*Easing* is a method of distorting time to control apparent motion in animation. It is most commonly used for slow-in, slow-out. By easing time, animated transitions are smoother and exhibit more plausible motion.

The easing types in this module implement the ease method, which takes a normalized time *t* and returns the corresponding “eased” time *tʹ*. Both the normalized time and the eased time are typically in the range [0,1], where 0 represents the start of the animation and 1 represents the end; some easing types, such as elastic, may return eased times slightly outside this range. A good easing type should return 0 if *t* = 0 and 1 if *t* = 1. See the easing explorer for a visual demonstration.

These easing types are largely based on work by Robert Penner.

If you use NPM, `npm install d3-ease`

. Otherwise, download the latest release. You can also load directly from d3js.org, either as a standalone library or as part of D3 4.0. AMD, CommonJS, and vanilla environments are supported. In vanilla, a `d3`

global is exported:

# *ease*(*t*)

Given the specified normalized time *t*, typically in the range [0,1], returns the “eased” time *tʹ*, also typically in [0,1]. 0 represents the start of the animation and 1 represents the end. A good implementation returns 0 if *t* = 0 and 1 if *t* = 1. See the easing explorer for a visual demonstration. For example, to apply cubic easing:

`var te = d3;`

Similarly, to apply custom elastic easing:

`// Before the animation starts, create your easing function. var customElastic = d3easeElastic; // During the animation, apply the easing function. var te = ;`

Linear easing; the identity function; *linear*(*t*) returns *t*.

Polynomial easing; raises *t* to the specified exponent. If the exponent is not specified, it defaults to 3, equivalent to cubicIn.

Reverse polynomial easing; equivalent to 1 - polyIn(1 - *t*). If the exponent is not specified, it defaults to 3, equivalent to cubicOut.

# d3.**easePoly**(*t*) <>

# d3.**easePolyInOut**(*t*) <>

Symmetric polynomial easing; scales polyIn for *t* in [0, 0.5] and polyOut for *t* in [0.5, 1]. If the exponent is not specified, it defaults to 3, equivalent to cubic.

Returns a new polynomial easing with the specified exponent *e*. For example, to create equivalents of linear, quad, and cubic:

`var linear = d3easePoly quad = d3easePoly cubic = d3easePoly;`

Quadratic easing; equivalent to polyIn.exponent(2).

Reverse quadratic easing; equivalent to 1 - quadIn(1 - *t*). Also equivalent to polyOut.exponent(2).

# d3.**easeQuad**(*t*) <>

# d3.**easeQuadInOut**(*t*) <>

Symmetric quadratic easing; scales quadIn for *t* in [0, 0.5] and quadOut for *t* in [0.5, 1]. Also equivalent to poly.exponent(2).

Cubic easing; equivalent to polyIn.exponent(3).

Reverse cubic easing; equivalent to 1 - cubicIn(1 - *t*). Also equivalent to polyOut.exponent(3).

# d3.**easeCubic**(*t*) <>

# d3.**easeCubicInOut**(*t*) <>

Symmetric cubic easing; scales cubicIn for *t* in [0, 0.5] and cubicOut for *t* in [0.5, 1]. Also equivalent to poly.exponent(3).

Sinusoidal easing; returns sin(*t*).

Reverse sinusoidal easing; equivalent to 1 - sinIn(1 - *t*).

# d3.**easeSin**(*t*) <>

# d3.**easeSinInOut**(*t*) <>

Symmetric sinusoidal easing; scales sinIn for *t* in [0, 0.5] and sinOut for *t* in [0.5, 1].

Exponential easing; raises 2 to the exponent 10 * (*t* - 1).

Reverse exponential easing; equivalent to 1 - expIn(1 - *t*).

# d3.**easeExp**(*t*) <>

# d3.**easeExpInOut**(*t*) <>

Symmetric exponential easing; scales expIn for *t* in [0, 0.5] and expOut for *t* in [0.5, 1].

Circular easing.

Reverse circular easing; equivalent to 1 - circleIn(1 - *t*).

# d3.**easeCircle**(*t*) <>

# d3.**easeCircleInOut**(*t*) <>

Symmetric circular easing; scales circleIn for *t* in [0, 0.5] and circleOut for *t* in [0.5, 1].

Elastic easing, like a rubber band. The amplitude and period of the oscillation are configurable; if not specified, they default to 1 and 0.3, respectively.

# d3.**easeElastic**(*t*) <>

# d3.**easeElasticOut**(*t*) <>

Reverse elastic easing; equivalent to 1 - elasticIn(1 - *t*).

Symmetric elastic easing; scales elasticIn for *t* in [0, 0.5] and elasticOut for *t* in [0.5, 1].

Returns a new elastic easing with the specified amplitude *a*.

Returns a new elastic easing with the specified period *p*.

Anticipatory easing, like a dancer bending his knees before jumping off the floor. The degree of overshoot is configurable; it not specified, it defaults to 1.70158.

Reverse anticipatory easing; equivalent to 1 - backIn(1 - *t*).

# d3.**easeBack**(*t*) <>

# d3.**easeBackInOut**(*t*) <>

Symmetric anticipatory easing; scales backIn for *t* in [0, 0.5] and backOut for *t* in [0.5, 1].

Returns a new back easing with the specified overshoot *s*.

Bounce easing, like a rubber ball.

# d3.**easeBounce**(*t*) <>

# d3.**easeBounceOut**(*t*) <>

Reverse bounce easing; equivalent to 1 - bounceIn(1 - *t*).

Symmetric bounce easing; scales bounceIn for *t* in [0, 0.5] and bounceOut for *t* in [0.5, 1].