Newton's Principia Mathematica


    2.2.1 • Public • Published



    Minimal tweening engine which operates on numbers and arrays.

    var tweenr = require('tweenr')()
    var data = { opacity: 0, position: [15, 25] }, { 
        opacity: 1, 
        position: [20, 50], 
        ease: 'expoOut', 
        duration: 1, //in seconds 
        delay: 0.25


    I love the simplicity of GreenSock's animation tools, but I don't agree with some of their practices (i.e. polluting tweened objects with variables) and find myself only using a tiny fraction of their entire codebase.

    Some features of tweenr:

    • common set of eases
    • works in node and the browser
    • small, focused and modular design; e.g. tween-ticker is a good fit for modular components
    • interpolates numbers and arrays (i.e. vectors, colors)
    • can tween multiple elements at once
    • tweens are cancellable
    • triggers complete, start, update, cancelling events
    • extensible and optimizable tween types: tween-array, tween-chain, etc



    tweenr = require('tweenr')([opt])

    Creates a new instanceof Tweenr and attaches itself to an application-wide render loop (to minimize animation frame requests). By default, this includes a common set of eases. Options:

    • eases can be specified to provide a new set of easing functions, defaults to the eases module
    • defaultEase the default easing function, or a string to use as a lookup into the eases object. defaults to a linear function

    tween =

    If only one argument is given, this method pushes a new tween onto the stack, returning that tween for chaining. Same as tweenr.push(tween).

    tween =, opt)

    A convenience version of to() which handles the most common case: object tweening. If the second argument, opt is truthy and an object, this method creates a new object tween and pushes it onto the stack.

    The tween modifies element, which can be an array of objects, or a single object. opt can be the following:

    • delay in time units, default 0
    • duration in time units, default 0
    • ease is a string (lookup for the eases passed at constructor) or an ease function, defaults to tweenr.defaultEase

    Any other properties to opt will be tweened if they are consistent with element and also if they are a number or an array.

    var elements = [
        { x: 25, shape: [10, 5] },
        { x: 15, opacity: 0 }
    var tween =, { 
        opacity: 1,
        shape: [5, 0],
        duration: 3,
        delay: 0.25
        after tween is finished, element will equal:
            { x: 25, shape: [5, 0] },
            { x: 15, opacity: 1 }

    tween =

    If no arguments are given, this method creates an "empty" or dummy tween that can be cancelled. This is similar to the way noop functions are used to avoid conditionals in functional programming.


    Pushes a generic tween object onto the stack. Like but more explicit.

    var array = require('tween-array')
    tweenr.push(array(start, end, { duration: 5 }))
        .on('complete', doSomething)


    Disposes this instance, removing it from the application-wide frame loop.

    tweenr.on('tick', fn)

    Attaches a function to this tweenr's tick. The event is triggered by the application-wide frame loop with a delta parameter in seconds.

    This event will stop after tweenr.dispose().


    Clears and cancels all tweens stored in this tweenr instance. Returns this for chaining.


    A value (default 1.0) which scales the delta time per frame, allowing you to slow down or speed up an instance of tweenr.


    The return value of is a tween with the following:


    Cancels the tween, removing it from the queue on the next tick without applying any further interpolation.

    tween.on(event, func)

    The returned tween is an event emitter with the following events:

    • start triggered when the tween is first started
    • cancelling triggered before the tween completes, initiating from a call to cancel()
    • complete triggered when the tween is completed
    • update triggered after the tween updates its values


    MIT, see for details.


    npm i tweenr

    DownloadsWeekly Downloads






    Last publish


    • mattdesl