JavaScript Calculator Challenge Lab

Learning Goals

• Practice doing math with JavaScript
• Practice writing functions that do things with numbers
• Practice parsing strings as numbers

Introduction

In this lab, we're going to practice writing functions and manipulating numbers in JavaScript. First, though, we need to review some basic math. In this lab, we're going to learn about various arithmetic operators. What's an operator, you say? It's a symbol that operates on one or more (usually two) objects — + is a good example. The + operator says "add what's to the left of + and what's to the right of + together."

If you haven't already, fork and clone this lab into your local environment. Remember to fork a copy into your GitHub account first, then clone from that copy. Once you've cloned it down, navigate into its directory in the terminal, then run code . to open the files in Visual Studio Code. (If you are using a different text editor, the command will be different.)

As you read through this lesson, you're going to be adding your solutions to index.js. You'll write a total of eight functions. Use the results of running npm test to guide you towards the right function names and functionality. Remember to run npm install first to install the dependencies.

Basic Math

The most fundamental math operations work as one might expect in JavaScript: + adds two numbers; - subtracts one number from another; * multiplies two numbers; and / divides one number by another.

Give it a try: type each of the following math examples into your terminal prompt after using the node command.

1 + 80; //=> 81
60 - 40; //=> 20
2 * 3.4; //=> 6.8
5.0 / 2.5; //=> 2

You can then exit the node prompt using .exit then navigate back to the lab directory in your terminal if necessary. Go ahead and run npm test and take a look at the first failing test:

  1) basic math functions
       'add()' is a valid function:
     ReferenceError: add is not defined
      at Context.<anonymous> (test/indexTest.js:10:12)
      at processImmediate (node:internal/timers:464:21)

The description of the test, 'add()' is a valid function, along with the error message, referenceError: add is not defined, tells us that we need to define a function add(). Go ahead and create the add() function in index.js. This test is only looking for the function to exist, so we can leave the code block empty for now:

function add() {
  // we'll fill this in shortly
}

Once you've added the empty function, run npm test again; the first test should be passing. Go ahead and get the next three tests passing as well before moving on.

Once you have the first four tests passing, the first error you get should look similar to this:

  1) basic math functions
       add(a, b) adds two numbers and returns the result:
     Error: Expected undefined to equal 1078
      at assert (node_modules/expect/lib/assert.js:29:9)
      at Expectation.toEqual (node_modules/expect/lib/Expectation.js:81:30)
      at Context.<anonymous> (test/indexTest.js:26:23)
      at processImmediate (node:internal/timers:464:21)

Note: the test document is randomly creating numbers to pass as arguments to the functions in this lab, so the specific values you see in your errors will be different from what's shown in this lesson, and will be different each time you run the tests!

The tests have found our add() function, getting us past the first test, and now we need to write the code inside the function to return what the test is expecting. Let's break down the information from our error:

add(a, b) adds two numbers and returns the result:
Error: Expected undefined to equal 1078

The first line tells us what our function needs to do, and the second line tells us the specific error that is being thrown. Any time you see the error "Expected undefined to equal" something, that means the test is expecting your function to return a value but it's currently not returning anything (undefined). Let's see if we can just get that specific error cleared. We won't worry about exactly what we're returning just yet:

function add() {
  return "something";
}

Running the tests now we get:

  1) basic math functions
       add(a, b) adds two numbers and returns the result:
     Error: Expected 'something' to equal 853
      at assert (node_modules/expect/lib/assert.js:29:9)
      at Expectation.toEqual (node_modules/expect/lib/Expectation.js:81:30)
      at Context.<anonymous> (test/indexTest.js:26:23)
      at processImmediate (node:internal/timers:464:21)

Great! So now our function is returning "something" instead of nothing! So the next step is to look more closely at the test's description of the function to figure out what should be returned:

add(a, b) adds two numbers and returns the result

The add(a, b) tells us the test is trying to pass two arguments to our function, so let's get that set up:

function add(a, b) {
  return "something";
}

The rest of the description tells us we need to add the two arguments together and return that value:

function add(a, b) {
  return a + b;
}

With this code, both tests for the add() function should now be passing!

