How to escape slashes in a Javascript string?

You must encode the value returned by do_shortcode() using the rules of the target language (JavaScript).

The best PHP way to encode a string to generate JavaScript code is json_encode().

Your code should be:

var myvariable = <?php print json_encode(do_shortcode('[shortcode]')); ?>;

The quotes surrounding the string are not needed any more. If given a string, json_encode() produces a representation of the input string that is a valid JavaScript string source code.

For example, if do_shortcode('[shortcode]') returns <div><a href="#"><span>Link text</span></a></div> then the line of code above produces:

var myvariable = "<div><a href=\"#\"><span>Link text<\/span><\/a><\/div>"

which is valid JavaScript code that puts the aforementioned string into the JavaScript variable myvariable.

Thank you for your answers. This is the code in file-a.php (functions.php, WordPress):

function shortcode_function() { 
    print '<p>Test</p>'; 
}
add_shortcode( 'shortcode', 'shortcode_function' );

This is the code in file-b.php (footer.php, WordPress):

<script>
    jQuery(document).ready(function($) {
    var myvariable = "<?php print do_shortcode('[shortcode]'); ?>";
    $( '.footer' ).prepend( myvariable );
    });
</script>

As you can see this returns a broken < p > tag.

<script>
        jQuery(document).ready(function($) {
        var myvariable = "<p>Test";
        $( '.footer' ).prepend( 'myvariable );
        });
    </script>
</body>
</html>

This is the result with json_encode(), with and without quotes, and nothing is visible on the public page:

var myvariable = "<?php print json_encode(do_shortcode('[shortcode]')); ?>";
var myvariable = "<p>Test""";

var myvariable = <?php print json_encode(do_shortcode('[shortcode]')); ?>;
var myvariable = <p>Test"";

In practice, I am trying to print some dynamic data before the footer area, data extracted from the DB through PHP queries. Dynamic data is correctly printed, but with an incorrect HTML markup.

This is my solution. It works in its basic form, and now I am expanding it.

I have escaped slashes with a backslash in functions.php>

print "<p class='red'>RED<\/p>";

This returns the following code in footer.php:

var str = "<p class='red'>RED<\/p>";

All I have done is sanitizing the string, like so:

var sanitized_str = str.replace(/\\/g, '');

The Javascript variable...

sanitized_str

...is renderered properly, with a correct HTML markup visible online when it is printed like so:

$( '.footer' ).prepend( sanitized_str );

All tags are improperly closed in the page markup (ex. </p> but the public outcome is sanitized and clearly visible.