Abstracted functional easing provider, based on Penner's equations.

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Functional Easing
browser support
Build Status
A functional, highly generic easing provider. Works especially well with Animation Timer which already provides the normalised 0 to 1 time elapsed input value used by this module (in addition to providing easy looping, bouncing, rewinding and pausing of animations. Check it out! It's cool!)


  • Refactored, optimised Penner Easing Equations (they now only mutate the time value)
  • Supports custom easing functions.
  • Can be used functionally or procedurally
  • Takes linear time progressed value of 0 to 1 as input and outputs only the eased time value.
  • Designed to providing easing for any interpolation/tween functions that take a normalised time value as input.
  • Covered by tests.

Built in Easing functions

All easing functions support in, out and in-out.
  • cubic
  • quad
  • quart
  • quint
  • expo
  • sine
  • circ
  • back (configurable)
  • elastic (configable)
  • bounce


$ npm install --save functional-easing


// some imports...
var AnimationTimer = require('animation-timer').AnimationTimer;
var Easer = require('functional-easing').Easer;

// make a new easer
var easer = new Easer()

var origin = new Vector2D(0,0);
var destination = new Vector2D(200,100);

// make a new easer. Wrap our tick handler with easer..
var animation = new AnimationTimer()
  // here we wrap our tick handler with our easer..
  .on('tick', easer(function(time){
    // every tick, this function will be called and 'time' will be already 'eased'
    var tweenedVector = Vector2D.lerp(origin, destination, time);
    // do something with the vector...



Create a new instance of Easer. Easers on their own don't do very much.
var Easer = require('functional-easing').Easer;
var easer = new Easer();


Generates an easing function from one of the presets. The options are...
  • in-cubic, out-cubic, in-out-cubic
  • in-quad, out-quad, in-out-quad
  • in-quart, out-quart, in-out-quart
  • in-quint, out-quint, in-out-quint
  • in-expo, out-expo, in-out-expo
  • in-sine, out-sine, in-out-sine
  • in-circ, out-circ, in-out-circ
  • in-back, out-back, in-out-back - configurable
  • in-elastic, out-elasic, in-out-elastic - configurable
  • in-bounce, out-bounce, in-out-bounce

Returns an easingFunction.
var easer = new Easer().using('in-out-sine');


Wraps external, custom easing functions.
Returns an easingFunction.
var easer =
  new Easer()
    .using(function(time){ return doSomethingWithTime(time); });

easingFunction API


Easing functions can be used procedurally by passing a time value between 0 - 1, in which case it simply returns the eased value.
var easer = new Easer().using('in-expo');

easer(0.5); // returns 0.03125
easer(0); // returns 0
easer(1); // returns 1


Easing functions can be used functionally. When passed a function func, it returns another function which when called will pass the output of the easing function as the first parameter to func. It also passes the original unmodified time value.
var easer = new Easer().using('in-expo');

var easingFunction = easer(function(easedTime, linearTime){
  // easedTime is the output of the easing function.
  console.log(easedTime, linearTime);

easingFunction(0.5); // 0.03125, 0.5
easingFunction(0.8); // 0.25000 (etc), 0.8

easingFunction.withParameters( ... )

Whether used functionally or procedurally, you can bind additional parameters to be appended to the parameters passed to easing functions. Time is always the first parameter to be passed.
The primary use for this is for configuring the back and elastic easing functions, but it also helps support custom functions.
// configuring some presets
var easer = new Easer()
  .withParameters(0.8); // in-back, out-back and in-out-back accept an additional 'overshoot' parameter

var easer = new Easer()
  .withParameters(0.5, 0.8); // in-elastic, out-elastic and in-out-elastic accept additional 'amplitude' and 'period' parameters.

// some custom thing
var easer = new Easer()
  .using(function(time, scale){
    return time * scale;  

// procedurally..
easer(0.5); // 1

// functionally...
var easingFunction = easer(function(t){  

easingFunction(0.5); // 1


All easing functions were refactored using information from Robert Penner's easing website, Actionscript code from Gizma and Elastic/Back and Bounce code from dZone.
All I have done is refactor the code to remove the beginning, change and duration parameters so that each function merely manipulates time and does not directly tween values. I've added a comprehensive suit of unit tests to guarantee that the refactored functions work identically to the original actionscript functions when used with the parameters beginning = 0, change=1, duration=1.
Math.easeInQuad = function (t, b, c, d) {
  t /= d;
  return c*t*t + b;

'in-quad' : function (time){
  return time * time;

Bezier Curve based easing

CSS3 transition easing curves are all based on Bezier curves. Previously I wrote a module that emulates the CSS3 transition curves but this depends upon look up tables and so depending on the number of samples being generated can have a noticable start-up cost. I'm looking into the effectiveness of using interpolation to smooth out the gaps and thus need less samples, but this type of easing curve will continue to be a seperate module. Of course this module will accept the old module as a custom easing function:
var BezierEasing = require('gm-easing').Easer;
var Easer = require('functional-easing').Easer;

var css3EaseIn = new BezierEasing().using('ease-in');

var easer = new Easer().using(css3EaseIn);

var animation = new AnimationTimer()
  .on('tick', easer(function(time){

    // time has been eased using css3EaseIn


Development and Tests