Approach()

/// Approach(a, b, amount)
// Moves "a" towards "b" by "amount" and returns the result
// Nice bcause it will not overshoot "b", and works in both directions
// Examples:
//      speed = Approach(speed, max_speed, acceleration);
//      hp = Approach(hp, 0, damage_amount);
//      hp = Approach(hp, max_hp, heal_amount);
//      x = Approach(x, target_x, move_speed);
//      y = Approach(y, target_y, move_speed);

if (argument0 < argument1)
{
    argument0 += argument2;
    if (argument0 > argument1)
        return argument1;
}
else
{
    argument0 -= argument2;
    if (argument0 < argument1)
        return argument1;
}
return argument0;
doesnt even work

it says it undefined value when trying to run the code
here is an updated code for newer GMS2 versions like (v2024.06.2.162)

function Approach(_value,_value_to,_amount)
{

    if (_value < _value_to)
    {
        _value += _amount;

        if (_value > _value_to)
            return _value_to;

    }
    else
    {
        _value -= _amount;
        if (_value < _value_to)
            return _value_to;

    }
    return _value;
}

Keep in mind you can't write the function and expect it to alter a value on its own. You have to write the whole instruction, like this:

y = Approach(y, target_y, move_speed);
and not
Approach(y, target_y, move_speed);