Generic helpers

Add generic helper functions.

Generic helper functions

Generic helper functions are reusable tools that work with any type. The type parameter is specified with <T>, making the function both type-safe and flexible. In a library, helpers like identity, first element, and filtering are typically written as generics.

function identity<T>(x: T): T {
  return x;
}

Common helpers

Common generic helpers include first (first array element), pick (select fields from an object), and groupBy (grouping). These functions form the core toolkit of the library and are used repeatedly in application code.

function first<T>(arr: T[]): T | undefined {
  return arr[0];
}

Exercise firstElement generic helper

Define a generic function named firstElement: <T>(arr: T[]): T | undefined. Its body should be return arr[0]. Print the result of calling firstElement(["a", "b"]) with console.log.

Show example solution

function firstElement<T>(arr: T[]): T | undefined {
  return arr[0];
}
console.log(firstElement(["a", "b"]));

Test yourself

What is the advantage of a generic helper function?
- Eliminates the need to specify types
- Works only with numbers
- Works with any type while preserving type safety
- Runs faster at runtime
Show answer

Works with any type while preserving type safety — Generic functions work with any type via the <T> type parameter, but the compiler preserves type safety.

First element index

function first<T>(arr: T[]): T | undefined {
  return arr[____];
}

Show answer

function first<T>(arr: T[]): T | undefined {
  return arr[0];
}

Generic helper concepts

<T>

Type parameter
identity

Returns what it takes
first

First array element
T | undefined

Maybe absent

Next lesson: Declaration file

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