javascriptcastingtypescript

Casting a number to a string in TypeScript


Which is the the best way (if there is one) to cast from number to string in Typescript?

var page_number:number = 3;
window.location.hash = page_number; 

In this case the compiler throws the error:

Type 'number' is not assignable to type 'string'

Because location.hash is a string.

window.location.hash = ""+page_number; //casting using "" literal
window.location.hash = String(number); //casting creating using the String() function

So which method is better?


Solution

  • "Casting" is different than conversion. In this case, window.location.hash will auto-convert a number to a string. But to avoid a TypeScript compile error, you can do the string conversion yourself:

    window.location.hash = ""+page_number; 
    window.location.hash = String(page_number); 
    

    These conversions are ideal if you don't want an error to be thrown when page_number is null or undefined. Whereas page_number.toString() and page_number.toLocaleString() will throw when page_number is null or undefined.

    When you only need to cast, not convert, this is how to cast to a string in TypeScript:

    window.location.hash = <string>page_number; 
    // or 
    window.location.hash = page_number as string;
    

    The <string> or as string cast annotations tell the TypeScript compiler to treat page_number as a string at compile time; it doesn't convert at run time.

    However, the compiler will complain that you can't assign a number to a string. You would have to first cast to <any>, then to <string>:

    window.location.hash = <string><any>page_number;
    // or
    window.location.hash = page_number as any as string;
    

    So it's easier to just convert, which handles the type at run time and compile time:

    window.location.hash = String(page_number); 
    

    (Thanks to @RuslanPolutsygan for catching the string-number casting issue.)