Unexpected Symbol ' var'

I’m really new to coding, and I’m trying to make AI in a game that randomly moves around in Unity. I’m trying to use a script I found, but it’s giving me the error of “Unexpected symbol ’ var’”
Does anybody know how to fix this?

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
var stuff: Transform;
var vel: Vector3;
var switchDirection: float = 3;
var curTime: float = 0;
 
function Start()
{
    SetVel();
}

function SetVel()
{
    if (Random.value > .5)
    {
        vel.x = 10 * 10 * Random.value;
    }
    else
    {
        vel.x = -10 * 10 * Random.value;
    }
    if (Random.value > .5)
    {
        vel.z = 10 * 10 * Random.value;
    }
    else
    {
        vel.z = -10 * 10 * Random.value;
    }
}

function Update()
{
    if (curTime < switchDirection)
    {
        curTime += 1 * Time.deltaTime;
    }
    else
    {
        SetVel();
        if (Random.value > .5)
        {
            switchDirection += Random.value;
    else {
                switchDirection -= Random.value;
            }
            if (switchDirection < 1)
            {
                switchDirection = 1 + Random.value;
            }
            curTime = 0;
        }
    }

    function FixedUpdate();
{
        stuff.rigidbody.velocity = vel;
    }]

I’m no expert but you are using JavaScript in the body but the header looks like what goes on C#. If the error line is line 4 - the first time var is used then that’s the problem. try it with out the first three lines
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
and start with:
var stuff: Transform;
this is a java script