🤖 Narrowing

TypeScript tiny narrowing helpers you better use.

Install

npm i narrowing  # yarn add narrowing

Usage

let a: unknown;

if (isXXX(a)) {
  // TypeScript know your type here!
}

Functions

Basic:

• isString
• isNumber
• isBigInt
• isBoolean
• isSymbol
• isUndefined
• isNull
• isNil
• isFunction
• isInstance
• isArray
• isObject

Advanced:

These functions help you make advanced type guards.

• has
• kind
• literal
• schema (💪 cool)

basic examples

import {
  isArray,
  isBigInt,
  isBoolean,
  isFunction,
  isInstance,
  isNil,
  isNull,
  isNumber,
  isObject,
  isString,
  isSymbol,
  isUndefined
} from 'narrowing';

let a: unknown;

if (isString(a)) a.toLocaleLowerCase();
if (isNumber(a)) a.toFixed();
if (isBigInt(a)) a.toString();
if (isBoolean(a)) a.valueOf();
if (isSymbol(a)) a.toString();
if (isUndefined(a)) {
  a; // undefined
}
if (isNull(a)) {
  a; // null
}
if (isNil(a)) {
  a; // null | undefined
}

function testFunc(a: string, b: number): boolean {
  return true;
}

if (isFunction<typeof testFunc>(a)) {
  a('11', 1); // no error
}

if (isInstance(a, Date)) {
  a.getFullYear();
}

class TestClass {
  m() {}
}

if (isInstance(a, TestClass)) {
  a.m();
}

if (isArray<string>(a)) {
  a[0].trim();
}

let b: TestClass | undefined | null;
// b.m(); // Error: Object is possibly 'null' or 'undefined'.ts(2533)
if (!isNil(b)) {
  b.m(); // no Error any more
}

if (isObject(a)) {
  //   let a: {
  //     [key: string]: unknown;
  //   };
  a;
}

has()

Check if a type has a property

TypeScript Handbook / Using type predicates

type Bird = { fly: () => {} };
type Cat = { run: () => {}; meow: () => {} };
type Dog = { run: () => {} };

let pet = {} as any;

// save these type guards somewhere and reuse them
const isBird = has<Bird>('fly');
const isDogOrCat = has<Dog | Cat>('run');
const isCat = has<Cat>('run', 'meow');

if (isBird(pet)) {
  pet.fly(); // Bird
}

if (isDogOrCat(pet)) {
  pet.run(); // Dog | Cat
}

if (isCat(pet)) {
  pet.meow(); // Cat
}

kind()

TypeScript handbook / Discriminated unions

interface Square {
  kind: 'square';
  size: number;
}
interface Rectangle {
  kind: 'rectangle';
  width: number;
  height: number;
}
interface Circle {
  kind: 'circle';
  radius: number;
}

const isSquare = kind<Square>('square');
const isRectangle = kind<Rectangle>('circle');
const isCircle = kind<Circle>('circle');

let s = {} as any;

if (isSquare(s)) {
  console.log(s.size);
}

if (isRectangle(s)) {
  console.log(s.height);
}

if (isCircle(s)) {
  console.log(s.radius);
}

literal()

const is404 = literal(404);
let code = 200;
if (is404(code)) {
  // code's type should be 404 , not number
  // let code: 404
  code;
}

this is useful when you see schema()

schema()

Basic schema validation

let message: unknown = {
  code: 200,
  msg: 'success',
  records: [
    { id: 1, name: 'aaa' },
    { id: 2, name: 'bbb' },
    { id: 3, name: 'ccc' }
  ]
};

const isSuccess = schema({
  code: literal(200),
  msg: isString,
  records: isArray
});

if (isSuccess(message)) {
  // let message: {
  //     code: 200;
  //     msg: string;
  //     records: unknown[];
  // }
  message;
}

schema supports a type argument for associating a schema with an existing type

interface TestInterface {
  id: number;
  name: string;
}

const isTestInterface = schema<TestInterface>({
  id: isNumber,
  name: isString
});

if (isTestInterface(message)) {
  // let message: TestInterface
  message;
}

every()

Runtime array type validation. Checks each element of an array.

let arr: unknown[] = [1, 2, 3];
if (every(isNumber)(arr)) {
  let typeCheck: number[] = arr;
}

Works with any narrowing validator, including schemas.

interface TestInterface {
  id: number;
  name: string;
}

const isTestInterface = schema<TestInterface>({
  id: isNumber,
  name: isString
});

let arr: unknown[] = [{ id: 1, name: 'aaa' }];

if (every(isTestInterface)(arr)) {
  let typeCheck: TestInterface[] = arr;
}

Version

• 1.1.0
  • add has()
• 1.1.1
  • has() accept multiple params
• 1.2.1
  • add kind()
• 1.3.0
  • add isObject(), isValidObject()
• 1.4.0
  • replace ~~isValidObject()~~ with schema()
  • add literal()
• 1.5.0
  • add every()