CODEWITHSUNDEEP
javascripttypescript
user11680003
user11680003

Reputation:

No type conversion is performed by a type assertion?

I'm new to Typescript, just a question on type assertion.

Below is an example from my textbook:

function calculateTax(amount: number, format: boolean): string | number {
   const calcAmount = amount * 1.2;
   return format ? `$${calcAmount.toFixed(2)}` : calcAmount;
}

let taxNumber: string | number = calculateTax(100, false);
let taxString: string | number = calculateTax(100, true);

and the book which says:

No type conversion is performed by a type assertion, which only tells the compiler what type it should apply to a value for the purposes of type checking. SO they are equivalent to these statements

let taxNumber: number = calculateTax(100, false) as number;
let taxString: string = calculateTax(100, true) as string;

I'm confused, so calculateTax(100, false) is string | number,when we do the assertion by append as number to it, calculateTax(100, false) as number becomes 'nu,ber' type, that's how taxNumber can be consider as number as let taxNumber: number, so why it is no type conversion involved?

Upvotes: 0

Views: 107

Answers (1)

Nicholas Tower
Nicholas Tower

Reputation: 84922

so why it is no type conversion involved?

They're pointing out that when you run the code, whatever the javascript code returns is what it returns. Doing as number doesn't change the behavior of the code, it's just a way of telling typescript "i know better than you, so don't check my types here".

If you truly do know better than typescript does, then type assertions are ok. But if you make a mistake (eg, if you say it's a number when it isn't), you'll have a bug in your code that typescript won't be able to tell you about.

By the way, you can do safer types on this function by using overloads:

function calculateTax(amount: number, format: true): string;
function calculateTax(amount: number, format: false): number;
function calculateTax(amount: number, format: boolean): string | number {
   const calcAmount = amount * 1.2;
   return format ? `$${calcAmount.toFixed(2)}` : calcAmount;
}

let taxNumber: number = calculateTax(100, false);
let taxString: string = calculateTax(100, true);

Upvotes: 2

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