EffectTiming.easing

The EffectTiming dictionary's easing property in the Web Animations API specifies the timing function used to scale the time to produce easing effects, where easing is the rate of the animation's change over time.

A string defining the timing function to use for easing transitions during the animation process. Accepts several pre-defined DOMString values, a steps() timing function like steps(5, end), or a custom cubic-bezier value like cubic-bezier(0.42, 0, 0.58, 1). Defaults to linear. Available values include:

linear

A constant rate of change, neither accelerating nor deccelerating. 

cubic-bezier(<number>, <number>, <number>, <number>)

Specifies a cubic Bézier timing function. The four numbers specify points P1 and P2 of the curve as (x1, y1, x2, y2). Both x values must be in the range [0, 1] or the definition is invalid.

ease

A decelerated rate of change, going from fast to slow. Equivalent to cubic-bezier(0.25, 0.1, 0.25, 1).

ease-in

An accelerated rate of change, going from slow to fast. Equivalent to cubic-bezier(0.42, 0, 1, 1).

ease-out

Another decelerated rate of change, going from fast to slow. Equivalent to cubic-bezier(0, 0, 0.58, 1).

ease-in-out

This rate of change speeds up in the middle. Equivalent to cubic-bezier(0.42, 0, 0.58, 1).

frames(<integer>)

Specifies a frames timing function, which breaks the animation down into a number of equal time intervals but also displays the start (0%) and end (100%) states for an equal amount of time to the other intervals. The browser flips to a different static frame when each interval is reached, rather than animating smoothly. See GitHub for a simple example that illustrates the difference between steps() and frames().

steps(<integer>[, [ start | end ] ]?)

Specifies a step timing function, which breaks the animation down into a number of equal time intervals. The browser flips to a different static frame when each interval is reached, rather than animating smoothly. The first parameter specifies the number of intervals in the function. It must be a positive integer (greater than 0). The second parameter, which is optional, specifies the point at which the change of values occur within the interval. If the second parameter is omitted, it is given the value end.

step-start

Equivalent to steps(1, start)

step-end

Equivalent to steps(1, end).

In the Red Queen's Race example, we animate Alice and the Red Queen by passing an easing of steps(7, end) to animate():

// Define the key frames
var spriteFrames = [
  { transform: 'translateY(0)' },
  { transform: 'translateY(-100%)' }
];

// Get the element that represents Alice and the Red Queen
var redQueen_alice_sprite = document.getElementById('red-queen_and_alice_sprite');

// Animate Alice and the Red Queen using steps()
var redQueen_alice = redQueen_alice_sprite.animate(
spriteFrames, {
  easing: 'steps(7, end)',
  direction: "reverse",
  duration: 600,
  playbackRate: 1,
  iterations: Infinity
});

• Web Animations API
• Element.animate(), KeyframeEffect(), and AnimationEffect.updateTiming() all accept an object of timing properties including this one.
• The value of this property corresponds to the one in EffectTiming (which is returned from AnimationEffect.getTiming() and AnimationEffect.getComputedTiming()).
• CSS's animation-timing-function and transition-timing-function.