jQuery easing function — variables' comprehension

Question

How does the easing function for jQuery work? Take for example:

easeInQuad = function (x, t, b, c, d) {
    return c*(t/=d)*t + b;
};

How d

Answer 1

A concrete example,

Suppose our initial value is 1000 and we want to reach 400 in 3s:

var initialValue = 1000,
    destinationValue = 400,
    amountOfChange = destinationValue - initialValue, // = -600
    duration = 3,
    elapsed;

Let's go from 0s to 3s:

elapsed = 0
$.easing.easeInQuad(null, elapsed, initialValue, amountOfChange, duration)
//> 1000

elapsed = 1
$.easing.easeInQuad(null, elapsed, initialValue, amountOfChange, duration)
//> 933.3333333333334

elapsed = 2
$.easing.easeInQuad(null, elapsed, initialValue, amountOfChange, duration)
//> 733.3333333333334

elapsed = 3
$.easing.easeInQuad(null, elapsed, initialValue, amountOfChange, duration)
//> 400

So compared to the synopsis:

$.easing.easeInQuad = function (x, t, b, c, d) {...}

we can deduce:

 x       t          b              c            d
 |       |          |              |            |
null  elapsed  initialValue  amountOfChange  duration

---

NB1: One important thing is that elapsed(t) and duration(d) should be expressed in the same unit, eg: here 'seconds' for us, but could be 'ms' or whatever. This is also true for initialValue(b) and amountOfChange(c), so to sum-up:

 x       t          b              c            d
 |       |          |              |            |
null  elapsed  initialValue  amountOfChange  duration
         ^          ^              ^            ^
         +----------|----=unit=----|------------+
                    +----=unit=----+

NB2: Like @DamonJW, I don't know why x is here. It does not appear in Penner's equations and does not seem to be used in result: let always set him to null