CSS-animations

CSS animations make it possible to do simple animations without JavaScript at all.

JavaScript can be used to control CSS animations and make them even better, with little code.

CSS transitions

The idea of CSS transitions is simple. We describe a property and how its changes should be animated. When the property changes, the browser paints the animation.

That is, all we need is to change the property, and the fluid transition will be done by the browser.

For instance, the CSS below animates changes of background-color for 3 seconds:

          .animated {
  transition-property: background-color;
  transition-duration: 3s;
}
        

Now if an element has .animated class, any change of background-color is animated during 3 seconds.

Click the button below to animate the background:

<button id="color">Click me</button>

<style>
  #color {
    transition-property: background-color;
    transition-duration: 3s;
  }
</style>

<script>
  color.onclick = function() {
    this.style.backgroundColor = 'red';
  };
</script>

There are 4 properties to describe CSS transitions:

transition-property
transition-duration
transition-timing-function
transition-delay

We’ll cover them in a moment, for now let’s note that the common transition property allows declaring them together in the order: property duration timing-function delay, as well as animating multiple properties at once.

For instance, this button animates both color and font-size:

<button id="growing">Click me</button>

<style>
#growing {
  transition: font-size 3s, color 2s;
}
</style>

<script>
growing.onclick = function() {
  this.style.fontSize = '36px';
  this.style.color = 'red';
};
</script>

Now, let’s cover animation properties one by one.

transition-property

In transition-property, we write a list of properties to animate, for instance: left, margin-left, height, color. Or we could write all, which means “animate all properties”.

Do note that, there are properties which can not be animated. However, most of the generally used properties are animatable.

transition-duration

In transition-duration we can specify how long the animation should take. The time should be in CSS time format: in seconds s or milliseconds ms.

transition-delay

In transition-delay we can specify the delay before the animation. For instance, if transition-delay is 1s and transition-duration is 2s, then the animation starts 1 second after the property change and the total duration will be 2 seconds.

Negative values are also possible. Then the animation is shown immediately, but the starting point of the animation will be after given value (time). For example, if transition-delay is -1s and transition-duration is 2s, then animation starts from the halfway point and total duration will be 1 second.

Here the animation shifts numbers from 0 to 9 using CSS translate property:

نتيجة

script.js

style.css

index.html

stripe.onclick = function() {
  stripe.classList.add('animate');
};

#digit {
  width: .5em;
  overflow: hidden;
  font: 32px monospace;
  cursor: pointer;
}

#stripe {
  display: inline-block
}

#stripe.animate {
  transform: translate(-90%);
  transition-property: transform;
  transition-duration: 9s;
  transition-timing-function: linear;
}

<!doctype html>
<html>

<head>
  <meta charset="UTF-8">
  <link rel="stylesheet" href="style.css">
</head>

<body>

  Click below to animate:

  <div id="digit"><div id="stripe">0123456789</div></div>

  <script src="script.js"></script>
</body>

</html>

The transform property is animated like this:

          #stripe.animate {
  transform: translate(-90%);
  transition-property: transform;
  transition-duration: 9s;
}
        

In the example above JavaScript adds the class .animate to the element – and the animation starts:

stripe.classList.add('animate');

We could also start it from somewhere in the middle of the transition, from an exact number, e.g. corresponding to the current second, using a negative transition-delay.

Here if you click the digit – it starts the animation from the current second:

نتيجة

script.js

style.css

index.html

stripe.onclick = function() {
  let sec = new Date().getSeconds() % 10;
  stripe.style.transitionDelay = '-' + sec + 's';
  stripe.classList.add('animate');
};

#digit {
  width: .5em;
  overflow: hidden;
  font: 32px monospace;
  cursor: pointer;
}

#stripe {
  display: inline-block
}

#stripe.animate {
  transform: translate(-90%);
  transition-property: transform;
  transition-duration: 9s;
  transition-timing-function: linear;
}

