打字稿是否仅在类型声明后为变量赋值时才返回变量类型？

Question

In type script, is it possible to get the type of a variable before assigning any value to it?

let a: number;
console.log(typeof a);

returns undefined

where as

let a: number;
a = 10;
console.log(typeof a);

returns number

Answer 1

No.

Types are erased in TypeScript. They don't exist at runtime.

In fact, TypeScript has no runtime semantics at all . TypeScript cannot possibly change the result of the code from what the result would be in plain ECMAScript.

What you are using in your code snippet is the ECMAScript typeof operator, it has nothing to do with TypeScript, and it doesn't know anything about TypeScript.