JustUp = function (key)
{
    if (key._justUp)
    {
        key._justUp = false;

        return true;
    }
    else
    {
        return false;
    }
}
The justUp value allows you to test if this Key has just been released or not.

When you check this value it will return `true` if the Key is up, otherwise `false`.

You can only call JustUp once per key release. It will only return `true` once, until the Key is pressed down and released again.
This allows you to use it in situations where you want to check if this key is up without using an event, such as in a core game loop.

@function Phaser.Input.Keyboard.JustUp
@since 3.0.0

@param {Phaser.Input.Keyboard.Key} key - The Key to check to see if it's just up or not.

@return {boolean} `true` if the Key was just released, otherwise `false`.