<!doctype html>
<html>

<head>
  <meta charset="UTF-8">
  <link rel="stylesheet" href="style.css">
</head>

<body>

  Click below to animate:
  <div id="digit"><div id="stripe">0123456789</div></div>

  <script src="script.js"></script>
</body>
</html>

JavaScript does it with an extra line:

          stripe.onclick = function() {
  let sec = new Date().getSeconds() % 10;
  // for instance, -3s here starts the animation from the 3rd second
  stripe.style.transitionDelay = '-' + sec + 's';
  stripe.classList.add('animate');
};
        

transition-timing-function

The timing function describes how the animation process is distributed along its timeline. Will it start slowly and then go fast, or vice versa.

It appears to be the most complicated property at first. But it becomes very simple if we devote a bit time to it.

That property accepts two kinds of values: a Bezier curve or steps. Let’s start with the curve, as it’s used more often.

Bezier curve

The timing function can be set as a Bezier curve with 4 control points that satisfy the conditions:

First control point: (0,0).
Last control point: (1,1).
For intermediate points, the values of x must be in the interval 0..1, y can be anything.

The syntax for a Bezier curve in CSS: cubic-bezier(x2, y2, x3, y3). Here we need to specify only 2nd and 3rd control points, because the 1st one is fixed to (0,0) and the 4th one is (1,1).

The timing function describes how fast the animation process goes.

The x axis is the time: 0 – the start, 1 – the end of transition-duration.
The y axis specifies the completion of the process: 0 – the starting value of the property, 1 – the final value.

The simplest variant is when the animation goes uniformly, with the same linear speed. That can be specified by the curve cubic-bezier(0, 0, 1, 1).

Here’s how that curve looks:

…As we can see, it’s just a straight line. As the time (x) passes, the completion (y) of the animation steadily goes from 0 to 1.

The train in the example below goes from left to right with the permanent speed (click it):

نتيجة

style.css

index.html

.train {
  position: relative;
  cursor: pointer;
  width: 177px;
  height: 160px;
  left: 0;
  transition: left 5s cubic-bezier(0, 0, 1, 1);
}

<!doctype html>
<html>

<head>
  <meta charset="UTF-8">
  <link rel="stylesheet" href="style.css">
</head>

<body>

  <img class="train" src="https://js.cx/clipart/train.gif" onclick="this.style.left='450px'">

</body>

</html>

The CSS transition is based on that curve:

          .train {
  left: 0;
  transition: left 5s cubic-bezier(0, 0, 1, 1);
  /* JavaScript sets left to 450px */
}
        

…And how can we show a train slowing down?

We can use another Bezier curve: cubic-bezier(0.0, 0.5, 0.5 ,1.0).

The graph:

As we can see, the process starts fast: the curve soars up high, and then slower and slower.

Here’s the timing function in action (click the train):

نتيجة

style.css

index.html

.train {
  position: relative;
  cursor: pointer;
  width: 177px;
  height: 160px;
  left: 0px;
  transition: left 5s cubic-bezier(0.0, 0.5, 0.5, 1.0);
}

<!doctype html>
<html>

<head>
  <meta charset="UTF-8">
  <link rel="stylesheet" href="style.css">
</head>

<body>

  <img class="train" src="https://js.cx/clipart/train.gif" onclick="this.style.left='450px'">

</body>

</html>

CSS:

          .train {
  left: 0;
  transition: left 5s cubic-bezier(0, .5, .5, 1);
  /* JavaScript sets left to 450px */
}
        

There are several built-in curves: linear, ease, ease-in, ease-out and ease-in-out.

The linear is a shorthand for cubic-bezier(0, 0, 1, 1) – a straight line, which we described above.

Other names are shorthands for the following cubic-bezier:

`ease`^*	`ease-in`	`ease-out`	`ease-in-out`
`(0.25, 0.1, 0.25, 1.0)`	`(0.42, 0, 1.0, 1.0)`	`(0, 0, 0.58, 1.0)`	`(0.42, 0, 0.58, 1.0)`