数组

4.1 用字面语法来创建数组

eslint:no-array-constructor

// bad
const items=new Array()

// good
const items=[]

4.2 使用Array.push而不是直接赋值来给数组添加元素

//bad 
someStack([someStack.length])='fdsafdsf'

//good
someStack.push('fdsafgdsf')

4.3 用数组的扩展运算符...来复制数组

// bad
const len=items.length
const itemsCopy=[]
let i

for(i=0;i<len;i+=1){
  itemsCopy[i]=items[i]
}

// good
const itemsCopy=[...items]

4.4 如果要讲一个类数组对象转换成数组，使用扩展运算符...代替Array.from

const foo=document.querySelectorAll('.foo')

//good
const nodes=Array.from(foo)

//best
const nodes=[...foo]

4.5 用Array.from而不是扩展运算符...来映射迭代器，因为这样避免了创建中间数组

//bad
const baz=[...foo].map(bar)

//good
const baz=Array.from(foo,bar)

4.6 在数组的回调函数里使用return声明。如果函数体包含了一个单一声明，这样的话返回的是一个没有副作用的表达式，此时可以省去return

eslint:array-callback-return

// good
[1, 2, 3].map((x) => {
  const y = x + 1;
  return x * y;
});

// good
[1, 2, 3].map(x => x + 1);

// bad - no returned value means `memo` becomes undefined after the first iteration
[[0, 1], [2, 3], [4, 5]].reduce((memo, item, index) => {
  const flatten = memo.concat(item);
  memo[index] = flatten;
});

// good
[[0, 1], [2, 3], [4, 5]].reduce((memo, item, index) => {
  const flatten = memo.concat(item);
  memo[index] = flatten;
  return flatten;
});

// bad
inbox.filter((msg) => {
  const { subject, author } = msg;
  if (subject === 'Mockingbird') {
    return author === 'Harper Lee';
  } else {
    return false;
  }
});

// good
inbox.filter((msg) => {
  const { subject, author } = msg;
  if (subject === 'Mockingbird') {
    return author === 'Harper Lee';
  }

  return false;
});

4.7 如果一个数组有多行，则在打开和关闭数组的括号前面使用换行符

// bad
const arr = [
  [0, 1], [2, 3], [4, 5],
];

const objectInArray = [{
  id: 1,
}, {
  id: 2,
}];

const numberInArray = [
  1, 2,
];

// good
const arr = [[0, 1], [2, 3], [4, 5]];

const objectInArray = [
  {
    id: 1,
  },
  {
    id: 2,
  },
];

const numberInArray = [
  1,
  2,
];