Aniket Jha
Late Night Coder

Late Night Coder

Allow to enter particular characters only

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Aniket Jha
·Jul 12, 2022·

3 min read

Allow to enter particular characters only

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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  
    );  
 }

Demo 👨‍💻

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