Add a descriptive id for each heading

Make the table of contents sticky and responsive

Create a table of contents from the headings

Extract the headings from the MDX

Use Flexbox to create a responsive footer

Use Flexbox to create the layout

Why doesn't an object copy maintain its identity?

Why is `Math.random()` a bad value for the `key` prop?

Implicit returns aren't always implied

How exactly does && work in JSX?

Spot the a11y errors in this sign-in form

Access an object by a variable property