Type Guard Functions for Various Data Types

isDefined.ts

function isDefined<T>(arg: T | undefined | null): arg is T {
    return arg !== undefined && arg !== null;
}

isEmpty.ts

/**
 * Checks if the given argument is empty.
 *
 * ⚠️ Note that this values are not considered empty:
 * - Boolean arguments
 * - The number zero "0"
 *
 * @param arg - The argument to check.
 * @returns `true` if the argument is empty, `false` otherwise.
 */
function isEmpty(arg: unknown): boolean {
  if (typeof arg === "boolean") {
    return false;
  }

  if (arg === undefined || arg === null) {
    return true;
  }

  if (typeof arg === "string") {
    return arg.trim().length === 0;
  }

  if (typeof arg === "number") {
    return isNaN(arg);
  }

  if (Array.isArray(arg)) {
    return arg.length === 0;
  }

  if (arg instanceof Map || arg instanceof Set) {
    return arg.size === 0;
  }

  if (typeof arg === "object") {
    return Object.keys(arg).length === 0;
  }

  throw new Error(`Argument could not be parsed: ${String(arg)}`);
}

isNil.ts

/**
 * Checks if a value is null or undefined.
 * 
 * @param value - The value to check.
 * @returns `true` if the value is null or undefined; otherwise, return `false`.
 */
function isNil(arg: unknown): arg is null | undefined {
    return arg === null || arg === undefined;
}

isPlainObject.ts

/**
 * This function checks if the given argument is a plain object or not.
 *
 * @template T - This is the type parameter, which defaults to `Record<string, unknown>`.
 * @param arg - The argument that needs to be checked.
 * @returns `true` if the argument is a plain object, `false` otherwise.
 */
function isPlainObject<T = Record<string, unknown>>(arg: unknown): arg is T {
    return typeof arg === 'object' && arg !== null;
}

isStringifiable.ts

function isStringifiable(arg: unknown): arg is string | number | boolean | null | { toString(): string } {
    return (
        typeof arg === 'string' ||
        typeof arg === 'number' || 
        typeof arg === 'boolean' || 
        arg === null ||
        (typeof arg === 'object' && arg !== null && typeof arg.toString === 'function')
    );
}