变量声明

声明变量时，请使用 const、let 关键字，如果没有写关键字，变量就会暴露在全局上下文中，这样很可能会和现有变量冲突，另外，也很难明确该变量的作用域是什么。这里推荐使用 const 来声明变量，我们需要避免全局命名空间的污染。eslint: no-undef prefer-const

// bad
demo = new Demo()

// good
const demo = new Demo()

将所有的 const 和 let 分组

// bad
let a
const b
let c
const d
let e

// good
const b
const d
let a
let c
let e

变量不要进行链式赋值

原因：变量链式赋值会创建隐藏的全局变量

// bad
(function example() {
  // JavaScript interprets this as
  // let a = ( b = ( c = 1 ) );
  // The let keyword only applies to variable a; variables b and c become
  // global variables.
  let a = b = c = 1
}())

console.log(a) // throws ReferenceError
console.log(b) // 1
console.log(c) // 1

// good
(function example() {
  let a = 1
  let b = a
  let c = a
}())

console.log(a) // throws ReferenceError
console.log(b) // throws ReferenceError
console.log(c) // throws ReferenceError

// the same applies for `const`

不允许出现未被使用的变量，eslint: no-unused-vars

原因：声明但未被使用的变量通常是不完全重构犯下的错误.这种变量在代码里浪费空间并会给读者造成困扰

// bad

var some_unused_var = 42

// Write-only variables are not considered as used.
var y = 10
y = 5

// A read for a modification of itself is not considered as used.
var z = 0
z = z + 1

// Unused function arguments.
function getX (x, y) {
  return x
}

// good

function getXPlusY (x, y) {
  return x + y
}

const x = 1
const y = a + 2

alert(getXPlusY(x, y))

// 'type' is ignored even if unused because it has a rest property sibling.
// This is a form of extracting an object that omits the specified keys.
const { type, ...coords } = data
// 'coords' is now the 'data' object without its 'type' property.