Can I type-cast a variable in block scope?

Question

I have this code:

function foo(input: Blob): void;
function foo(input: string): void;
function foo(input: any) {
    if (typeof input === "string") {

    }   
    else if (input instanceof Blob) {

    }
}

Here, I want to type-cast input in block scope rather than reassign it to another variable.

if (typeof input === "string") {
    declare input: string;
}   
else if (input instanceof Blob) {
    declare input: Blob;
}

rather than:

if (typeof input === "string") {
    var str: string = input;
}   
else if (input instanceof Blob) {
    var blob: Blob = input;
}

Is this possible in TypeScript?

Answer 1

I think the latest version of TypeScript does this automatically. But you have to use an if statement together with typeof, instanceof or a special function.

function foo(input: string|blob|whatever) {
    if(typeof input === 'string') {
        // input is now a string
        input.blobfunction(); // Error: blobfunction() is not a method of string
    } else if(input instanceof blob) {
        // input is now a blob
    } else if(isWhatever(input)) {
        // input is now a whatever
    }
}

// Probably a good idea to make this a static 'whatever.isInstance()'
function isWhatever(input: string|blob|whatever): input is whatever {
    return true; // put your test for whatever here
}

See: https://www.typescriptlang.org/docs/handbook/advanced-types.html