Generics for Reusable Enterprise Code

Build reusable typed repositories, API clients, form helpers, and event utilities without losing domain safety.

46 min

3 sections

generics

constraints

repositories

sdk

01. Generic Functions Preserve Relationships

Section 1 of 3

Generics are useful when the input and output types are related. Do not add generic parameters just to look advanced. Use them when the function should preserve a caller-provided type.

typescript

function groupBy<T, K extends string | number>(
  items: readonly T[],
  getKey: (item: T) => K
): Record<K, T[]> {
  return items.reduce((groups, item) => {
    const key = getKey(item);
    groups[key] = [...(groups[key] ?? []), item];
    return groups;
  }, {} as Record<K, T[]>);
}

Exercise

Build a typed indexBy helper

Practice

Create a generic indexBy function that turns an array of objects into a lookup by a selected string key.

Object Modeling and Domain Types

Classes, Services, and Decorator Tradeoffs

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