A brief discussion on how to use console in Node.js

How to use the console in Node.js? This article will introduce you to how to use the console in Node.js and learn about most of the methods in the console class. I hope it will be helpful to you!

In this article, we will learn how to use most of the methods in the Node.js console class more efficiently. [Recommended learning: "nodejs Tutorial"]

Prerequisite

This tutorial is in Chrome browser version 70.0.3538.77 and Node.js 8.11 Verified in version .3.

Use console.log, console.info, and console.debug

# The ##console.log method prints to standard output, whether it is a terminal or a browser console. It outputs a string by default, but can be combined with a template string to modify what it returns. The

console.log(string, substitution)

console.info and console.debug methods are operationally identical to console.log.

You can use

console.debug by default in the Firefox browser console, but to use it in Chrome you must Set the log level to Verbose in the All Levels menu.

The parameters in the template string are passed to

util.format which will process these parameters with the corresponding conversion value Replace each replacement token.

Supported replacement tokens are.

%s

const msg = `Using the console class`;
console.log('%s', msg);
console.log(msg);

This code will output the following.

OutputUsing the console class
Using the console class

%s is the default replacement mode.

%d,%f,%i,%o

const circle = (radius = 1) => {
  const profile = {};
  const pi = 22/7;
  profile.diameter = 2 * pi * radius;
  profile.circumference = pi * radius * 2;
  profile.area = pi * radius * 2;
  profile.volume = 4/3 * pi * radius^3;

  console.log('This circle has a radius of: %d cm', radius);
  console.log('This circle has a circumference of: %f cm', profile.diameter);
  console.log('This circle has an area of: %i cm^2', profile.area);
  console.log('The profile of this cirlce is: %o', profile);
  console.log('Diameter %d, Area: %f, Circumference %i', profile.diameter, profile.area, profile.circumference)
}

circle();

This code will output the following content.

OutputThis circle has a radius of: 1 cm
This circle has a circumference of: 6.285714285714286 cm
This circle has an area of: 6 cm^2
The profile of this cirlce is: {diameter: 6.285714285714286, circumference: 6.285714285714286, area: 6.285714285714286, volume: 7}
Diameter 6, Area: 6.285714285714286, Circumference 6

• %d will be replaced by a number (integer or float).
• %f will be replaced by a float value.
• %i will be replaced by an integer.
• %o will be replaced by an object.

%o is especially convenient because we don’t need to use JSON.stringify to expand our object since it displays by default All properties of the object.

Please note that you can use as many token substitutions as you like. They are simply replaced in the order you pass the arguments.

%c

This replacement token applies CSS styles to the text being replaced.

console.log('LOG LEVEL: %c OK', 'color: green; font-weight: normal');
console.log('LOG LEVEL: %c PRIORITY', 'color: blue; font-weight: medium');

console.log('LOG LEVEL: %c WARN', 'color: red; font-weight: bold');
console.log('ERROR HERE');

This code will output the following.

After the

%c replacement tag, the text we pass to console.log will be affected by the style, but the previous The text remains as is, without styles.

Use console.table

The first parameter passed to it is the data to be returned in the form of a table. The second is an array of selected columns to display.

console.table(tabularData, [properties])

This method will format the input passed to it into a table and then log the input object after the table representation.

Array

If an array is passed to it as data, each element in the array will be a row in the table.

const books = ['The Silmarillion', 'The Hobbit', 'Unfinished Tales'];
console.table(books);

For a simple array with a depth of 1, the first column in the table has the header index. Under the Index heading in the first column is the index of the array, and the items in the array are listed under the Value heading in the second column.

