Sometimes we want to make validations on an input box for characters that user can type. And other characters will not be entertained in the input box.
Question: Make a input box that accepts digits and spaces only. Also, care for copy-paste (Ctrl +V ) of invalid characters.
The first step is to register an event on the input tag. But what event type? 🤔 We are typing characters into it so keypress the event looks fine.
<input type="text" id="input"/>
const input = document.getElementById('input');
var currentInputValue = '';
input.addEventListener('keypress', function inputKeypressHandler(e) {
// keypress event on input box is listened here and this function is triggered
// which key is pressed, keyPressed = e.which || e.keyCode;
const key = e.which || e.keyCode;
// key code corresponds to digits 0-9 or space then okay👍🏼
// 0-9 key code is 48-57
// space keycode is 32
const SPACE = 32; // May be just stored somewhere
const ZERO = 48;
const NINE = 57;
// valid digit is b/w 0-9 thus invalid will be lt 0 or gt 9
const isNotValidDigit = key < ZERO || key > NINE;
// if key is not a space or not a digit prevent this event
if (key != SPACE || ( isNotValidDigit ) ) {
e.preventDefault();
}
});
This is a pretty good solution but this doesn’t prevent paste cheats. And that is because the keypress event only records key pressed inside the input box. A better event type is required. input it runs on all input ways including copy paste and drag.
var currentInputValue = '';
input.addEventListener('input', function inputInputEventHandler(e) {
// use target of event to get value of input
const target = e.target;
// we can use regex to check if current input value
// is valid input
const DIGITS_SPACE_REGEX = /^[0-9\s]*$/;
// test if target.value or value of input box now is valid.
// if value is valid then update currentInputValue
// target.value else its is not value and we will
// ignore this target.value and replace it with
// previously valid value currentInputValue
DIGITS_SPACE_REGEX.test(target.value)
? ( currentInputValue = target.value )
: ( target.value = currentInputValue );
});
This solves our problem of paste but there is one problem here. Say you paste something Ctrl/Cmd + V your current cursor position will be lost and moved to starting. This must not happen and you must be able to retain cursor position.
// Track cursor position
// cursor position is changed when you type something
const cursorState = {};
input.addEventListener('keydown', function inputKeydownHandler(e) {
const target = e.target;
// record the start and end
cursorState.selectionStart = target.selectionStart;
cursorState.selectionEnd = target.selectionEnd;
});
now in input handler
// modify
DIGITS_SPACE_REGEX.test(target.value)
? ( currentInputValue = target.value )
: ( target.value = currentInputValue );
// to
if (DIGITS_SPACE_REGEX.test(target.value)) {
currentValue = target.value;
}
else {
target.value = current.value;
// restore cursor state
target.setSelectionRange(
cursorState.selectionStart,
cursorState.selectionEnd
);
}
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