Make a variable start at 10 and count down until 0

I want to make it so that when a key is pressed a variable starts counting down from 10 to 0.
C# or Java Script is fine.
(I looked in other questions similar to this, but couldn’t find what I was looking for.)

float countdownValue=10;
float countdown;

public Ienumerator StartCountdown()
{
	countdown = countdownValue;
	while (countdown >0)
	{
		yield return new WaitForSeconds(1.0f);
 		countdown --;
	}
}

StartCoroutine(StartCountdown());

for (int i = 10; i >= 0; i–)
{

}

In the end, this is what worked. C#:

    if (Input.GetKeyDown("a"))
    {
    			countdown();
    		StartCoroutine(countdown());
      }
    IEnumerator countdown()
    		{
    yield return new WaitForSeconds(10.0f);
    Debug.Log ("It's been 10 seconds!");
    }