Why 'X' Y = 'XY' in TypeScript ? Expect an error-CodePudding

I am trying to understand TypeScript and how it works so this is a very simple example :

const X: string = '5';
const Y: number = 6;
console.log(X Y) // output '56' ! why not an error ? and why not 11 ?

With JavaScript, such a result is acceptable, but since TypeScript is designed to avoid logic errors, why it don't mind if we try to append a "string" to a "number". Logically speaking, this makes no sense. also why the default procedure is to treat Y as string why not try to convert X to number.

CodePudding user response：

One of the tenets of TypeScript is that "your existing working JavaScript code is also TypeScript code" [link].

There's plenty of existing JavaScript code with bits like this:

const x = '5';
const y = 6;
console.log(x   y); // '56'

and that do expect the behavior of JavaScript. It would be a significant barrier to migration if TypeScript forced developers to change the above. (And it's a nonstarter to suggest that it should output 11; we couldn't trust TypeScript if renaming a file from .js to .ts could introduce bugs detectable only at runtime.)

Now, you might want to object that your code is different, in that you explicitly annotate the variables with their types. But TypeScript provides type inference, and it would be a horrible mess if

const x : string = '5';

produced a variable with slightly different behavior than

const x = '5';

Language design always involves tradeoffs, and TypeScript has decided that some JavaScript quirks are worth keeping for compatibility.

CodePudding user response：

In TypeScript, the operator is used for both addition and concatenation of strings. In this example, since one of the operands (X) is a string, the operator is treated as a concatenation operator and concatenates the string '5' with the number 6, resulting in the string '56'.

TypeScript does not raise an error in this case because it is following the behavior of JavaScript, which also uses the operator for both addition and concatenation. This can be confusing, but it is a feature inherited from JavaScript, not a design decision made by TypeScript.

If you want to ensure that the variables are treated as numbers, you can explicitly convert them to numbers using the Number() function or using the symbol to convert it to number.

console.log(Number(X)   Y) 
// it will output `11`
console.log( X   Y) 
// it will also output 11

It's always good practice to make sure that you're working with the correct types, and TypeScript provides several ways to do this such as type casting and type assertion.