All files / assert / instance_of.ts

// Copyright 2018-2025 the Deno authors. MIT license.
// This module is browser compatible.
import { AssertionError } from "./assertion_error.ts";

/** Any constructor */
// deno-lint-ignore no-explicit-any
export type AnyConstructor = new (...args: any[]) => any;
/** Gets constructor type */
export type GetConstructorType<T extends AnyConstructor> = InstanceType<T>;

/**
 * Make an assertion that `obj` is an instance of `type`.
 * If not then throw.
 *
 * @example Usage
 * ```ts ignore
 * import { assertInstanceOf } from "@std/assert";
 *
 * assertInstanceOf(new Date(), Date); // Doesn't throw
 * assertInstanceOf(new Date(), Number); // Throws
 * ```
 *
 * @typeParam T The expected type of the object.
 * @param actual The object to check.
 * @param expectedType The expected class constructor.
 * @param msg The optional message to display if the assertion fails.
 */
export function assertInstanceOf<
  // deno-lint-ignore no-explicit-any
  T extends abstract new (...args: any[]) => any,
>(
  actual: unknown,
  expectedType: T,
  msg = "",
): asserts actual is InstanceType<T> {
  if (actual instanceof expectedType) return;

  const msgSuffix = msg ? `: ${msg}` : ".";
  const expectedTypeStr = expectedType.name;

  let actualTypeStr = "";
  if (actual === null) {
    actualTypeStr = "null";
  } else if (actual === undefined) {
    actualTypeStr = "undefined";
  } else if (typeof actual === "object") {
    actualTypeStr = actual.constructor?.name ?? "Object";
  } else {
    actualTypeStr = typeof actual;
  }

  if (expectedTypeStr === actualTypeStr) {
    msg =
      `Expected object to be an instance of "${expectedTypeStr}"${msgSuffix}`;
  } else if (actualTypeStr === "function") {
    msg =
      `Expected object to be an instance of "${expectedTypeStr}" but was not an instanced object${msgSuffix}`;
  } else {
    msg =
      `Expected object to be an instance of "${expectedTypeStr}" but was "${actualTypeStr}"${msgSuffix}`;
  }

  throw new AssertionError(msg);
}