How do I sanitize the str_replace function in javascript variables

esc_js() is intended for escaping data for use within an HTML attribute.

If you want to escape data for use within an inline script, wp_json_encode() should be sufficient.

For example:

var disabledDays = <?php echo wp_json_encode( $iva_disable_days ); ?>;

This outputs:

var disabledDays = ["4\/7\/2018","11\/18\/2017"];

If you check the variable in your dev tools console, you will see that it is being parsed correctly:

However – if you absolutely do not want escaped slashes (not recommended), the second param to wp_json_encode() is a bitmask of options:

var disabledDays = <?php echo wp_json_encode( $iva_disable_days, JSON_UNESCAPED_SLASHES ); ?>;

This outputs:

var disabledDays = ["4/7/2018","11/18/2017"];

For a list of available options, check the PHP json_encode() docs.