React state

Example of state

In React, state refers to a special variable that stores the current situation of a component, especially after the initial return render, e.g.:

A state may contain a user's "wish list of trip destinations" for a travel app
- During the user's session, a user may add a new location to the bucket list
  - Thus, the state of the wish list changes to reflect that addition

Implementing state in React

As of now, we implement state with a React hook (a special built-in function) from the React library, so we begin with an import statement:

/* src/Component.js */

import { useState } from 'react'

Then, we define state variables and state setter functions by de-structuring the useState hook:

/* src/Component.js */

import { useState } from 'react'

export default function Component() {

  const [ title, setTitle ] = useState('')  

  return (
    <div>
      <h1>{title}</h1>
    </div>
  )
}

Note:

the state variable, title, can actually have any legal JavaScript variable name
the state setter function, setTitle , typically follows the convention of having set at the beginning of its name, plus the state variable to which it refers
- this typically gets used in a handler function (shown below) that gets called upon user interaction, e.g. in a form or button or "link as a button"
the call to useState typically takes on one argument which
- describes the state's initial value
- can be any primitive (string, number, boolean, object, array)
the state gets referenced in the return statement like any other variable in JSX with the curly braces

Updating the state in React

As mentioned in the previous section, we would typically update a state variable by anticipating an interaction from the user interface; that interaction would then trigger the handler function to use the state setter function:

/* src/Component.js */

import { useState } from 'react'

export default function Component() {

  const [ title, setTitle ] = useState('hello') 

  const handleClick = () => {
     setTitle('goodbye')
  } 

  return (
    <div>
      <h1>{title}</h1>
      <button onClick={handleClick}>change title</button>
    </div>
  )
}

There, we added a <button> with an onClick prop to handleClick which calls setTitle with an argument to change the title from the initial "hello" to "goodbye"!

Note:

onClick exists a pre-determined attribute/prop name for click-type events
- a click is the same thing as touch on mobile devices or an enter keypress
handleClick can take on any name but we use handle as a prefix by convention

Updating state more dynamically with events

That last example showed a state update for pedagogical purposes but it might look a tad over-simplistic; let's look at another example with a user-driven input text field:

/* src/NewComponent.js */

import { useState } from 'react'

export default function NewComponent() {

  const [ title, setTitle ] = useState('hello')  

  const handleChange = (event) => {
    setTitle(event.target.value)
  }

  return (
    <div>
      <h1>{title}</h1>
      <label htmlFor="custom">Customize name: </label>
      <input type="text" name="custom" onChange={handleChange} />
    </div>
  )
}

With that, the heading changes based on the user input (still not an amazingly cool example but shows how a seemingly static page can change with the help of state!)

Note:

an onChange event triggers the handler handleChange

<label> tags must use the attribute htmlFor instead of for to avoid confusion with the keyword for in JavaScript
we used the handleChange following the naming convention in the previous example
- the handleChange takes on a parameter event supplied by the input field
- event.target refers to the input field while
  - event.target.value refers to the text in that field