Question

I am learning about javascript from the book “JavaScript Bible”, but I have some difficulties.
I’m trying to understand this code:

function checkIt(evt) {
    evt = (evt) ? evt : window.event
    var charCode = (evt.which) ? evt.which : evt.keyCode
    if (charCode > 31 && (charCode < 48 || charCode > 57)) {
        status = "This field accept only numbers."
        return false
    }
    status = ""
    return true
}

Someone can explain to me?

Answer 1

I assume that you just want us to explain what the code does. If so, see below:

// Create a function named checkIt, which takes an argument evt.
function checkIt(evt) {

    // If the user has passed something in the argument evt, take it.
    // Else, take the window.event as evt.
    evt = (evt) ? evt : window.event;

    // Get the Character Code. It can be either from evt.which or evt.keyCode,
    // depending on the browser.
    var charCode = (evt.which) ? evt.which : evt.keyCode;

    // If the Character is not a number, the do not allow the user to type in.
    // The reason for giving 31 to 48 and greater than 57 is because, each key
    // you type has its own unique character code. From 31 to 48, are the numeric
    // keys in the keyboard.
    if (charCode > 31 && (charCode < 48 || charCode > 57)) {

        // Give a status informing the user.
        status = "This field accept only numbers.";

        // Prevent the default action of entering the character.
        return false;
    }

    // Clear the status
    status = "";

    // Allow the user to enter text.
    return true;
}

ASCII Code Reference

_{(source: cdrummond.qc.ca)}

_{(source: cdrummond.qc.ca)}

PS: I edited your code adding semi-colons ; which were missing.