Why concatenating string and undefined does Not raise an error in TypeScript

For example, this code is not valid:

function fn1(a: number, b?: number) {
  console.log(a + b);  // error 'b' is possibly 'undefined'.(18048)
}
fn1(5);

But this code is valid:

function fn2(a: string, b?: string) {
  console.log(a + b);  // valid code
}
fn2('Hi');

Why doesn't Typescript in strict mode raise an error in the second case?

The authoritative answer to this question can be found in a comment on microsoft/TypeScript#2093 by the development lead of the TypeScript team at Microsoft:

> There's a huge amount of code out there that does stuff like this:
>
> console.log("Foo is " + x);
>
> where x might be a function, undefined or null, in which case explicitly calling toString() would be a bad idea. We made the decision that adding anything to a string is so often done that it is not ever an error.
>
> [This suggestion] doesn't meet the bar for introducing a breaking change in the language.

So that's the answer: whether or not you agree with it, the TypeScript team didn't want to break existing real-world code that concatenates random stuff to strings.

In that issue and similar issues (such as microsoft/TypeScript#7989, microsoft/TypeScript#42983, and microsoft/TypeScript#48962), it is mentioned that those who really want to see this functionality and are willing to use ESLint could enable the @typescript-eslint/restrict-plus-operands rule which will issue errors when using + to concatenate values of different types. So there are options out there, even if they are not part of TypeScript directly.

Because in concatenation, if the first value is string JavaScript will force the second value to be string that is why the compiler does not care if b may be undefined, since you can concatenate anything with a string.

However, if you try to make a of type number, or try any other operation the error will occur :

function fn2(a: number, b?: string) {
  console.log(a + b);  // 'b' is possibly 'undefined'
}
fn2(5);

I have a different thought

I think string + undefined is acceptable because it is a valid JS operation

but number cannot concatenate undefined surprise me because it is also a valid JS operation