Any state associated with the old tree is lost.Īny components below the root will also get unmounted and have their state destroyed. Component instances receive UNSAFE_componentWillMount() and then componentDidMount(). When building up a new tree, new DOM nodes are inserted into the DOM. Component instances receive componentWillUnmount(). When tearing down a tree, old DOM nodes are destroyed. Going from to, or from to, or from to - any of those will lead to a full rebuild.

Whenever the root elements have different types, React will tear down the old tree and build the new tree from scratch. The behavior is different depending on the types of the root elements. When diffing two trees, React first compares the two root elements. In practice, these assumptions are valid for almost all practical use cases. The developer can hint at which child elements may be stable across different renders with a key prop.Two elements of different types will produce different trees.Instead, React implements a heuristic O(n) algorithm based on two assumptions: If we used this in React, displaying 1000 elements would require in the order of one billion comparisons. However, the state of the art algorithms have a complexity in the order of O(n 3) where n is the number of elements in the tree. There are some generic solutions to this algorithmic problem of generating the minimum number of operations to transform one tree into another. React then needs to figure out how to efficiently update the UI to match the most recent tree. On the next state or props update, that render() function will return a different tree of React elements. When you use React, at a single point in time you can think of the render() function as creating a tree of React elements. This article explains the choices we made in React’s “diffing” algorithm so that component updates are predictable while being fast enough for high-performance apps. This makes writing applications a lot easier, but it might not be obvious how this is implemented within React. React provides a declarative API so that you don’t have to worry about exactly what changes on every update.

Building generator algorithm