Type Guard Functions for Various Data Types

isDefined.ts

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

isEmpty.ts

/**
 * Checks if the given argument is empty.
 *
 * 🚨 The values listed below are not considered empty (they are falsy, not empty):
 * - The boolean `false`
 * - The number zero `0`
 *
 * @param arg - The argument to check. It can be of any type.
 *
 * @returns `true` if the argument is empty, `false` otherwise.
 */
export default 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() === '';
    }

    if (typeof arg === 'number') {
        /**
         * `isNaN` checks whether the value is not a number or cannot be converted 
         * into a number. `Number.isNaN` only checks if the value is equal to NaN.
         */
        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(`The given argument could not be parsed: ${String(arg)}`);
}

isNil.ts

/**
 * Checks if a value is null or undefined.
 * 
 * @param arg - The value to check.
 * @returns `true` if the value is null or undefined; otherwise, return `false`.
 */
export default 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 `unknown`.
 * @param arg - The argument that needs to be checked.
 * @returns Returns `true` if the argument is a plain object, `false` otherwise.
 */
export default function isPlainObject<T>(arg: T): arg is T {
  return (
    typeof arg === 'object' &&
    arg !== null &&
    !Array.isArray(arg) &&
    Object.getPrototypeOf(arg) === Object.prototype
  );
}

isStringifiable.ts

export default 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')
    );
}