Notes from Interviews

Pure function

function pure (num) {
  return num * 5;
}

• Always returns same result
• Never depends on any state/data/change
• Always return something
• Test cases will be straighforward

High Order Function

• Takes another function as an argument
• Returns function

High Order Component

• Takes a component as an input and returns a new compnent
• Idealy enhance the component
• Usually a pure function and returns a new component
• Use cases: loader while fetching data

Undefined VS null

Undefined	Null
Declared, but not yet defined	No value, on purpose
The type is undefined	The type is object
Equal to NaN	Equal to 0

THIS

In a method

refer to the owner object (me)

var me = {
  firstName: 'John',
  lastName: 'Doe',
  fullName: function () {
    return this.firstName + ' ' + this.lastName;
  }
}

Alone & Independent

refer to the global object (window)

var x = this.name;

In a function

refer to the global object

function myFunction () {
  return this.name;
}

In a function “strict mode”

undefined

"use strict"
function myFunction () {
  return this.name;
}

In Event Handlers

refer to the HTML Element that received the event (button)

<button onclick="this.style.display='none'">
  Button
</button>

call() & apply() - Explict binding

force the function(a) to refer to the object(b)

var a = {
  fullName: function () {
    return this.firstName + ' ' + this.lastName;
  }
}
var b = {
  firstName: 'John',
  lastName: 'Doe',
}
a.fullName.call(b);