Learning Journal

1/2/2024[edited at: 1/11/2024]

Expressing Constraints with Generics in TypeScript

function listToDict<T extends HasId>(list: T[]): Dict<T> {\n  const dict: Dict<T> = {};\n\n  for (const item of list) {\n    dict[item.id] = item;\n  }\n\n  return dict;\n}\nfunction listToDict<T extends HasId>(list: T[]): Dict<T> {\n  const dict: Dict<T> = {};\n\n  for (const item of list) {\n    dict[item.id] = item;\n  }\n\n  return dict;\n}\n

In the block of code above, if we only use T as the type of list, TypeScript will complain that T does not have a property called id. On the other hand, if we only use HasId, we only get the property id on IDE's suggestion when typing dict.. TypeScript doesn't know the type of the value of dict is T. We need to use T extends HasId to express the constraint that T must have a property called id and it has some other properties.