This is a JavaScript library for scrollytelling, which is dynamically changing charts (or triggering whatever) as text scrolls into view. It implements best practices for scrollytelling, which means built-in keyboard shortcuts, no scrolljacking and reliable "sticky" behaviour.

TwoStep was developed at The Wall Street Journal and has been used in stories such as:

• How This Tech Rally Is Different From 1999
• Then and Now: The Big Shift at Work

TwoStep is highly flexible, and can be used in range of designs.

• Basic working example
• One-column layout, with text scrolling over the top
• Two instances on the same page
• With swipe carousel on mobile
• With static charts on mobile

These demos are also meant as starting points for new projects.

1. Include jQuery, Waypoints, FixTo and TwoStep on the page.

2. Set up HTML and CSS as best fits your project. See source code from the demos above for inspiration.

3. In your JavaScript:

var ts = new TwoStep({
    elements: document.querySelectorAll('.slide-item'),
    onChange: function(event) {
        console.log(event.index);
    }
});

Only elements is required. All others are optional.

• elements: Array/NodeList of DOM nodes (i.e. your narrative text).
• narrative: Array of functions corresponding to elements. Each called with event object as argument.
• onChange: Called when any/every element is activated. Called with event object as argument.
• stick: DOM node to stick in right rail (i.e. your sticky chart).
• offset: Object to manually set the offset for both directions. Must include both an up and down key and both values should be strings (i.e. {up:"50%",down:"0"})

Whenever a narrative function or onChange is called, it’s passed an event object as an argument.

var ts = new TwoStep({
    elements: document.querySelectorAll('.item'),
    onChange: function(event) {
        console.log(event);
    }
});

Here’s what you can find in a typical event object:

{
    index: 0,
    direction: 'up',     // or 'down', or null
    element: < element > // DOM node corresponding to index
}

Check event.direction:

var ts = new TwoStep({
    elements: document.querySelectorAll('.item'),
    onChange: function(event) {
        if (event.direction === 'up') {
            // do something
        } else if (event.direction === 'down') {
            // do something else
        }
    }
});

• .goTo(index, scroll): Activate item at index. If scroll is true, will animate to position. Returns a promise which resolves when scrolling is complete.
• .disable(): Prevent waypoints from firing and unstick stuck element, if present
• .enable(): Return to standard behaviour

Make sure you have Node.js installed. Then run:

npm install
npm run start

To run tests, open tests.html in your browser and wait a couple of seconds.

• Graph Scroll
• Scrollama
• The Pudding’s article How to implement scrollytelling with six different libraries

For something a bit different, see scrollWatcher, also made by WSJ.

v1.0.0 (November 27, 2017)

• Initial public release

ISC