This is the case of nested arrays.

const authorsAndBooks = [['Tolkien', 'Lord of The Rings'],['Rutger', 'Utopia For Realists'], ['Sinek', 'Leaders Eat Last'], ['Eyal', 'Habit']];
console.table(authorsAndBooks);

对象

对于深度为1的对象，对象的键会列在索引标题下，而对象中的值则列在第二列标题下。

const inventory = { apples: 200, mangoes: 50, avocados: 300, kiwis: 50 };
console.table(inventory);

对于嵌套的对象。

const forexConverter = { asia: { rupee: 1.39, renminbi: 14.59 , ringgit: 24.26 }, africa: { rand: 6.49, nakfa: 6.7 , kwanza:0.33 }, europe: { swissfranc: 101.60, gbp: 130, euro: 115.73 } };
console.table(forexConverter);

还有一些嵌套的对象。

const workoutLog = { Monday: { push: 'Incline Bench Press', pull: 'Deadlift'}, Wednesday: { push: 'Weighted Dips', pull: 'Barbell Rows'}};
console.table(workoutLog);

这里，我们指定只想看到列推下的数据。

console.table(workoutLog, 'push');

要对某一列下的数据_进行排序_，只需点击该列标题。

很方便，是吗？

试着把一个带有一些数值的对象作为数组传给console.table!

使用console.dir

传给这个函数的第一个参数是要记录的对象，而第二个参数是一个包含选项的对象，这些选项将定义结果输出的格式，或者对象中的哪些属性将被显示。

返回的是一个由node的util.expect函数格式化的对象。

输入对象中的嵌套或子对象可在披露三角形下展开。

console.dir(object, options);
// where options = { showHidden: true ... }

让我们看看这个动作。

const user = {
  details: {
    name: {
      firstName: 'Immanuel',
      lastName: 'Kant'
    },
    height: `1.83m"`,
    weight: '90kg',
    age: '80',
    occupation: 'Philosopher',
    nationality: 'German',
    books: [
      {
        name: 'Critique of Pure Reason',
        pub: '1781',
      },
      {
        name: 'Critique of Judgement',
        pub: '1790',
      },
      {
        name: 'Critique of Practical Reason',
        pub: '1788',
      },
      {
        name: 'Perpetual Peace',
        pub: '1795',
      },
    ],
    death: '1804'
  }
}

console.dir(user);

这里是Chrome浏览器的控制台。

使用console.dirxml

这个函数将为传递给它的XML/HTML渲染一棵交互式树。如果无法渲染节点树，它默认为一个Javascript对象。

console.dirxml(object|nodeList);

与console.dir ，渲染的树可以通过点击披露三角形来扩展，在其中可以看到子节点。

它的输出类似于我们在浏览器的Elements标签下发现的输出。

这是我们从维基百科页面传入一些HTML时的情况。

const toc = document.querySelector('#toc');
console.dirxml(toc);

让我们从这个网站上的一个页面传入一些HTML。

console.dirxml(document)

这就是我们传入一个对象时的情况。

试着在一些HTML上调用console.dir ，看看会发生什么。

使用console.assert

传递给函数的第一个参数是一个要测试是否为真值的值。所有传递的其他参数被认为是信息，如果传递的值没有被评估为真值，就会被打印出来。

Node REPL将抛出一个错误，停止后续代码的执行。

console.assert(value, [...messages])

下面是一个基本的例子。

console.assert(false, 'Assertion failed');

OutputAssertion failed: Assertion failed

现在，让我们找点乐子。我们将建立一个小型测试框架，使用console.assert

const sum = (a = 0, b = 0) => Number(a) + Number(b);

function test(functionName, actualFunctionResult, expected) {
  const actual = actualFunctionResult;
  const pass = actual === expected;
  console.assert(pass, `Assertion failed for ${functionName}`);
  return `Test passed ${actual} === ${expected}`;
}

console.log(test('sum', sum(1,1), 2)); // Test passed 2 === 2
console.log(test('sum', sum(), 0));    // Test passed 0 === 0
console.log(test('sum', sum, 2));      // Assertion failed for sum
console.log(test('sum', sum(3,3), 4)); // Assertion failed for sum

使用console.error 和console.warn

这两个基本上是相同的。它们都会打印传递给它们的任何字符串。

然而，console.warn 在信息传递之前会打印出一个三角形的警告符号。

console.warn(string, substitution);

而console.error ，在信息传递前打印出一个危险符号。

console.error(string, substitution);

