JustDown = function (key)
{
    if (key._justDown)
    {
        key._justDown = false;

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

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

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

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

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

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