TypeScript 中最初未定义的变量的正确类型是什么？

Question

I have a variable we'll call a in this example:

let a;

I only want to assign it a value if it meets one of the requirements in the switch statement:

switch(someOtherVar) {
  case "a": {
    a = "123";
    break;
  }
  case "b": {
    a = "456";
    break;
  }
}

I then want to check if it has a value like so:

if (a) {
  // ...do something
}

This means a can be a string or undefined , and it starts undefined . If I don't declare the type then it throws the following error:

Variable 'a' implicitly has an 'any' type, but a better type may be inferred from usage.ts(7043)

I'm not sure what the correct way to declare the type would be in this instance:

1. let a: string | undefined; // this seems to be the best way
2. let a = undefined; // this declares it as any which I don't want
3: let a: string; // is this correct if it can be undefined?

Answer 1

The correct type is string | undefined string | undefined . Under srictNullChecks , the type string does not contain the undefined value or the null value. These two values have their own types undefined and null . So to have a variable that is string or undefined using a union is appropriate.

Note that control flow analysis does a pretty good job of inferring the type for your specific use case if the variable is not referenced in a closure

let a = undefined; // any here 

switch(someOtherVar) {
  case "a": {
    a = "123";
    break;
  }
  case "b": {
    a = "456";
    break;
  }
}
// function x() { console.log(a)}; // if referenced in another function you get an error

a; // but the type of a is string | undefined 

Playground Link

Answer 2

Your first suggestion is correct because a can't be any .

You second suggestion still has the type any . You would need to it as follows but that's basically the same as your first suggestion.

let a: string | undefined = undefined;

Your third suggestion will cause problems because the engine does not expect the value to be undefined.

If you know it's a string, you could declare it as empty string and later check the length of the string.

let a: string = "";

if (a.length > 0) {
  // do something...
}