8个有用的JS技巧

摘要：不幸的是，它们不能很好地处理非基本类型的数组。还可以接受第二个参数，作用类似于数组的方法，用来对每个元素进行处理，将处理后的值放入返回的数组。

为了保证的可读性，本文采用意译而非直译。

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这些技巧可能大家大部分都用过了，如果用过就当作加深点映像，如果没有遇到过，就当作学会了几个技巧。

使用 grid ，需要重新创建原始数据，并且每行的列长度可能不匹配， 为了确保不匹配行之间的长度相等，可以使用Array.fill方法。

let array = Array(5).fill("");
console.log(array); // outputs (5) ["", "", "", "", ""]

ES6 提供了从数组中提取惟一值的两种非常简洁的方法。不幸的是，它们不能很好地处理非基本类型的数组。在本文中，主要关注基本数据类型。

const cars = [
    "Mazda", 
    "Ford", 
    "Renault", 
    "Opel", 
    "Mazda"
]
const uniqueWithArrayFrom = Array.from(new Set(cars));
console.log(uniqueWithArrayFrom); // outputs ["Mazda", "Ford", "Renault", "Opel"]

const uniqueWithSpreadOperator = [...new Set(cars)];
console.log(uniqueWithSpreadOperator);// outputs ["Mazda", "Ford", "Renault", "Opel"]

对象合并是很常见的事情，我们可以使用新的ES6特性来更好，更简洁的处理合并的过程。

// merging objects
const product = { name: "Milk", packaging: "Plastic", price: "5$" }
const manufacturer = { name: "Company Name", address: "The Company Address" }

const productManufacturer = { ...product, ...manufacturer };
console.log(productManufacturer); 
// outputs { name: "Company Name", packaging: "Plastic", price: "5$", address: "The Company Address" }

// merging an array of objects into one
const cities = [
    { name: "Paris", visited: "no" },
    { name: "Lyon", visited: "no" },
    { name: "Marseille", visited: "yes" },
    { name: "Rome", visited: "yes" },
    { name: "Milan", visited: "no" },
    { name: "Palermo", visited: "yes" },
    { name: "Genoa", visited: "yes" },
    { name: "Berlin", visited: "no" },
    { name: "Hamburg", visited: "yes" },
    { name: "New York", visited: "yes" }
];

const result = cities.reduce((accumulator, item) => {
  return {
    ...accumulator,
    [item.name]: item.visited
  }
}, {});

console.log(result);
/* outputs
Berlin: "no"
Genoa: "yes"
Hamburg: "yes"
Lyon: "no"
Marseille: "yes"
Milan: "no"
New York: "yes"
Palermo: "yes"
Paris: "no"
Rome: "yes"
*/

另一种数组 map 的实现的方式，不用 Array.map。

Array.from 还可以接受第二个参数，作用类似于数组的map方法，用来对每个元素进行处理，将处理后的值放入返回的数组。如下：

const cities = [
    { name: "Paris", visited: "no" },
    { name: "Lyon", visited: "no" },
    { name: "Marseille", visited: "yes" },
    { name: "Rome", visited: "yes" },
    { name: "Milan", visited: "no" },
    { name: "Palermo", visited: "yes" },
    { name: "Genoa", visited: "yes" },
    { name: "Berlin", visited: "no" },
    { name: "Hamburg", visited: "yes" },
    { name: "New York", visited: "yes" }
];

const cityNames = Array.from(cities, ({ name}) => name);
console.log(cityNames);
// outputs ["Paris", "Lyon", "Marseille", "Rome", "Milan", "Palermo", "Genoa", "Berlin", "Hamburg", "New York"]

不再需要根据一个条件创建两个不同的对象，可以使用展开运算符号来处理。

nst getUser = (emailIncluded) => {
  return {
    name: "John",
    surname: "Doe",
    ...emailIncluded && { email : "john@doe.com" }
  }
}

const user = getUser(true);
console.log(user); // outputs { name: "John", surname: "Doe", email: "john@doe.com" }

const userWithoutEmail = getUser(false);
console.log(userWithoutEmail); // outputs { name: "John", surname: "Doe" }

有时候一个对象包含很多属性，而我们只需要其中的几个，这里可以使用解构方式来提取我们需要的属性。如一个用户对象内容如下：

const rawUser = {
   name: "John",
   surname: "Doe",
   email: "john@doe.com",
   displayName: "SuperCoolJohn",
   joined: "2016-05-05",
   image: "path-to-the-image",
   followers: 45
   ...
}

我们需要提取出两个部分，分别是用户及用户信息，这时可以这样做:

let user = {}, userDetails = {};
({ name: user.name, surname: user.surname, ...userDetails } = rawUser);

console.log(user); // outputs { name: "John", surname: "Doe" }
console.log(userDetails); // outputs { email: "john@doe.com", displayName: "SuperCoolJohn", joined: "2016-05-05", image: "path-to-the-image", followers: 45 }

早期，如果属性名需要是动态的，我们首先必须声明一个对象，然后分配一个属性。这些日子已经过去了，有了ES6特性，我们可以做到这一点。

const dynamic = "email";
let user = {
    name: "John",
    [dynamic]: "john@doe.com"
}
console.log(user); // outputs { name: "John", email: "john@doe.com" }

在用例中，如果正在构建一个基于模板的helper组件，那么这一点就会非常突出，它使动态模板连接容易得多。

const user = {
  name: "John",
  surname: "Doe",
  details: {
    email: "john@doe.com",
    displayName: "SuperCoolJohn",
    joined: "2016-05-05",
    image: "path-to-the-image",
    followers: 45
  }
}

const printUserInfo = (user) => { 
  const text = `The user is ${user.name} ${user.surname}. Email: ${user.details.email}. Display Name: ${user.details.displayName}. ${user.name} has ${user.details.followers} followers.`
  console.log(text);
}

printUserInfo(user);
// outputs "The user is John Doe. Email: john@doe.com. Display Name: SuperCoolJohn. John has 45 followers."

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