编码技巧

函数

选自ryanmcdermott/clean-code-javascript: Clean Code concepts adapted for JavaScript

给函数命名

函数声明时作用域被提前了，这意味着在一个文件里函数很容易（太容易了）在其定义之前被引用。这样伤害了代码可读性和可维护性。如果你发现一个函数又大又复杂，这个函数妨碍这个文件其他部分的理解性，这可能就是时候把这个函数单独抽成一个模块了。别忘了给表达式显示的命名，不用管这个名字是不是由一个确定的变量推断出来的，这消除了由匿名函数在错误调用栈产生的所有假设，这在现代浏览器和类似babel编译器中很常见。具体讨论可以参考这里

// bad
function foo() {
  // ...
}

// bad
const foo = function () {
  // ...
};

// good
// lexical name distinguished from the variable-referenced invocation(s)
// 函数表达式名和声明的函数名是不一样的
const short = function longUniqueMoreDescriptiveLexicalFoo() {
  // ...
};

使用默认参数而不是||

||有很多情况比如'',false,0,null,NaN都会绕过

Bad:

function createMicrobrewery(name) {
  const breweryName = name || "Hipster Brew Co.";
  // ...
}

Good:

function createMicrobrewery(name = "Hipster Brew Co.") {
  // ...
}

使用Object.assign设置默认值

Bad:

const menuConfig = {
  title: null,
  body: "Bar",
  buttonText: null,
  cancellable: true
};

function createMenu(config) {
  config.title = config.title || "Foo";
  config.body = config.body || "Bar";
  config.buttonText = config.buttonText || "Baz";
  config.cancellable =
    config.cancellable !== undefined ? config.cancellable : true;
}

createMenu(menuConfig);

Good:

const menuConfig = {
  title: "Order",
  // User did not include 'body' key
  buttonText: "Send",
  cancellable: true
};

function createMenu(config) {
  config = Object.assign(
    {
      title: "Foo",
      body: "Bar",
      buttonText: "Baz",
      cancellable: true
    },
    config
  );

  // config now equals: {title: "Order", body: "Bar", buttonText: "Send", cancellable: true}
  // ...
}

createMenu(menuConfig);

对入参进行解构，解构保存了这些属性的临时值/引用 

// bad
function getFullName(user) {
  const firstName = user.firstName;
  const lastName = user.lastName;

  return `${firstName} ${lastName}`;
}

// good
function getFullName(user) {
  const { firstName, lastName } = user;
  return `${firstName} ${lastName}`;
}

// best
function getFullName({ firstName, lastName }) {
  return `${firstName} ${lastName}`;
}

入参避免数组，采用对象

入参最好控制在3个以内，多了就采用对象

Bad:

function createMenu(title, body, buttonText, cancellable) {
  // ...
}

createMenu("Foo", "Bar", "Baz", true);

Good:

function createMenu({ title, body, buttonText, cancellable }) {
  // ...
}

createMenu({
  title: "Foo",
  body: "Bar",
  buttonText: "Baz",
  cancellable: true
});

不要使用arguments，用...替代 

// bad
function concatenateAll() {
  const args = Array.prototype.slice.call(arguments);
  return args.join('');
}

// good
function concatenateAll(...args) {
  return args.join('');
}

入参不要含有开关参数

开关参数就相关于告诉这个函数不仅仅做一件事情了，不符合单一原则。

Bad:

function createFile(name, temp) {
  if (temp) {
    fs.create(`./temp/${name}`);
  } else {
    fs.create(name);
  }
}

Good:

function createFile(name) {
  fs.create(name);
}

function createTempFile(name) {
  createFile(`./temp/${name}`);
}

单一职责，尽可能拆分

Bad:

function emailClients(clients) {
  clients.forEach(client => {
    const clientRecord = database.lookup(client);
    if (clientRecord.isActive()) {
      email(client);
    }
  });
}

Good:

function emailActiveClients(clients) {
  clients.filter(isActiveClient).forEach(email);
}

function isActiveClient(client) {
  const clientRecord = database.lookup(client);
  return clientRecord.isActive();
}

函数名要语义化

Bad:

function addToDate(date, month) {
  // ...
}

const date = new Date();

// It's hard to tell from the function name what is added
addToDate(date, 1);

Good:

function addMonthToDate(month, date) {
  // ...
}

const date = new Date();
addMonthToDate(1, date);

控制抽象级别为一层

如果抽象级别超过一层，就应该另立函数了

Bad:

function parseBetterJSAlternative(code) {
  const REGEXES = [
    // ...
  ];

  const statements = code.split(" ");
  const tokens = [];
  REGEXES.forEach(REGEX => {
    statements.forEach(statement => {
      // ...
    });
  });

  const ast = [];
  tokens.forEach(token => {
    // lex...
  });

  ast.forEach(node => {
    // parse...
  });
}

Good:

function parseBetterJSAlternative(code) {
  const tokens = tokenize(code);
  const syntaxTree = parse(tokens);
  syntaxTree.forEach(node => {
    // parse...
  });
}

function tokenize(code) {
  const REGEXES = [
    // ...
  ];

  const statements = code.split(" ");
  const tokens = [];
  REGEXES.forEach(REGEX => {
    statements.forEach(statement => {
      tokens.push(/* ... */);
    });
  });

  return tokens;
}

function parse(tokens) {
  const syntaxTree = [];
  tokens.forEach(token => {
    syntaxTree.push(/* ... */);
  });

  return syntaxTree;
}

避免造成副作用

副作用包括修改外部变量、文件读写等等

Bad:

// Global variable referenced by following function.
// If we had another function that used this name, now it'd be an array and it could break it.
let name = "Ryan McDermott";

function splitIntoFirstAndLastName() {
  name = name.split(" ");
}

splitIntoFirstAndLastName();

console.log(name); // ['Ryan', 'McDermott'];

Good:

function splitIntoFirstAndLastName(name) {
  return name.split(" ");
}

const name = "Ryan McDermott";
const newName = splitIntoFirstAndLastName(name);

console.log(name); // 'Ryan McDermott';
console.log(newName); // ['Ryan', 'McDermott'];

注意数组的引用，不要直接修改原数组：

Bad:

const addItemToCart = (cart, item) => {
  cart.push({ item, date: Date.now() });
};

Good:

const addItemToCart = (cart, item) => {
  return [...cart, { item, date: Date.now() }];
};

不要造成原型污染

Bad:

Array.prototype.diff = function diff(comparisonArray) {
  const hash = new Set(comparisonArray);
  return this.filter(elem => !hash.has(elem));
};

Good:

class SuperArray extends Array {
  diff(comparisonArray) {
    const hash = new Set(comparisonArray);
    return this.filter(elem => !hash.has(elem));
  }
}

将条件语句封装

Bad:

if (fsm.state === "fetching" && isEmpty(listNode)) {
  // ...
}

Good:

function shouldShowSpinner(fsm, listNode) {
  return fsm.state === "fetching" && isEmpty(listNode);
}

if (shouldShowSpinner(fsmInstance, listNodeInstance)) {
  // ...
}

避免反条件判断

Bad:

function isDOMNodeNotPresent(node) {
  // ...
}

if (!isDOMNodeNotPresent(node)) {
  // ...
}

Good:

function isDOMNodePresent(node) {
  // ...
}

if (isDOMNodePresent(node)) {
  // ...
}