charlypoly

function isString(value?: any): value is string;

charlypoly

function isPremiumUser(user: User): user is PremiumUser {
  return user.plan === 'premium'
}



function getPremiumOptions(user: User | PremiumUser): PremiumOptions | null {
  if (isPremiumUser(user)) {
    user
    // `user` is of type `PremiumUser`

charlypoly

function isPremiumUser(user: User): user is PremiumUser {
  return user.plan === 'premium'
}

charlypoly

type UUID = string;

interface User {
   id: UUID;
   first_name: string;
   last_name: string;
   plan: 'free' | 'premium'
}

interface PremiumOptions {

charlypoly

function doSomething(x: string | number | boolean) {
  const isString = typeof x === "string"
  const isNumber = typeof x === "number"
  const isStringOrNumber = isString || isNumber

  if (isStringOrNumber) {
    x
    // `x` is of type `string | number`

    if (typeof x === "number" && x > 0) {

charlypoly

import { format as fmtDate, parseISO } from 'date-fns'


function formatDate(date: Date | string | number, format = 'dd-mm-YYYY') {
  let d = null
  if (typeof date === 'string') {
    date
    // `date` is of type `string`
    
    d = parseISO(date)

charlypoly

function example(x: string | number, y: string | boolean) {
  if (x === y) {
    x
    // `x` is of type `string`

    y
    // `y` is of type `string`
  } else {
    x
    // `x` is of type `string | number`

charlypoly

interface Person {
  name: string
  address?: {
    city: string
    zipcode: string
  }
}

function getCity(p: Person): string | undefined {
  if (p.address) {

charlypoly

type Mutation {
  update_user(id: String!, user: UserInputType): User
}

input UserInputType {
  first_name: String
  last_name: String
  picture_path: String
  job_title: String
  email: String

charlypoly

import * as React from 'react';
/* imports ... */
import { client } from '../../core/apollo';

const jsonSchema: JSONSchema6 = require('./core/apollo-form-json-schema.json');
// this is common for the whole application, brang here for the example.
const AlineSidebarForm = configure<ApolloFormMutationNames>({
    client,
    jsonSchema,
    theme: alineSidebarTheme