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const fs = require('fs')
const { promisify } = require('util')

const readFile = promisify(fs.readFile)

/**
 * Parses text for environment variables and saves them to an object as keys.
 *
 * Empty lines and lines starting with `#` are skipped.
 *

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function spy (fn = () => {}) {
  f.calls = []
  function f (...args) {
    const returnValue = fn(...args)
    f.calls.push({ args, returnValue })
    return returnValue
  }
  return f
}

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// Walk an object graph, calling the visit function with
// value, key, and object at each level.
function walk (o, visit) {
  visit(o, '', o)
  if (o && (Array.isArray(o) || Object(o) === o)) {
    let stack = [ o ]
    while (stack.length) {
      let obj = stack.shift()
      for (let k in obj) {
        const v = obj[k]

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Junior Server Developer:

Write a Nodejs+Express web server that responds with { message: 'Hello World!' } when it receives a request at GET /hello.

Junior React Developer:

Write a React component that abstracts embedding YouTube and Vimeo videos. The component should accept the prop src that is a string equal to the source of the video to load. The component will determine what video it is based on the domain of the source and then decide how to embed the video.

Intermediate Server Developer:

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/**
 * @param {any[]} items
 * @param {any} item
 * @param {{ (a: any, b: any) => number }} compare
 * @param {number} low
 * @param {number} high
 * @return {number}
 */
function binarySearch (items, item, compare, low = 0, high = items.length) {
  if (high <= low) {

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/**
 * Read a command line option. Option names must start with '-'.
 * 
 * Options:
 * 
 * `required`     Determines if the option must appear on the command line.
 * 
 * `multi`        Determines if the option can be specified more than once on the
 *                command line. Causes the return value to be an array.
 * 

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const walkObject = (function () {
  function walkObjectRec (obj, visit, { pointer = '', key = '', visited = new WeakMap(), parent = undefined } = {}) {
    if (obj === null || obj === undefined) {
      visit(obj, key, pointer, parent)
    } else if (typeof obj === 'object') {
      if (visited.has(obj)) {
        visited.set(obj, visited.get(obj) + 1)
      } else {
        visited.set(obj, 1)
        visit(obj, key, pointer, parent)

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/**
 * A simple service container.
 *
 * @example
 * const app = express()
 *
 * const appContainer = new ServiceContainer()
 * const config = Object.freeze({ })
 * appContainer.constant('Config', config)
 * appContainer.singleton('Db', () => new Db())

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/*
 * Simple EcmaScript templating system.
 *
 * Templates are just functions that accept an object of
 * properties and return a string.
 *
 * const Hello = (props = {}) => `
 *   Hello ${props.message || 'World'}!
 * `
 * console.log(Hello({ message: 'Mom' }))

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/**
 * Checks the type of a value. If the value is invalid then throws.
 * 
 * Errors:
 * 
 *    TypeError
 * 
 *    Thrown whenever a value has an invalid type
 * 
 *    Properties: