What does it mean exactly that "React updates DOM dynamically"?

Question

The ReactJS website states that,

When the data changes, React conceptually hits the "refresh" button, and knows to only update the changed parts.

What does this mean exactly?

How is this achieved?

How is this different from what AngularJS does?

Answer 1

Reactjs create a virtual DOM and does not touch to the real DOM. Then when some of DOM changed, it will use diff algorithm to find out what is different from previous state of DOM elements, finally just replace that changes. You can see this link for that algorithm to have more understanding. And you have to know what is shadow DOM as well.