让我们注意到，字符串替换可以用与console.log 方法相同的方式来应用。

下面是一个使用console.error 的迷你日志函数。

const sum = (a = 0, b = 0) => Number(a) + Number(b);

function otherTest(actualFunctionResult, expected) {
  if (actualFunctionResult !== expected) {
    console.error(new Error(`Test failed ${actualFunctionResult} !== ${expected}`));
  } else {
    // pass
  }
}

otherTest(sum(1,1), 3);

使用console.trace(label)

这个控制台方法将打印字符串Trace: ，后面是传递给函数的标签，然后是堆栈跟踪到函数的当前位置。

function getCapital(country) {
  const capitalMap = {
    belarus: 'minsk', australia: 'canberra', egypt: 'cairo', georgia: 'tblisi', latvia: 'riga', samoa: 'apia'
  };
  console.trace('Start trace here');
  return Object.keys(capitalMap).find(item => item === country) ? capitalMap[country] : undefined;
}

console.log(getCapital('belarus'));
console.log(getCapital('accra'));

使用console.count(label)

Count将开始并递增一个名为label 的计数器。

让我们建立一个单词计数器来看看它是如何工作的。

const getOccurences = (word = 'foolish') => {
  const phrase = `Oh me! Oh life! of the questions of these recurring, Of the endless trains of the faithless, of cities fill’d with the foolish, Of myself forever reproaching myself, for who more foolish than I, and who more faithless?`;

  let count = 0;
  const wordsFromPhraseArray = phrase.replace(/[,.!?]/igm, '').split(' ');
  wordsFromPhraseArray.forEach((element, idx) => {
    if (element === word) {
      count ++;
      console.count(word);
    }
  });
  return count;
}

getOccurences();

getOccurences('foolish');

在这里，我们看到foolish 这个词被记录了两次。该词在短语中每出现一次就记录一次。

[secondary_label]
foolish: 1
foolish: 2
2

我们可以用这个方法来查看一个函数被调用了多少次，或者我们的代码中的某一行被执行了多少次。

使用console.countReset(label)

顾名思义，这将重置一个计数器，该计数器有一个由console.count 方法设置的label 。

const getOccurences = (word = 'foolish') => {
  const phrase = `Oh me! Oh life! of the questions of these recurring, Of the endless trains of the faithless, of cities fill’d with the foolish, Of myself forever reproaching myself, for who more foolish than I, and who more faithless?`;

  let count = 0;
  const wordsFromPhraseArray = phrase.replace(/[,.!?]/igm, '').split(' ');
  wordsFromPhraseArray.forEach((element, idx) => {
    if (element === word) {
      count ++;
      console.count(word);
      console.countReset(word);
    }
  });
  return count;
}

getOccurences();

getOccurences('foolish');

[secondary_label]
foolish: 1
foolish: 1
2

我们可以看到，我们的getOccurences 函数返回2，因为在这句话中确实有两次出现foolish ，但由于我们的计数器在每次匹配时都被重置，所以它记录了两次foolish: 1 。

使用console.time(label) 和console.timeEnd(label)

console.time 函数启动一个定时器，并将label 作为参数提供给该函数，而console.timeEnd 函数停止一个定时器，并将label 作为参数提供给该函数。

console.time(&#39;<timer-label>&#39;);
console.timeEnd(&#39;<timer-label>&#39;);

我们可以通过向两个函数传递相同的label 名称来计算出运行一个操作所需的时间。

const users = ['Vivaldi', 'Beethoven', 'Ludovico'];

const loop = (array) => {
  array.forEach((element, idx) => {
    console.log(element);
  })
}

const timer = () => {
  console.time('timerLabel');
  loop(users);
  console.timeEnd('timerLabel');
}

timer();

我们可以看到计时器停止后显示的计时器标签与时间值相对应。

OutputVivaldi
Beethoven
Ludovico
timerLabel: 0.69091796875ms

循环函数花了0.6909ms完成了对数组的循环操作。

结论

最后，我们已经来到了本教程的结尾。

请注意，本教程没有涵盖console 类的非标准使用，如console.profile ，console.profileEnd ，和console.timeLog 。

更多编程相关知识，请访问：编程入门！！

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