@uplift-ltd/ts-helpers

Installation

npm i --save @uplift-ltd/ts-helpers

API

assertUnreachable

Invoke this assertion for an unreachable case.

import { assertUnreachable } from "@uplift-ltd/ts-helpers";

switch (condition) {
  case true:
    return bleh;
  default:
    assertUnreachable("invalid condition");
}

notEmpty

Filter and array so typescript knows there are no null or undefined values.

import { notEmpty } from "@uplift-ltd/ts-helpers";

const array: (string | null)[] = ["foo", "bar", null, "zoo", null];
const filteredArray: string[] = array.filter(notEmpty);

typedIncludes

Assert whether a variable is a member of an array in a type-safe way. This also avoids the "string" is not a member of TYPE errors.

const colors = ["blue", "green", "red"] as const;

// You avoid the error here, that myColor is string, not a member of colors
if (typedIncludes(colors, myColor)) {
  // typeof myColor = "blue" | "green" | "red"

  // You also get a type error here that yellow is not "blue" | "green" | "red"
  if (myColor === "yellow") {
  }
}

ArrayElement

Returns the type of an element of the array

type Item = ArrayElement<string[]>; // => string

type Item = ArrayElement<(ThisLineItem | ThatLineItem)[]>; // => (ThisLineItem | ThatLineItem)

Arrays

Ensures that the provided data is an array, and if it's not, wraps it in an array

type Item = ThisLineItem | ThisLineItem[];

const itemOrItems: Item;
const itemsArray: ThisLineItem[] = ensureArray(itemOrItems);

ArrayToUnion

Converts an array of strings into a discriminated type union.

const httpMethods = ["GET", "POST", "PUT", "PATCH", "OPTIONS", "HEAD", "DELETE"] as const; // note: `as const` is required
type HttpMethod = ArrayToUnion<typeof httpMethods>; // => "GET" | "POST" | "PUT" | "PATCH" | "OPTIONS" | "HEAD" | "DELETE"

DeepPartial

Deeply marks attributes as optional

interface RequiredFields {
  firstName: string;
  lastName: string;
  age: number;
  address: {
    street: string;
    postalCode: string;
    city: string;
    state: string;
    country: string;
  };
}

type OptionalFields = DeepPartial<RequiredFields>;
/*
 * => {
 *   firstName?: string;
 *   lastName?: string;
 *   age?: number;
 *   address?: {
 *     street?: string;
 *     postalCode?: string;
 *     city?: string;
 *     state?: string;
 *     country?: string;
 *   }
 * }
 *
 */

RequireAtLeastOne

Tell typescript to require at least one of the keys to be present.

import { RequireAtLeastOne } from "@uplift-ltd/ts-helpers";

interface Props {
    a?: boolean
    b?: boolean
    c: boolean
}

type ABProps = RequireAtLeastOne<Props, 'a' | 'b'>

<Component a c /> /* passes */
<Component b c /> /* passes */
<Component a b c /> /* passes */
<Component c /> /* throws */

RequireOnlyOne

Tell typescript to require only one (not all) of the keys to be present.

import { RequireOnlyOne } from "@uplift-ltd/ts-helpers";

interface Props {
    a?: boolean
    b?: boolean
    c: boolean
}

type ABProps = RequireOnlyOne<Props, 'a' | 'b'>

<Component a c /> /* passes */
<Component b c /> /* passes */
<Component a b c /> /* throws */
<Component c /> /* throws */

Unpromise

Returns the result type of a promise.

type Result = Unpromise<Promise<boolean>>; // => boolean

Can be combined with ReturnType to get the result of a function that returns a promise.

type TheTypeWeWant = { a: boolean };

type PromiseReturningFunction = () => Promise<TheTypeWeWant>;

type TheTypeWeHave = Unpromise<ReturnType<PromiseReturningFunction>>;

// TheTypeWeHave is now TheTypeWeWant

UnreachableCaseError

Throw this type of error for an unreachable case.

import { UnreachableCaseError } from "@uplift-ltd/ts-helpers";

switch (condition) {
  case true:
    return bleh;
  default:
    throw new UnreachableCaseError("invalid condition");
}

ValuesOf

Returns a type union of the values of a record/object

const Colors = {
  BLUE: "blue",
  RED: "red",
  GREEN: "green",
};

type Color = ValuesOf<typeof Colors>; // => "blue" | "red" | "green"

Relay

Helpers for working with Relay Connections

GetConnectionNode
Extracts the Node from a Relay Connection, even if some or all of the Connection is nullable

GetConnectionNodeArray
Helper that just returns an array of the Connection Node, also works if Connection is nullable

type User = {
  firstName: string;
  lastName: string;
  age: number;
};

// Non-nullable
type Organization = {
  users: {
    edges: { node: User }[];
  };
};

// Nullable
type NullableOrganization = {
  users?: {
    edges?: { node: User }[] | null;
  } | null;
};

Organization["users"]["edges"][0]["node"];
// => User

GetConnectionNode<Organization["users"]>;
// => User

GetConnectionNodeArray<Organization["users"]>;
// => Array<User>

GetConnectionNode<NonNullable<NullableOrganization["users"]>>;
// => User

GetConnectionNodeArray<NonNullable<NullableOrganization["users"]>>;
// => Array<User>

makeUnionMemberGuard

Filter on a discriminated union.

import { makeGraphqlUnionGuard, makeUnionMemberGuard } from "@uplift-ltd/ts-helpers";

type A = {
  __typename: "A";
  x: number;
};

type B = {
  __typename: "B";
  y: number;
};

type AB = A | B;

const a = { __typename: "A", x: 1 } as const;
const b = { __typename: "B", y: 2 } as const;

const ab: AB[] = [a, b];

ab.filter(makeUnionMemberGuard("__typename", "A")); // A[]

ab.filter(makeUnionMemberGuard("__typename", "B")); // B[]

ab.filter(makeGraphqlUnionMemberGuard("A")); // A[]

ab.filter(makeGraphqlUnionMemberGuard("B")); // B[]