Maybe monad implemented in TypeScript. Base of the work of https://codewithstyle.info/advanced-functional-programming-in-typescript-maybe-monad/

maybe.spec.ts

import { Maybe } from './maybe'

describe('Maybe', () => {
  it('should handle a value', () => {
    const maybe = Maybe.some('test')
    expect(maybe.getOrElse('')).toBe('test')
  })

  it('should handle an undefined value', () => {
    const maybe = Maybe.fromValue<string>(undefined)
    expect(maybe.getOrElse('test')).toBe('test')
  })

  it('should a false value', () => {
    const maybe = Maybe.fromValue<boolean>(false)
    expect(maybe.getOrElse(true)).toBe(false)
  })

  it('should handle a string value', () => {
    const maybe = Maybe.fromValue('test')
    expect(maybe.getOrElse('')).toBe('test')
  })

  it('should handle an empty string value', () => {
    const maybe = Maybe.fromValue('')
    expect(maybe.getOrElse('test')).toBe('')
  })

  it('should return a default value with a different type', () => {
    const maybe = Maybe.fromValue<string>(null)
    expect(maybe.getOrElse(null)).toBe(null)
  })

  it('should handle a null value', () => {
    const maybe = Maybe.fromValue<string>(null)
    expect(maybe.getOrElse('test')).toBe('test')
  })

  it('should handle a numeric value', () => {
    const maybe = Maybe.fromValue(42)
    expect(maybe.getOrElse(0)).toBe(42)
  })

  it('should handle the zero value as valid', () => {
    const maybe = Maybe.fromValue(0)
    expect(maybe.getOrElse(1)).toBe(0)
  })

  it('should tap a value', () => {
    const maybe = Maybe.fromValue('value')
    let actual = false

    maybe.tap(() => {
      actual = true
    })

    expect(actual).toBe(true)
  })

  it('should not tap a value', () => {
    const maybe = Maybe.none()
    let actual = false

    maybe.tap(() => {
      actual = true
    })

    expect(actual).toBe(false)
  })

  it('should throw an error if the value is not valid', () => {
    expect(() => {
      Maybe.some(null)
    }).toThrowError()
  })

  it('should handle a callback as a default value', () => {
    const mock = jest.fn()
    const maybe = Maybe.fromValue(null)
    maybe.getOrExecute(mock)
    expect(mock).toHaveBeenCalled()
  })

  it('should check if it has a value', () => {
    const maybe = Maybe.fromValue('hello')
    expect(maybe.has()).toBe(true)
  })

  it('should check if it does not have a value', () => {
    const maybe = Maybe.fromValue<string>(null)
    expect(maybe.has()).toBe(false)
  })

  it('should handle none value', () => {
    const maybe = Maybe.none()
    expect(maybe.getOrElse('test')).toBe('test')
  })

  it('should get or throw', () => {
    const maybe = Maybe.none()
    expect(() => {
      maybe.getOrThrow(new Error('foo'))
    }).toThrowError('foo')
  })

  it('should be able to map existing values', () => {
    const maybeMap = Maybe.some({ a: 'a' })
    expect(maybeMap.map(e => e.a).getOrElse('b')).toBe('a')
  })

  it('should be able to map non existing values', () => {
    type Type = { foo: Maybe<{ bar: string }> }
    const maybeMap = Maybe.some<Type>({ foo: Maybe.none() })
    expect(
      maybeMap
        .getOrExecute(() => {
          throw new Error()
        })
        .foo.map(x => x.bar)
    ).toEqual(Maybe.none())
  })

  it('should be able to flat map existing values', () => {
    type Type = { foo: Maybe<{ bar: string }> }
    const maybeMap = Maybe.fromValue<Type>({ foo: Maybe.some({ bar: 'qux' }) })
    expect(maybeMap.flatMap(x => x.foo).map(x => x.bar)).toEqual(Maybe.some('qux'))
  })

  it('should be able to flat map non existing values', () => {
    type Type = { foo: Maybe<{ bar: string }> }
    const maybeMap = Maybe.none<Type>()
    expect(maybeMap.flatMap(x => x.foo)).toEqual(Maybe.none())
  })
})

maybe.ts

type CallbackFunction<T = unknown> = (...params: unknown[]) => T

export class Maybe<T> {
  private constructor(private value: T | null) {}

  static some<T>(value: T): Maybe<T> {
    if (!this.isValid(value)) {
      throw new Error('Provided value must not be empty')
    }
    return new Maybe(value)
  }

  static none<T>(): Maybe<T> {
    return new Maybe<T>(null)
  }

  static fromValue<T>(value: T | undefined | null): Maybe<T> {
    return this.isValid(value) ? Maybe.some(value as T) : Maybe.none<T>()
  }

  private static isValid(value: unknown | null | undefined): boolean {
    return !!value || this.isNumberZero(value) || this.isFalse(value) || this.isEmptyString(value)
  }

  private static isNumberZero<R>(value: R): boolean {
    return typeof value === 'number' && value === 0
  }

  private static isEmptyString<R>(value: R): boolean {
    return typeof value === 'string' && value === ''
  }

  private static isFalse<R>(value: R): boolean {
    return typeof value === 'boolean' && !value
  }

  has(): boolean {
    return this.value !== null
  }

  getOrElse<R = T>(defaultValue: T | R): T | R {
    return this.value === null ? defaultValue : this.value
  }

  getOrExecute(defaultValue: CallbackFunction<T>): T {
    return this.value === null ? defaultValue() : this.value
  }

  map<R>(f: (wrapped: T) => R): Maybe<R> {
    if (this.value === null) {
      return Maybe.none<R>()
    } else {
      return Maybe.some(f(this.value))
    }
  }

  tap(f: (wrapped: T) => void): Maybe<T> {
    if (this.value !== null) {
      f(this.value)
    }

    return Maybe.fromValue(this.value)
  }

  flatMap<R>(f: (wrapped: T) => Maybe<R>): Maybe<R> {
    if (this.value === null) {
      return Maybe.none<R>()
    } else {
      return f(this.value)
    }
  }

  getOrThrow(error?: Error): T {
    return this.value === null
      ? (() => {
          if (error !== undefined) {
            throw error
          }
          throw new Error()
        })()
      : this.value
  }
}

Came here from here:
https://codewithstyle.info/advanced-functional-programming-in-typescript-maybe-monad/