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);

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);

• %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);

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?'
);

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);
});

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);
});

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();

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)

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();

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

你可以使用 转义序列 在控制台中为文本的输出着色。转义序列是一组用于标识颜色的字符。

¥You can color the output of your text in the console by using escape sequences. An escape sequence is a set of characters that identifies a color.

示例：

¥Example:

console.log('\x1b[33m%s\x1b[0m', 'hi!');

你可以在 Node.js REPL 中尝试，它将以黄色打印 hi!。

¥You can try that in the Node.js REPL, and it will print hi! in yellow.

但是，这是执行此操作的底层方法。为控制台输出着色的最简单方法是使用库。Chalk 就是这样一个库，除了着色之外，它还可以帮助实现其他样式功能，例如使文本加粗、斜体或加下划线。

¥However, this is the low-level way to do this. The simplest way to go about coloring the console output is by using a library. Chalk is such a library, and in addition to coloring it also helps with other styling facilities, like making text bold, italic or underlined.

你将其与 npm install chalk 一起安装，然后就可以使用它了：

¥You install it with npm install chalk, then you can use it:

const chalk = require('chalk');

console.log(chalk.yellow('hi!'));

使用 chalk.yellow 比尝试记住转义码更方便，并且代码更具可读性。

¥Using chalk.yellow is much more convenient than trying to remember the escape codes, and the code is much more readable.

查看上面发布的项目链接以获取更多使用示例。

¥Check the project link posted above for more usage examples.

创建进度条

¥Create a progress bar

进度 是一个很棒的包，可以在控制台中创建进度条。使用 npm install progress 安装

¥Progress is an awesome package to create a progress bar in the console. Install it using npm install progress

此代码段创建了一个 10 步进度条，每 100 毫秒完成一步。当柱状图完成时，我们清除间隔：

¥This snippet creates a 10-step progress bar, and every 100ms one step is completed. When the bar completes we clear the interval:

const ProgressBar = require('progress');

const bar = new ProgressBar(':bar', { total: 10 });
const timer = setInterval(() => {
  bar.tick();
  if (bar.complete) {
    clearInterval(timer);
  }
}, 100);