$ npm install @github/query-selector

import {closest, getAttribute, namedItem, query, querySelectorAll} from '@github/query-selector'

// Find an element by selector and type, or throw if not found.
const image: HTMLImageElement = query(document, '.avatar', HTMLImageElement)
image.src = '/hubot.png'

// Find the parent by selector and type, or throw if not found.
const parent: HTMLDetailsElement = closest(image, '.container', HTMLDetailsElement)
parent.open = true

// Filter all children by selector and type.
const inputs: Array<HTMLInputElement> = querySelectorAll(document, 'input', HTMLInputElement)
for (const input of inputs) {
  input.value = ''
}

// Retrieve the attribute's value or throw.
const url: string = getAttribute(image, 'data-url')

// Find the form's `input[name=login]` field or throw if not found.
const form: HTMLFormElement = query(document, 'form', HTMLFormElement)
const input: HTMLInputElement = namedItem(form, 'login')

const el = document.querySelector('.expected-element')
// el may be null!
el.classList.add('selected')
el.setAttribute('title', 'hello')

const el = document.querySelector('.expected-element')
if (el) {
  el.classList.add('selected')
  el.setAttribute('title', 'hello')
}

const el = document.querySelector('.expected-element')
if (el) {
  // Element might not have a value property!
  el.value = 'hello'
}

const el = document.querySelector('.expected-element')
if (el instanceof HTMLInputElement) {
  el.value = 'hello'
}

npm install
npm test