user pressed key in javascript code example

Example 1: js keyboard input

document.addEventListener('keydown', function(event) {
    if(event.keyCode == 37) {
        alert('Left was pressed');
    }
    else if(event.keyCode == 39) {
        alert('Right was pressed');
    }
});

Example 2: html onkeypress

<input type="text" onkeypress="myFunction()">