run down with your code and receive it normally. It is recommended to check the code to see if it is caused by manual miswriting of the variable name.

// app.js
const express = require('express')
const app = express()

app.get('/test', function (req, res) {
    res.send(req.query.search_value)
})

app.listen(3000, () => console.log('Example app listening on port 3000!'))

// index.js
<script>
    window.onload = function () {
        initData();
    }

    function initData() {
        $.ajax({
            type: "get",
            url: "http://localhost:3000/test?search_value=" + 555,
            dataType: "jsonp",
            jsonp: "jsonpCallback",
            success: function (res) {
                console.log(res)
            }
        })    
    }   
</script>

result:

eval / new function