main.js

let display = document.getElementById('display');

let buttons = Array.from(document.getElementsByClassName('button'));

buttons.map(button => {
    button.addEventListener('click', (e) => {
        switch (e.target.innerText) {
            // Here we added break after end of case C because it was leaving C
            // we stopped its execution after printing'' so it left default case
            case 'C':
                display.innerText = '';
                break;
            case '←':
                display.innerText = display.innerText.slice(0, -1);
                break;
            // here eval is used to evaluate mathematical operations
            case '=':
                try {
                    display.innerText = eval(display.innerText);
                    break;
                }
                catch {
                    display.innerText = 'Error!';
                    break;
                }
            case 'sin':
                {
                    var x = display.innerText;
                    x = x * Math.PI / 180;
                    display.innerText = Math.sin(x);
                    break;
                }
            case 'cos':
                {
                    var x = display.innerText;
                    x = x * Math.PI / 180;
                    display.innerText = Math.cos(x);
                    break;
                }
            case 'tan':
                {
                    var x = display.innerText;
                    x = x * Math.PI / 180;
                    display.innerText = Math.tan(x);
                    break;
                }
            case '√':
                    {
                        var x = display.innerText;
                        display.innerText = Math.sqrt(x);
                        break;
                    }    
            
            
            case 'cosec':
                {
                    var x = display.innerText;
                    x = x * Math.PI / 180;
                    display.innerText = 1 / Math.sin(x);
                    break;
                }
            case 'cot':
                {
                    var x = display.innerText;
                    x = x * Math.PI / 180;
                    display.innerText = 1 / Math.tan(x);
                    break;
                }
            case 'sec':
                {
                    var x = display.innerText;
                    x = x * Math.PI / 180;
                    display.innerText = 1 / Math.cos(x);
                    break;
                }
            case 'ln':
                {
                    var x = display.innerText;
                    display.innerText = Math.log(x);
                    break;
                }
            default:
                display.innerText += e.target.innerText;


        }
    });
});