How to assign a json object in a particular localhost route to a variable in javascript

I am quite new to learning web development and have been stuck for hours and my knowledge of Javascript is not that well. I have a route localhost:8080/api/employee/ so that when I enter this route in my browser it shows a JSON

[
    {
        "id":1,
        "employee_name":"Anders",
        "department_id":4
    },
    { 
        "id":3,
        "employee_name":"Brownea",
        "department_id":17
    },
    {
        "id":4,
        "employee_name":"Chow",
        "department_id":19
    }
]

In the more specific route, localhost:8080/api/employee/{id}, it shows for instance id: 1

{data: 
    {
        "id":1,
        "employee_name":"Anders",
        "department_id":4
    }
}

I am trying to assign a json object in a particular localhost route to a variable in javascript.

Currently the problem is the number 3

1. In the javascript, I have tried this:

const fileHandler = async () => {
    const res = await fetch("localhost:8080/api/employee/1")
    .then((res) => {
        console.log(res)
    }
}

In the console it display:

XHRGEThttp://localhost:8080/api/employee/1
CORS Missing Allow Origin

Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at http://localhost:8080/api/employee/1. (Reason: CORS header ‘Access-Control-Allow-Origin’ missing). Status code: 404.
Uncaught (in promise) TypeError: NetworkError when attempting to fetch resource. EmployeeRegis.js:34

2. I then try to do this in the javascript:

const res = await fetch("localhost:8080/api/employee/1", {
    method: "GET",
    mode: "no-cors",
})
.then((res) => {
    console.log(res)
}

The console shows:

XHRGEThttp://localhost:8080/api/employee/1
[HTTP/1.1 404 Not Found 7ms]

Response { type: "opaque", url: "", redirected: false, status: 0, ok: false, statusText: "", headers: Headers, body: null, bodyUsed: false }
EmployeeRegis.js:34

3. I also try the mm107 answer and do this:

const res = await fetch("http://localhost:8080/api/employee/1", {
    method: "GET",
    mode: "no-cors",
}).then(res => {
    console.log(res)
    console.log(res.json())
    res.json()
}).then(data => {
    console.log(data)
})

It still return some error in console:

Response { type: "opaque", url: "", redirected: false, status: 0, ok: false, statusText: "", headers: Headers, body: null, bodyUsed: false }
EmployeeRegis.js:37

Promise { <state>: "pending" }
<state>: "rejected"
<reason>: SyntaxError: JSON.parse: unexpected end of data at line 1 column 1 of the JSON data
​​columnNumber: 0
fileName: ""
lineNumber: 0
message: "JSON.parse: unexpected end of data at line 1 column 1 of the JSON data"
stack: ""
<prototype>: SyntaxError.prototype { stack: "", … }
<prototype>: Promise.prototype { … }
EmployeeRegis.js:39

undefined EmployeeRegis.js:43

Uncaught (in promise) SyntaxError: JSON.parse: unexpected end of data at line 1 column 1 of the JSON data 2

I am using Golang as the backend, here is the handler (using GoFiber and gorm):

func HandlerEmployeeID(ctx *fiber.Ctx) error {
    employeeID := ctx.Params("id")

    var emply entity.Employee

    error := database.DB.First(&emply, "id = ?", employeeID).Error
    if error != nil {
        return ctx.Status(404).JSON(fiber.Map{
            "message": "failed get employee by the id",
        })
    }
    // Previously in number 1 and 2, I pathetically return 404 status not found
    // return ctx.Status(404).JSON(fiber.Map{
    //  "message": "successfully get employee by the id",
    //  "data":    emply,
    // })
    
    // In the number 3, I have update it like this
    return ctx.JSON(fiber.Map{
        "message": "successfully get employee by the id",
        "data":    emply,
    })
}

What I want on console.log(resSomething) returns this:

{data: 
    {
        "id":1,
        "employee_name":"Anders",
        "department_id":4
    }
}

How to do that?

Updated:

I just know that the mode: "no-cors" will not result in my expected response body. But, if I am not using that mode, it will keep returning an error in the console. I have tried to run parallelly lcp --proxyUrl http://localhost:8080/api/employee/1 but the CORS issue is still not handled.