js_interview_solutions.md

Javascript interviews

Part one - Core JS

Clojures

for (var i = 0; i < 5; i++) {
  setTimeout(function() {
    console.log(i)
  }, 1000)
}

What will this log, and when?

This program will log the number 5, 5 times, after 1 second Understanding the event loop and closures would have allowed you to answer this question

This

var pet = {
  name: 'meow',
  sayName: function() {
    console.log(this.name)
  }
}

var speak = pet.sayName
speak()

1. What is logged to the console
2. How can we fix this

If this program is run in the browser, outside 'strict mode'. this inside sayName will be equal to the window object. The program will most likely log undefined. If this program runs in 'strict mode', or in node. The value of this will be undefined, and the program will throw an error Why? When a function is called as a method on an object, the this variable is bound to that object. In the code above, speak is not called as a method on pet, so the this variable is not bound to pet. Fix? The solution is to add .bind(pet) to the declaration of speak

You should know

1. How the this parameter gets assigned in Javascript
2. How the methods, bind, call and apply work

Event loop

var mockRequest = function(callback) {
  var error = null

  setTimeout(function() {
    callback(error, { user: 'eoin' })
  }, 0)
}

var result
mockRequest(function(_, data) {
  result = data
})

if (result.user !== 'eoin') {
  throw new Error('test failed')
}

Question: what happens when you execute this code

When you execute this code the line if (result.user !== 'eoin') { throws the error: Type Error: Cannot read property 'name' of undefined

You should understand

• That the synchronous code executes before the async code (How the event loop works!)
• How callbacks (or/and higher order functions) work
• That trying to access a property of a variable with the value of undefined throws an error

Async basics

var getUser = function() {
  var userId = window.localStorage.getItem('currentUserId')

  return api.request('/user', userId, function(err, data) {
    if (err) {
      return err
    }
    return  data
  })
}

The candidate should be able to see that the callback is being used incorrectly The canditate should be able to see that the function getUser requires a callback paramiter The canditate should be able to fix the code

Generally, you should be familiar with callbacks.

Final Question

var generateId = ((counter = 0) => () => { return counter++ })()

What is the typeof generateId what is output from console.log(generateId()); console.log(generateId());

I leave this last one as an exercise for the reader

• ES6 default values
• Higher order functions
• ES6 arrow functions
• Immediately invoked function expressions (IIFE)
• Closures in Javascript