1 · Model, solve and test · 25 MIN

Transform a discriminated result safely

Transform a discriminated result safely

A discriminated union puts the success and failure shapes behind a shared literal field. Check ok before accessing the associated value or error. This preserves failure information without throwing and gives the compiler a reason to permit each property access. Generic mapping can generalize this pattern, but the exercise uses one concrete transformation so runtime behavior is easy to test. TypeScript types still do not validate a network payload.

Use the literal ok field as the branch discriminator.

Read the example

type Result={ok:true;value:number}|{ok:false;error:string};
function solve(input:Result):Result{if(input.ok)return {ok:true,value:input.value*2};return {ok:false,error:input.error}}
console.log(JSON.stringify(solve({"ok":true,"value":3})));

{"ok":true,"value":6}

Your challenge

Input is {ok:true,value:number} or {ok:false,error:string}. Double a successful value and preserve a failure unchanged in a new result object.

Solution cost: O(1) result transformation. time · O(1) returned object. space

Common trap

Accessing value without narrowing assumes every result is successful.