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JavaScript Short Project: Calculator

Overview

In this project, we’re building a simple calculator that allows users to perform basic arithmetic operations such as addition, subtraction, multiplication, and division. The calculator has a simple user interface, consisting of an input field and several buttons that correspond to the different numbers and operations that can be performed.

The goal of this project is to provide an introduction to working with JavaScript events and DOM manipulation, as well as some of the challenges that arise when building user interfaces that involve complex user interactions.

Technologies Used

  • IDE: Visual Studio Code
  • Languages: JavaScript, HTML, and CSS

Challenges

One of the primary challenges involved in building a calculator is handling user input and converting that input into the appropriate mathematical operations. This involves not only capturing the user’s input but also validating and processing that input in a way that ensures the calculator produces accurate results.

Another challenge is building a user interface that is intuitive and easy to use. This requires careful consideration of factors such as button placement, color schemes, and feedback mechanisms that provide users with a clear sense of how the calculator is responding to their input.

JavaScript Methods Used

To overcome the challenges involved in building a calculator, we will use a variety of JavaScript methods and techniques. These include:

DOM manipulation:

We’ll use DOM manipulation techniques to update the calculator display in real-time as the user enters their input. This will involve updating the text content of various HTML elements such as the input field and the calculator display.

let buttonItems = document.querySelectorAll('button');
let entry = document.getElementById('entry');

This code defines two variables, buttonItems and entry, which are used to interact with the buttons and input fields in the calculator. Here is an explanation of each line:

  1. let buttonItems = document.querySelectorAll('button'); This line uses the document.querySelectorAll() method to select all of the <button> elements in the HTML document. It then assigns this collection of elements to the buttonItems variable as a NodeList. This variable can be used to interact with each button individually, such as adding event listeners or changing their content.
  2. let entry = document.getElementById('entry'); This line uses the document.getElementById() method to select the <input> element with the ID of 'entry'. It assigns this element to the entry variable, which can be used to read or modify the value of the input field. For example, the calculator can append digits and operators to this input field as the user presses the buttons.

Event listeners:

We’ll use event listeners to capture the user’s input when they click on a button or press a key on their keyboard. This will allow us to track the user’s input in real-time and update the calculator display accordingly.

buttonItems[i].addEventListener('click', function(){})

This code is using the addEventListener() method to add a click event listener to each button in the buttonItems NodeList. Here is an explanation of each part of the code:

  1. buttonItems[i]: This selects the button element at the i-th index of the buttonItems NodeList. This index value is determined by the for loop which iterates through all of the buttons.
  2. .addEventListener('click', function(){}): This method adds an event listener to the selected button element, which listens for a click event. When the button is clicked, the second argument, a function, is executed. This function can contain any code that should run when the button is clicked.

Putting these two parts together, the buttonItems[i].addEventListener('click', function(){}) code adds a click event listener to each button in the calculator, allowing the JavaScript code to respond when each button is clicked.

Array manipulation:

We’ll use arrays to store the user’s input as they type it into the calculator. This will allow us to easily perform operations on that input, such as joining the array elements together to form a mathematical expression.

let entryList = [];

This code declares a variable entryList and initializes it with an empty array. Here is an explanation of what is happening in this line of code:

  1. let entryList: This declares a new variable named entryList. The let keyword is used to indicate that this variable is block-scoped, meaning that it is only accessible within the block of code where it was declared.
  2. = []: This initializes the entryList variable with an empty array. An array is a data structure in JavaScript that can hold multiple values of different types. In this case, entryList is an empty array, meaning it has no elements yet.

By initializing entryList as an empty array, this code sets up a place to store the user’s input values as they click the calculator buttons. The array will be populated with button values as the user clicks them, and can be used later to calculate the result of the user’s input.

join() and eval() methods:

The join() method is an array method that joins all the elements of an array into a string. It takes an optional argument that specifies the separator to be used between the elements. If no separator is specified, the elements are joined with a comma by default.

In the calculator code, join() is used to combine the array of input values into a string that can be evaluated by the eval() method. For example, if the input values are [2, ‘+’, 3], join() is used to combine them into the string “2+3”.

The eval() method is a global function in JavaScript that evaluates a string of code as if it were a JavaScript statement. It can be used to perform calculations on a string of input values, as in the calculator code.

In the calculator code, eval() is used to perform the calculation on the string of input values generated by join(). For example, if the input values are [2, ‘+’, 3], join() is used to generate the string “2+3”, which is then passed to eval() to calculate the result.

let expression = entryList.join('');
let result = eval(expression);

This code creates a string expression from the values stored in the entryList array using the join() method, and then evaluates that expression as a mathematical equation using the eval() function. Here is an explanation of what is happening in these lines of code:

  1. let expression = entryList.join('');: This line of code creates a new variable named expression and initializes it to the result of calling the join() method on the entryList array. The join() method concatenates all the elements in an array into a single string, with an optional separator between each element. In this case, the separator is an empty string (''), meaning that the elements will be concatenated with no spaces or other characters between them. This creates a string that represents the mathematical expression entered by the user.
  2. let result = eval(expression);: This line of code creates a new variable named result and initializes it to the result of calling the eval() function on the expression string. The eval() the function takes a string argument that represents a JavaScript expression or statement, and evaluates it as if it were actual code. In this case, the expression string contains a mathematical equation that the user entered by clicking the calculator buttons. The eval() function parses and evaluates this equation, and returns the result as a number.

Together, these two lines of code create a simple calculator that allows the user to enter a series of mathematical operations using buttons and then calculates the result of those operations when the user clicks the = button. The join() method is used to create a string expression from the user’s input, and the eval() function is used to evaluate that expression as a mathematical equation.

Conditional statements:

We’ll use conditional statements to handle the different types of user input and perform the appropriate operations based on that input. For example, when the user clicks the equals button, we’ll use a conditional statement to evaluate the mathematical expression and display the result.

if (buttonItems[i].textContent === 'Reset')
{
     // statements
}
else if (buttonItems[i].textContent === '=')
{
     // statements
}
else
{
    // statements
}

This is an if-else statement that is used to determine what happens when a user clicks on a button in the calculator.

The first condition if (buttonItems[i].textContent === 'Reset') checks if the button clicked is the “Reset” button. If it is, it resets the entryList array to an empty array and sets the value of the input element to an empty string, effectively resetting the calculator.

The second condition else if (buttonItems[i].textContent === '=') checks if the button clicked is the “=” button. If it is, it converts the entryList array to a string using the join() method, evaluates the resulting string as a mathematical expression using the eval() function, and displays the result in the input element.

Finally, the third condition else covers all other cases where the button clicked is a numeric button or an operator button (+, -, *, /). In this case, the value of the button is added to the entryList array, and its value is also displayed in the input element, allowing the user to build a mathematical expression.

See my code

JavaScript_Short_Projects/Calculator at main · michellealzola/JavaScript_Short_Projects (github.com)

See the site in Action

Calculator (michellealzola.github.io)

Conclusion

Building a calculator is a fun and challenging project that requires a combination of JavaScript programming skills and user interface design principles. By using the JavaScript methods and techniques outlined in this article, you should be well on your way to creating a calculator that is both functional and easy to use.

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