JavaScript Fix the Scope Lab

Objectives

• Explore scope in JavaScript
• Exercise bug-finding skills related to scope issues

Instructions

In this lab, you'll need to debug some issues related to function scope in JavaScript: variables might not be what we expect, certain variables might be unavailable, and in general, havoc might be set loose in the current code.

As usual, start by running the specs and reading the results. Then fix some things, run the tests and repeat.

myAnimal and yourAnimal

The first two functions, myAnimal and yourAnimal, both return a variable animal, but the tests for the two expect different values. The tests are designed so that:

• you cannot hard code return 'cat'
• yourAnimal must return a variable named animal.
• yourAnimal should not reassign the existing animal declared on the first line (in the global scope).

After the initial tests are run for myAnimal and yourAnimal, myAnimal is tested again to ensure that the original globally scoped animal variable is not altered.

Hint: Remember that variables declared inside a function are within the function's scope.

add2

The third function in index.js, add2() takes in a number and returns that number plus two. Well, at least, that is what it should do. What is wrong?

funkyFunction and theFunk

The final task of this lab is to figure out what code is needed at the end. The provided code does not need to change, but something else needs to be added.

A Hint About ()

Remember the purpose of using () is to call functions in JavaScript. It essentially tells our code to execute the function. A function called without a () (i.e. functionName instead of functionName()), will return the function definition but NOT run it. You'll see the return value as [Function: functionName].

To get our code to execute that function, we instead call functionName(), which executes the code within that function.

As an example, the third test here at first returns: Error: Expected [Function: funkyFunction] to equal 'FUNKY!'

We can make the function execute by saying this: var theFunk = funkyFunction() But this returns ANOTHER FUNCTION definition! If you recall from the previous lesson, a closure is when a function is returned by another function, allowing the returned (inner) function to have access to variables declared inside the scope of the original (outer) function.

// An example closure
function outer() {
  var example = "Greetings "
  return function inner(name) {
    return example + name
  }
}

// greeting is assigned the definition of the inner function
var greeting = outer()

// we can then call greeting and pass in a string
greeting('Proffesor Falken')

// when called, greeting will combine the provided string with the 'example' variable defined in the outer function
// => "Greetings Proffesor Falken"

Notice in the above code, outer() is calle once, and its return value is assigned to the greeting variable. The return value is the inner function definition. We never explicitly call inner() because its definition gets assigned to greeting. Instead, we call greeting()!

Resources

• MDN: Scope

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