Scope Lesson Notes

scope.md

Scope

Objectives

• Describe what global scope is
• Describe what local (or lexical) scope is
• Explain why global scope should be avoided
• Describe what variable shadowing is
• Explain how variables are hoisted
• Explain how an IIFE can help with scope

Guiding Questions

Watch this video about scope: https://www.youtube.com/watch?v=SBwoFkRjZvE

• Turn to your neighbor and explain what scope refers to in javascript. How are global and local variables different?

  Your answer...

• What is a global variable? When are they useful? How can they be problematic?

  Your answer...

• Looking at the following code, identify all the local and global variables:

  var a = 4
  
  function foo(x) {
    var b = x
  }
  
  if(true) {
    var c = false
  }
  
  var d = 123

• What does it mean when a variable is shadowed?

  What will we see logged to the console?

  var foo = 5;
  
  function logFoo(foo) {
    console.log(foo);
  }
  
  logFoo(2);

  What are the values of a and b when this snippet is run?

  var a = 'red'
  var b = 'blue'
  function go() {
    a = 'yellow'
    var b = 'green'
  }
  go()

• What things get hoisted in javascript?

  What would we see printed out?

  function hoist() {
    a = 20;
    var b = 100;
  }
  
  hoist();
  
  console.log(a); 
  console.log(b); 

• How can IIFEs be used to avoid global scope issues?

  Your answer...

For the following questions, refer to this snippet of code:

var people = [
  { name: 'Rufus Harvey' },
  { name: 'Hector Ferguson' },
  { name: 'Pamela Simon' }
]

function sayHello () {
  for (var i = 0; i < people.length; i++) {
    const person = people[i]
    console.log(`Hello, ${person.name}!`)
  }

  console.log(`I said hello to ${i} people.`)
}

sayHello()

• Which variables are global?

  Your answer...

• Which variables are scoped to the overall function?

  Your answer...

• Which variables are scoped to a specific block?

  Your answer...

• Can they sayHello() function invocation be moved to above the function's definition? Why or why not?

  Your answer...