代码片段

array.length

set中有这么段代码

if (Array.isArray(target) && isValidArrayIndex(key)) {
    target.length = Math.max(target.length, key)
    target.splice(key, 1, val)
    return val
  }

target.length = Math.max(target.length, key) 测试

  const a = [1,2,3]
  a.length = 5 // 不加这一行 [1, 2, 3, 7]
  a.splice(5, 1, 7)
  console.log(a) // [1, 2, 3, empty × 2, 7]