Script to calcualte the player's distance?

I need a script that says how far the player has travelled from an initial starting point, and to display that as a “score”. Preferably javascript, I’m not comfortable with C++

Just cumulate the delta distance between two updates.

// JS
var previousPosition : Vector3; // variable to keep the previous position
var cumulatedDistance : float;

function Awake()
{
    // initialize to the current position
    previousPosition = transform.position;
}

function Update()
{
    // add the distance covered during the last frame
    cumulatedDistance += (transform.position - previousPosition).magnitude;
    // update the 'previous' position to the current one
    previousPosition = transform.position;
}

C# version looks almost the same.

// C#
private Vector3 previousPosition; // variable to keep the previous position
public float cumulatedDistance;

void Awake()
{
    // initialize to the current position
    previousPosition = transform.position;
}

void Update()
{
    // add the distance covered during the last frame
    cumulatedDistance += (transform.position - previousPosition).magnitude;
    // update the 'previous' position to the current one
    previousPosition = transform.position;
}

This solution is very basic. You may need to check first if the distance covered isn’t too small to prevent the distance value to jitter. For example:

// JS
var covered : float (transform.position - previousPOsition).magnitude;
if (covered < 0.001f)
{
    // don't cumulate because the speed is less than 6 cm/s (at 60 FPS)
}
else
{
    cumulatedDistance += covered;
}

Since calculating a distance uses a square root (a quite expensive calculation), it may be better to calculate it less often. You will loose some precision (the less frequent the less precision), but will save performance. For example you can use Start() as a coroutine:

// C# (sorry I'm not familiar with coroutine in JS)
public float cumulatedDistance;

IEnumerator Start()
{
    Vector3 previousPosition = transform.position;
    this.cumulatedDistance = 0;

    while (true)
    {
        // add the distance covered since the 'previous' position
        this.cumulatedDistance += (transform.position - previousPosition).magnitude;
        // update the 'previous' position to the current one
        previousPosition = transform.position;

        yield return new WaitForSeconds(1.0f);           
    }
}