How to escape string from PHP for javascript?

lets imagine a form editor, it can edit available values. If the data contains " character (double quote) it "destroys" HTML code. I meant, lets check the code: so I generate HTML:

onclick="var a = prompt('New value: ', '<?php echo addslashes($rec[$i]); ?>'); if (a != null)....

and it results in

onclick="var a = prompt('New value: ', 'aaaa\"aaa'); if (a != null) { v....

and this makes JS work impossible, so that it ruins the code. With single qoute ' it works OK. mysql real escape does the same. How to escape any string so that it won't ruin javascript?

json_encode looked OK, but I must be doing something wrong, its still bad: heres a screenshot how Firefox sees it - it inserts a "bad" double quote! The value is just a simple number:

and I did used:

('Ird be az új nevet:', <?php echo json_encode($rec['NAME']); ?>); if (a) { 

The value of the onclick attribute should be escaped like any other HTML attribute, using htmlspecialchars(). Actual Javascript strings inside the code should be encoded using json_encode(). For example:

$message = 'Some \' problematic \\ chars " ...';
$jscode = 'alert('.json_encode($message).');';
echo '<a onclick="' . htmlspecialchars($jscode) . '">Click me</a>';

That being said... onclick (or any other event) attributes are so 2005. Do yourself a favor and separate your javascript code from your html code, preferably to external file, and attach the events using DOM functions (or jQuery, which wraps it up nicely)

onclick="var a = prompt('New value: ', 'aaaa\"aaa'); if (a != null) { v....

Your problem is highlighted in bold. You can't quote a variable declaration you shouldn't need to escape the double quote once this is removed since it is within single quotes. Should look like this -

function newFunc()  {
var a = prompt('New value: ', 'aaaa"aaa'); 
if (a != null) { v....

function new_func() {
    var a = prompt('new value:','<?php code; ?>');
    if (a) { <!--javascript code--> } else { <!--javascript code--> }

I'm really just re-wording what @Marshall House says here, but:

In HTML, a double quote (") will always end an attribute, regardless of a backslash - so it sees: onclick="var a = prompt('New value: ', 'aaaa\". The solution that @Marshall offers is to separate your code out into a function. This way you can print escaped PHP into it without a problem.


    // This is a function, wrapping your code to be called onclick.
    function doOnClickStuff() {
        // You should no longer need to escape your string. E.g.:
        //var a = prompt('new value:','<?php echo $rec[$i]; ?>');
        // Although the following could be safer
        var a = prompt('new value:',<?php json_encode($rec[$i]); ?>);
        if (a) { <!--javascript code--> }
        else { <!--javascript code--> }
<someelement onclick="doOnClickStuff();"> <!-- this calls the javascript function doOnClickStuff, defined above -->