Although TypeScript is strongly typed, can you explain why the code below outputs 12 instead of 3?
function add_numbers(a: number, b: number){
return a + b;
}
var a = '1';
var b = 2;
var result = add_numbers(<number><any>a, b);
console.log(result);
While Typescript offers strong typing at compile time, it is important to note that it may not always prevent runtime errors if the code is poorly written. By overriding types using <any>, you risk tricking the compiler and allowing for normal javascript behavior such as adding a string to a number during function execution.
Removing the casting done with <number><any> will reveal TypeScript's ability to catch and flag errors in your code.
The TypeScript type system includes a special type called any. This type allows you to bypass the usual strict typing rules and essentially tell the compiler to back off. The any type is like a wild card that can be assigned any value and can also be assigned to any other type in the system. Check out the example below for a demonstration:
var flexibility: any;
// Can hold any type of value
flexibility = 'hello';
flexibility = 42;
// Can be assigned to any type
var numberVar: number;
flexibility = numberVar;
numberVar = flexibility;
For more information, you can refer to this link: https://basarat.gitbooks.io/typescript/content/docs/types/type-system.html
When it comes down to it, TypeScript is always transformed into JavaScript. This means that essentially you are combining a string '1' with the number 2. JavaScript will automatically convert the number to a string because adding a string and a number directly is not feasible.
If you define a type for the variables being sent to the function, the TypeScript transpiler will detect an error:
var a: number = '1';
var b: number = 1;
add_numbers(a, b);
Error:
Type mismatch found on line one
var a: string = '1';
var b: number = 1;
add_numbers(a, b);
Error:
Incorrect parameter type in function call
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