A Quick Note about Hard Coding

Think back to the errors we were getting above, e.g., Expected undefined to equal 1078. One thing that might be tempting to do when addressing an error like this is to explicitly return exactly what the test is looking for:

function add(a, b) {
  return 1078;
}

This is what's known as hard coding and is virtually never what you should do! If you think about it, it doesn't really make sense. We've created an add() function that can take any two numbers as arguments but will always return 1078. This makes it a pretty useless function. Instead, we want to create functions that will return the correct answer for whatever argument or arguments we pass in.

Go ahead and tackle the next three tests on your own. You should follow a process very similar to what we did above. Once you have those passing, continue to the next section.

Math + Assignment

Recall that we can increment (++) and decrement (--) a number if it's assigned to a variable. Please follow along in the terminal using the node command.

let number = 5;

number++; //=> 5... hmmmm

number; //=> 6 -- the number was incremented after it was evaluated

number--; //=> 6

number; //=> 5

We can also put the increment and decrement operators before the number, in which case the number is evaluated after the operator is executed:

--number; //=> 4

number; //=> 4

++number; //=> 5

number; //=> 5

But generally, you will see them placed after the number (and we recommend that that's where you put them). If you're interested in the difference, take a look at the MDN documentation.

And, while we're on the subject, you'll usually only want to use these incrementors and decrementors when the shorthand makes what you're writing easier to read (more on when exactly in a later lesson). Instead, it's best to use the basic arithmetic operators combined with =.

Follow along in the terminal. You can pick up where we left off with the previous examples. Start by setting number equal to 5.

• += modifies the value to the operator's left by adding to it the value to the operator's right:

number += 3; //=> 8

• -= modifies the value to the operator's left by subtracting from it the value to the operator's right:

number -= 2; //=> 6

• *= modifies the value to the operator's left by multiplying it by the value to the operator's right:

number *= 10; //=> 60

• /= modifies the value to the operator's left by dividing it by the value to the operator's right:

number /= 5; //=> 12

Note that these methods modify the variable in place. So, if we have two functions that depend on the same external variable, the order in which they are called matters. Follow along in the terminal:

//reset number
number = 10;

function add5() {
  return (number += 5);
}

function divideBy3() {
  return (number /= 3);
}

divideBy3(); //=> 3.3333333333333335

add5(); //=> 8.333333333333334

// reset number
number = 10;

add5(); //=> 15

divideBy3(); //=> 5

Because these methods are more explicit, we prefer += to ++ and -= to -- (usually).

Okay, now we're ready to write solutions for the next two functions: increment(n) and decrement(n). As indicated by the tests, these methods should take a number as an argument, increment/decrement the provided value by one, and return the result.

Parsing Numbers

Sometimes, we'll receive a number wrapped in quotes. We recognize it as a number, but JavaScript will think it's a string. Luckily, JavaScript gives us tools to turn these strings into proper numbers (that is, numbers that JavaScript understands).

parseInt()

The first such tool is the function parseInt(), which accepts two arguments: the value to parse and the base of the value being parsed (called the radix). Usually you will want to work with base 10, so a typical call to parseInt() looks like:

parseInt("2", 10); //=> 2

What happens if we pass a representation of a non-integer to parseInt()? Let's try it:

parseInt("2.2222", 10);

If we enter the above in the REPL console, we will see that parseInt() forces the parsed number to be an integer — which makes sense when we think about it, right?

What happens, though, if we pass utter nonsense to parseInt()? Go ahead and try it — something like:

parseInt("nonsense!", 10);

What did it return? NaN? What is that?

NaN stands for "Not a Number" — pretty handy, right? This is the number (in the JavaScript sense) that JavaScript returns when it can't determine a valid value for a numeric operation.

Note: You may see parseInt() used without a radix specified. In most cases, the radix will default to 10, but not always! To be safe, you should always specify a radix.

parseFloat()

Above, we saw that parseInt() lops off everything after the decimal point and only returns integers. If we want to preserve decimals, we'll need to use parseFloat().

Unlike parseInt(), parseFloat() accepts only a single argument, the thing to be parsed. We can use it like so:

parseFloat("80.123999"); // 80.123999

You now have the information you need to write the final two functions, makeInt(string) and preserveDecimal(string). makeInt(string) should take in a string, parse it into a base 10 integer and return it. preserveDecimal(string) should take in a string, parse it into a float and return it.

After you have all the tests passing, remember to commit and push your changes up to GitHub, then submit your work to Canvas using CodeGrade. If you need a reminder, go back to the Complete Your First Software Engineering Assignment lesson to review the process.

Resources

• MDN - parseInt()

• MDN - parseFloat()