Introduction

typechecked is a runtime type checker for typescript & javascript. It allows you to define types and check if a value matches that type. In typescript, it enables you to narrow the type of a value based on a type check.

Installation

npm install typechecked

Concept

Typechecked is based on the idea of a simple function, that accepts an unknown value and returns it if it matches a given type. If the value does not match the type, an error is thrown.

function value(value: unknown): KnownType {
  // throw if value does not match type
  return value as KnownType;
}

The library provides a set of these functions for all primitive types, arrays, objects, tuples, unions, intersections, and more. Types that are more domain specific can easily be defined by the consumer.

Exported functions

import { isString } from 'typechecked';
import { isNumber } from 'typechecked';
import { isInteger } from 'typechecked';
import { isBigInt } from 'typechecked';
import { isBoolean } from 'typechecked';
import { isNully } from 'typechecked';
import { isNull } from 'typechecked';
import { isUndefined } from 'typechecked';
import { isSymbol } from 'typechecked';
import { isDate } from 'typechecked';
import { isObject } from 'typechecked';
import { isFunction } from 'typechecked';

Pipe functions

Pipe functions are used to combine multiple type checks into one. They are useful when you want to check if a value matches other conditions apart from just the type. Or if you want to transform a value from the original type before returning it.

import { tcpipe, tcpipe_t } from 'typechecked';

tcpipe

Used when piping functions but the value must be of one type.

const validator = tcpipe(
  isString,
  (value) => value.toUpperCase(),
);

const value: string = validator('hello'); // value === 'HELLO'

tcpipe_t

Useful when the value is transformed to a different type.

const validator = tcpipe_t(
  isString,
  isHex,
  (value) => parseInt(value, 16),
);

const value: number = validator('FF1233'); // value === 16716339