);

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发生的事情是 console.count() 将计算打印字符串的次数，并在其旁边打印计数：

¥What happens is that console.count() will count the number of times a string is printed, and print the count next to it:

你可以只计算苹果和橘子：

¥You can just count apples and oranges:

const oranges = ['orange', 'orange'];
const apples = ['just one apple'];

oranges.forEach(fruit => {
  console.count(fruit);
});
apples.forEach(fruit => {
  console.count(fruit);
});

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重置计数

¥Reset counting

console.countReset() 方法重置与 console.count() 一起使用的计数器。

¥The console.countReset() method resets counter used with console.count().

我们将使用苹果和橙子示例来演示这一点。

¥We will use the apples and orange example to demonstrate this.

const oranges = ['orange', 'orange'];
const apples = ['just one apple'];

oranges.forEach(fruit => {
  console.count(fruit);
});
apples.forEach(fruit => {
  console.count(fruit);
});

console.countReset('orange');

oranges.forEach(fruit => {
  console.count(fruit);
});

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注意对 console.countReset('orange') 的调用如何将值计数器重置为零。

¥Notice how the call to console.countReset('orange') resets the value counter to zero.

打印堆栈跟踪

¥Print the stack trace

在某些情况下，打印函数的调用堆栈跟踪可能很有用，也许可以回答你是如何到达代码的那部分的问题？

¥There might be cases where it's useful to print the call stack trace of a function, maybe to answer the question how did you reach that part of the code?

你可以使用 console.trace() 执行此操作：

¥You can do so using console.trace():

const function2 = () => console.trace();
const function1 = () => function2();
function1();

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这将打印堆栈跟踪。如果我们在 Node.js REPL 中尝试此操作，则会打印以下内容：

¥This will print the stack trace. This is what's printed if we try this in the Node.js REPL:

Trace
    at function2 (repl:1:33)
    at function1 (repl:1:25)
    at repl:1:1
    at ContextifyScript.Script.runInThisContext (vm.js:44:33)
    at REPLServer.defaultEval (repl.js:239:29)
    at bound (domain.js:301:14)
    at REPLServer.runBound [as eval] (domain.js:314:12)
    at REPLServer.onLine (repl.js:440:10)
    at emitOne (events.js:120:20)
    at REPLServer.emit (events.js:210:7)

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计算所花费的时间

¥Calculate the time spent

使用 time() 和 timeEnd()，你可以轻松计算出一个函数运行所需的时间

¥You can easily calculate how much time a function takes to run, using time() and timeEnd()

const doSomething = () => console.log('test');
const measureDoingSomething = () => {
  console.time('doSomething()');
  // do something, and measure the time it takes
  doSomething();
  console.timeEnd('doSomething()');
};
measureDoingSomething();

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stdout 和 stderr

¥stdout and stderr

正如我们所见，console.log 非常适合在控制台中打印消息。这就是所谓的标准输出，或 stdout。

¥As we saw console.log is great for printing messages in the Console. This is what's called the standard output, or stdout.

console.error 打印到 stderr 流。

¥console.error prints to the stderr stream.

它不会出现在控制台中，但会出现在错误日志中。

¥It will not appear in the console, but it will appear in the error log.

为输出着色

¥Color the output

注意此部分资源是使用版本 22.11 设计的，该版本将 styleText 标记为“主动开发”。

¥This part of the resource is designed with version 22.11 which notes styleText as ‘Active development’.

在许多情况下，你会倾向于粘贴某些文本以在终端上获得不错的输出。

¥In many cases, you will be tempted to paste certain text to get a nice output at the terminal.

node:util 模块提供了 styleText 函数。让我们来看看如何使用它。

¥There is a styleText function provided by the node:util module. Let's discover how to use it.

首先，你需要从 node:util 模块导入 styleText 函数：

¥First of all, you need to import the styleText function from the node:util module:

import { styleText } from 'node:util';

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然后，你可以使用它来设置文本样式：

¥Then, you can use it to style your text:

console.log(
  styleText(['red'], 'This is red text ') +
    styleText(['green, bold'], 'and this is green bold text ') +
    'this is normal text'
);

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第一个参数是样式数组，第二个参数是你想要设置样式的文本。我们邀请你阅读文档

¥The first argument is an array of styles, and the second argument is the text you want to style. We invite you to read the docs

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