# Lesson3-6: State Management: Update state with setState
###### tags: `Recat`
{%youtube 81CCwyeiaWE%}
{%youtube qLa4LEvrYn4%}
[Here's the commit with the changes made in this video.](https://github.com/udacity/reactnd-contacts-app/commit/4b656124626a1ea127baa92530bf87f89d14389c)

# How State is Set
Earlier in this lesson, we saw how we can define a component's state at the time of initialization. Since state reflects mutable information that ultimately affects rendered output, a component may also update its state throughout its lifecycle using `this.setState()`. As we've learned, when local state changes, React will trigger a re-render of the component.

There are two ways to use `setState()`. The first is to merge state updates. Consider a snippet of the following component:

class Email extends React.Component {
 state = {
   subject: '',
   message: ''
 }
 // ...
});

Though the initial state of this component contains two properties (subject and message), they can be updated independently. For example:

this.setState({
 subject: 'Hello! This is a new subject'
})

This way, we can leave `this.state.message` as-is, but replace `this.state.subject` with a new value.

The second way we can use `setState()` is by passing in a function rather than an object. For example:

this.setState((prevState) => ({
 count: prevState.count + 1
}))

Here, the function passed in takes a single `prevState` argument. When a component's new state depends on the previous state (i.e., we are incrementing `count` in the previous state by 1), we want to use the functional `setState()`.