Calculator with Vanilla JavaScript

Tolulope — Mon, 01 Jun 2020 21:02:23 +0000

I will like to share how I made a simple calculator with JavaScript.

The calculator can perform basic arithmetic operations like addition, subtraction, multiplication and division of numbers.

Here is how I did it:

Step 1: Stored my selectors in variables to target the DOM elements

const erase = document.querySelector("#erase");
const user = document.querySelector("#user");
const ops = document.getElementsByClassName("ops");
const equals = document.querySelector(".equals_to");
const num = document.getElementsByClassName("num");
const period = document.getElementById("period");

Step 2: Declare global variables

I declared variables and assigned them their initial values.

let currentNum = ""; //to store clicked number
let operator = null; //initial value of the operator
let calculation = []; //array to store the numbers and operators
let previousNum = ""; //to store the last calculated result

Step 3: Function structure

Functions to update and calculate the numbers.

// To update the button number
const updateNum = (e) => {
} 
// To select and update operator
const selectOperator = (e) => {
} 
// To calculate the result
const getResult = (e) => {
}

Step 4: Onclick event listeners

I assigned functions accordingly to the elements onclick

for ( i = 0; i<num.length; i++) {
  num[i].addEventListener('click', updateNum);
}
for (let i = 0; i<ops.length; i++) {
  ops[i].addEventListener('click', selectOperator);
}

equals.addEventListener('click', getResult);
}

Step 5: Functions to store clicked numbers

To declare a variable to store the user clicked input;

const numText = e.target.innerText;

To first check if operator is null and previousNum has a value. If true, clear previousNum value;

const updateNum = (e) => {
  if (operator === "" && previousNum !== ""){
  previousNum = "";
  }

Also check if number clicked is a decimal, and confirm that currentNum does not already have a decimal. If true if does, set numText to null.

else if (numText === "." && currentNum.includes ('.')) {
   numText = null;
 }

Lastly check if numText is decimal and currentNum has no stored value. If true, set currentNum to 0;

if (currentNum === "" && numText === ".") {
    currentNum = "0.";
 user.innerHTML = currentNum;

now, append the value of numText to currentNum ;

else {
  currentNum += numText;
 user.innerHTML = currentNum;
 }
}

Declared an operator to take the text content of the button operator clicked
If the last value of the array is not equal to an operator, let operator be push to the array string;

Step 6: Functions to store operator clicked

First check if previousNum is empty, if it isn't, push the value to the calculation array, and clear previousNum empty. Then check the last value of the calculation array, if it is not an operator, push the operator to the array;

const selectOperator = (e) => {

if (previousResult !== "") {
    calculation.push(previousResult);
    if (calculation[calculation.length - 1] !== ("+" || "-" || "/" || "*")) {
      operator = e.target.innerText;
      calculation.push(operator);
    }
    previousResult = "";
  }
}

Next, check if currentNum has a value, if yes, push the value to the calculation array. Then check the last value of the calculation array, if it is not an operator, push the operator to the array;

 if (currentNum !== "") {
    calculation.push(currentNum);
    if (calculation[calculation.length - 1] !== ("+" || "-" || "/" || "*")) {
      operator = e.target.innerText;
      calculation.push(operator);
    }
  }

then clear currentNum

currentNum="";

Step 7: Function to carry out the calculation

This will be triggered when the equal button is clicked
First check if currentNum has a value. If yes, add the value to the calculation array. Then carry out the calculation;

function getResult(e) {
  if (currentNum !== "") {
    calculation.push(currentNum);
  }
  var result = eval(calculation.join("")).toString();

  user.innerHTML = result;
  previousResult = result;
  currentNum = "";
  calculation = [];
  operator = null;
}

Step 8: Erase function

Onclick of the erase button, set all variable to their initial values;

erase.onclick = () => {
user.innerHTML = "0";
currentNum = "";
pendingNum = "";
calculation = [];
}

You can view the full working code on:

Alright! I hope this brief article helped to build your own calculator. Thanks for reading and happy coding! I'm opened to questions and suggestions.

DEV Community: Tolulope

Calculator with Vanilla JavaScript