为什么连接两个字符串会出错？

I have a text field and an input type submit. When I click on the submit button I should get a url related to the input field. But I am getting Uncaught TypeError: undefined is not a function. Here's my code that explains more better:

<script>

    $(document).ready(function(e) {

        $('#search_btn').click(function() {
            console.log($('#thisiswhatineed').val()); // this is working perfectly
            console.log("<?php echo BASE_URL; ?>/ads/search/"); // this is also working perfectly
            var url_to_go = "<?php echo BASE_URL; ?>/ads/search/" . $('#thisiswhatineed').val(); 
            console.log(url_to_go); // this is not working and giving me the error mentioned above
        });

    });

</script>

In javascript, you concatenate strings with +, not .

So:

var url_to_go = 
   "<?php echo BASE_URL; ?>/ads/search/" + $('#thisiswhatineed').val(); 

Use + operator instead of . to concatenate two strings in JavaScript.

Instead of

var url_to_go = "<?php echo BASE_URL; ?>/ads/search/" . $('#thisiswhatineed').val();

use

var url_to_go = "<?php echo BASE_URL; ?>/ads/search/" + $('#thisiswhatineed').val();

In Javascript you use the plus symbol for concatenation.

var url_to_go = "<?php echo BASE_URL; ?>/ads/search/" + $('#thisiswhatineed').val();