Check out the code snippets below:
function functionA(x: string, y: number, z: SpecialType): void { }
const functionWrapper: (x, y, z) => functionA(x, y, z);
The parameters of functionWrapper are currently assigned the type any. Is there a way we can prompt tsc to infer their types based on how they are used? What aspect of the TypeScript type system is preventing this deduction?
TypeScript undergoes type analysis in a top-down manner, aligning with the program data flow where data is also passed top-down. The language scrutinizes control-flow utilizing information provided from the top rather than the bottom.
Let's examine a function as an example:
function functionMaker(x: string, y: string) {
return () => x + y;
}
In the above function, another function is being returned. While there is no explicit typing in the definition of the anonymous function being returned, TypeScript can deduce that functionMaker consistently returns a function of the type () => string.
Attempting to analyze it the other way around proves challenging, as TypeScript cannot anticipate how the arguments will be utilized. Consider the following scenario:
function functionA(x: string, y: number, z: SpecialType): void { }
function functionB(x: number): void { }
const functionWrapper: (x, y, z) => {
functionA(x, y, z); // x should be a string
functionB(x); // x should be a number
}
Here, TypeScript encounters two functions that each take one argument, but with different type requirements. Any attempt to resolve this dilemma would result in failure for either function.
To address this issue, we can create a generic function that acts as a wrapper for another function.
function functionA(x: string, y: number, z: SpecialType): void { }
const wrap = <X, Y, Z, R>(f: (x: X, y: Y, z: Z) => R) => (x: X, y: Y, z: Z): R => f(x,y,z);
const functionWrapper = wrap(functionA);
Our wrap function serves explicitly as a wrapper, tasked with inferring types from the given function and creating a new function with identical arguments and return type.
One key point to consider is that if Typescript were to automatically narrow the signature in your proposal, it would have a ripple effect across all call sites leading to failures.
function yourFunction(x) {
doSomethingThatTakesANumber(x);
}
// somewhere else
yourFunction(1);
// another location
yourFunction(-1);
Changing yourFunction to doSomethingThatTakesABoolean would make Typescript infer that x must be boolean, resulting in failure for both calls even though they are unrelated. Debugging such issues could prove to be quite challenging.
When it comes to writing a wrapper function with the same signature as the wrapped function, using typeof functionA can help avoid repetition. While Typescript can determine parameter types from the function type, it cannot analyze the body of the function itself.
const functionWrapper2: typeof functionA = (x, y, z) => functionA(x, y, z);
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