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Documented the difference between props and State

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Yash Joshi
2017-11-28 19:52:59 +05:30
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content/docs/faq-state.md
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@@ -6,22 +6,16 @@ layout: docs
category: FAQ
---
### What Should Go in State?
**State should contain data that a component's event handlers may change to trigger a UI update.** In real apps this data tends to be very small and JSON-serializable. When building a stateful component, think about the minimal possible representation of its state, and only store those properties in `this.state`. Inside of `render()` simply compute any other information you need based on this state. You'll find that thinking about and writing applications in this way tends to lead to the most correct application, since adding redundant or computed values to state means that you need to explicitly keep them in sync rather than rely on React computing them for you.
### What Shouldn’t Go in State?
`this.state` should only contain the minimal amount of data needed to represent your UI's state. As such, it should not contain:
* **Computed data:** Don't worry about precomputing values based on state — it's easier to ensure that your UI is consistent if you do all computation within `render()`. For example, if you have an array of list items in state and you want to render the count as a string, simply render `this.state.listItems.length + ' list items'` in your `render()` method rather than storing it on state.
* **React components:** Build them in `render()` based on underlying props and state.
* **Duplicated data from props:** Try to use props as the source of truth where possible. One valid use to store props in state is to be able to know its previous values, because props can change over time.
### What does setState do?
`setState()` schedules an update to a component's `state` object. When state changes, the component responds by re-rendering.
### What is the difference between state and props?
[state](docs/state-and-lifecycle) and [props](/docs/components-and-props) are plain JS objects which cause re-render whenever they change. While both hold information relating to the component, they are used differently and should be kept separate because:
* `props` contains information set by the parent component and should not be changed whereas `state` contains private information for the component to initialise, change, and use on it’s own.
### Why is `setState` is giving me the wrong value?
Calls to `setState` are asynchronous - don't rely on `this.state` to reflect the new value immediately after calling `setState`. Pass an updater function instead of an object if you need compute values based on the current state (see below for details).