TypeScript Tips

tips.ts

/**
 * ---
 * ---
 * Partial, Pick, Omit and Required
 * ---
 * ---
 */
interface Props {
  key1: string;
  key2: number;
  key3?: string;
}

// Make all properties in T optional
type PartialProps = Partial<Props>;
// From T, pick a set of properties whose keys are in the union K
type PickProps = Pick<Props, "key1" | "key3">;
// Construct a type with the properties of T except for those in type K.
type OmitProps = Omit<Props, "key3">;
// Make all properties in T required
type RequiredProps = Required<Props>;

/**
 * ---
 * ---
 * Readonly
 * ---
 * ---
 */

// Make all properties in T readonly
type Amounts = Readonly<number[]>;
const amounts: Amounts = [100, 300, 500, 1000];

// ERROR: Property 'push' does not exist on type 'readonly number[]'
amounts.push(1000);

/**
 * ---
 * ---
 * as const
 * ---
 * ---
 */

// Infer literal values
const COLORS = {
  red: "RED",
  blue: "BLUE",
  green: "GREEN",
} as const;

// Some other cool stuff you can do with this -> https://www.youtube.com/watch?v=hk6ZkD0Vg3w

/**
 * ---
 * ---
 * Index Signature
 * ---
 * ---
 */

enum InvoiceStatus {
  Draft = "DRAFT",
  Pending = "PENDING",
  Paid = "PAID",
  Failed = "FAILED",
}

interface InvoiceStatusByID {
  // Use dynamic keys with a specific type
  [id: string]: InvoiceStatus;
}

const invoiceStatusByID: InvoiceStatusByID = {
  "inv-1": InvoiceStatus.Draft,
  "inv-2": InvoiceStatus.Paid,
  "inv-3": "Something else",
};

/**
 * ---
 * ---
 * Literal Types
 * ---
 * ---
 */

type Status = "DRAFT" | "PENDING" | "PAID" | "FAILED";

// Enforce specific values for a parameter
const processInvoices = (status: Status) => {
  // ... do stuff
};

processInvoices("DRAFT");
processInvoices("Something else");