Callback implementation for only allowing the insertion of numbers on an input.

only_numbers.js

$('#selector').on('keydown', onlyNumber);

function onlyNumber(event){
    // Allow: backspace, delete, tab, escape, and enter
	if ( event.keyCode == 46 || event.keyCode == 8 || event.keyCode == 9 || event.keyCode == 27 || event.keyCode == 13 ||
	// Allow: Ctrl+A
	(event.keyCode == 65 && event.ctrlKey === true) ||
	// Allow: home, end, left, right
	(event.keyCode >= 35 && event.keyCode <= 39) ||
	// Allow point
	(event.keyCode == 190)
	) {
		// let it happen, don't do anything
		return;
	} else {
	// Ensure that it is a number and stop the keypress
		if (event.shiftKey || (event.keyCode < 48 || event.keyCode > 57) && (event.keyCode < 96 || event.keyCode > 105 )) {
			event.preventDefault();
		}
	}
}

Con jQuery

//validaciones para valores NUMERICOS
$('.valida_numerico').keyup(function() {
    this.value = this.value.replace(/[^0-9]/g, '');
});

Con JavaScript

let elementosDOM = document.querySelectorAll('.valida_num_entero');
//Recorremos cada uno de nuestros elementos HTML

elementosDOM.forEach(elemento => {
    elemento.addEventListener('keyup', function() {   
        this.value = this.value.replace(/[^0-9]/g, '');
    });
});

ejemplo:

<input class="valida_numerico" />