Item 57: Prefer Tail-Recursive Generic Types

Things to Remember

Aim to make your recursive generic types tail recursive. They're more efficient and have greater depth limits.
Recursive type aliases can often be made tail recursive by rewriting them to use an accumulator.

Code Samples

function sum(nums: readonly number[]): number {
  if (nums.length === 0) {
    return 0;
  }
  return nums[0] + sum(nums.slice(1));
}

console.log(sum([0, 1, 2, 3, 4]));

💻 playground

const arr = Array(7875).fill(1);
console.log(sum(arr));

💻 playground

function sum(nums: readonly number[], acc=0): number {
  if (nums.length === 0) {
    return acc;
  }
  return sum(nums.slice(1), nums[0] + acc);
}

💻 playground

type GetChars<S extends string> =
    S extends `${infer FirstChar}${infer RestOfString}`
    ? FirstChar | GetChars<RestOfString>
    : never;

type ABC = GetChars<"abc">;
//   ^? type ABC = "a" | "b" | "c"

💻 playground

type Long = GetChars<"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWX">;
//          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
//          Type instantiation is excessively deep and possibly infinite.

💻 playground

type ToSnake<T extends string> =
    string extends T
    ? string  // We want ToSnake<string> = string
    : T extends `${infer First}${infer Rest}`
    ? (First extends Uppercase<First>  // Is First a capital letter?
      ? `_${Lowercase<First>}${ToSnake<Rest>}`  // e.g. "B" -> "_b"
      : `${First}${ToSnake<Rest>}`)
    : T;

type S = ToSnake<'fooBarBaz'>;
//   ^? type S = "foo_bar_baz"

type Two = ToSnake<'className' | 'tagName'>;
//   ^? type Two = "class_name" | "tag_name"

💻 playground

type Long = ToSnake<'reallyDescriptiveNamePropThatsALittleTooLoquacious'>;
//          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
//          Type instantiation is excessively deep and possibly infinite.

💻 playground

type ToSnake<T extends string, Acc extends string = ""> =
  string extends T
  ? string  // We want ToSnake<string> = string
  : T extends `${infer First}${infer Rest}`
  ? ToSnake<
      Rest,
      First extends Uppercase<First>
      ? `${Acc}_${Lowercase<First>}`
      : `${Acc}${First}`
    >
  : Acc;

type S = ToSnake<'fooBarBaz'>;
//   ^? type S = "foo_bar_baz"

type Two = ToSnake<'className' | 'tagName'>;
//   ^? type Two = "class_name" | "tag_name"

type Long = ToSnake<'reallyDescriptiveNamePropThatsALittleTooLoquacious'>;
//   ^? type Long = "really_descriptive_name_prop_thats_a_little_too_loquacious"

💻 playground

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Uh oh!