All files / assert / false.ts

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

/** Assertion condition for {@linkcode assertFalse}. */
export type Falsy = false | 0 | 0n | "" | null | undefined;

/**
 * Make an assertion, error will be thrown if `expr` have truthy value.
 *
 * @example Usage
 * ```ts ignore
 * import { assertFalse } from "@std/assert";
 *
 * assertFalse(false); // Doesn't throw
 * assertFalse(true); // Throws
 * ```
 *
 * @param expr The expression to test.
 * @param msg The optional message to display if the assertion fails.
 */
export function assertFalse(expr: unknown, msg = ""): asserts expr is Falsy {
  if (expr) {
    throw new AssertionError(msg);
  }
}