⑤ ES7 数组&数值拓展

1 数组扩展：Array.prototype.includes(searchElement, fromIndex)

fromIndex 从指定索引位置开始查找

1.1 返回值：Boolean

const arr = ['es6', 'es7', 'es8']
console.log(arr.includes('es7')); // true
console.log(arr.includes('es9')); // false
console.log(arr.includes('es7', 1)); // true
console.log(arr.includes('es7', 2)); // false
console.log(arr.includes('es7', -1)); // false
console.log(arr.includes('es7', -2)); // true

1.2 includes VS indexOf

1. 检查基本数据类型

console.log(arr.indexOf('es7')); // 1
console.log(arr.indexOf('es7') > -1); // true

2. 检查引用数据类型 -- 检查不了

const arr = ['es6', ['es7', 'es8'], 'es9']
console.log(arr.includes(['es7', 'es8'])); // false
console.log(arr.indexOf(['es7', 'es8'])); // -1

3. 能否检测 NaN

const arr = ['es6', 'es7', NaN, 'es8']
console.log(arr.includes(NaN)); // true
console.log(arr.indexOf(NaN)); // -1

4. includes 和 indexOf 是严格检查

const arr = ['es6', 'es7', NaN, 'es8', 2]
console.log(arr.includes('2')); // false
console.log(arr.indexOf('2')); // -1

2 数组扩展：幂运算符

• 等同于 Math.pow()

2.1 ES5

• 2^10 -> 1024

function pow(x, y) {
  let res = 1
  for(let i = 0; i < y; i++) {
    res *= x
  }
  return res
}
console.log(pow(2, 10)); // 1024

console.log(Math.pow(2, 10)); // 1024

2.2 幂运算符**

console.log(2 ** 10); // 1024