在TS [ts（7053）]

Question

让我们看一下简单的代码：

let obj = {
  a:1,
  b:2,
  c:3
}
for (let i in obj) {
  console.log(obj[i]);
}

我们在obj[i]上收到错误消息：

Element implicitly has an 'any' type because expression of type 'string' 
can't be used to index type '{ a: number; b: number; c: number; }'.
  No index signature with a parameter of type 'string' was found 
on type '{ a: number; b: number; c: number; }'. ts(7053)

怎么了？