Node.js变量静态类型如何工作

Question

I sometimes see Node code with variable types declared, such as:

const st: string = "hello world";

What version of node supports this? Where can I find the documentation on this (this is "static typing" right?)? Can I use this in method variables?

Answer 1

That's not JavaScript, it's TypeScript ; see the link for documentation. TypeScript is basically JavaScript with a type system overlaid on it. It's then compiled to JavaScript by the TypeScript compiler. (The compiler also tends to support new JavaScript features before they're natively supported in JavaScript engines like the one in V8. For instance, TypeScript's compiler currently supports public class fields , which aren't in a JavaScript spec yet but are likely to be at some stage.)

Can I use this in method variables?

Yes, absolutely:

function foo() : void {
    let a : number = 42;
    let q : string = "Life, the Universe, and Everything";

    // ...
}

If you tried to assign "glarb" to a , the TypeScript compiler would give you an error because a is declared as number . Similarly, trying to return something from the function would raise an error because the function is declared to have a void return type (doesn't return anything).

That compiles to:

function foo() {
    var a = 42;
    var q = "Life, the Universe, and Everything";
    // ...
}

...which can run in any JavaScript environment.