Pure Functions

Pure Function

• Given the same input, will always return the same output
• Produces no side effects
• Relies on no external mutable state

Source: https://medium.com/javascript-scene/master-the-javascript-interview-what-is-a-pure-function-d1c076bec976#.f5pk6ttpb

Pure Function

function hello(name) {
  return `Hello ${name}!`
}

> hello('Amelia')
"Hello Amelia!"

Impure Function

var name = 'Amelia'

function hello() {
  return `Hello ${name}!`
}

Impure Function

> hello()
"Hello Amelia!"
> name = 'Diane'
> hello()
"Hello Diane!"

Impure Function

var person = {}

function hello(name) {
  person.name = name
  return `Hello ${name}!`
}

Impure Function

> person
{}
> hello('Amelia')
"Hello Amelia!"
> person
{ name: "Amelia" }

Client-side Templating

Component-oriented Architecture 

Source: https://facebook.github.io/react/docs/thinking-in-react.html

Source: https://facebook.github.io/react/docs/thinking-in-react.html

Tree of Components

• FilterableProductTable
  • SearchBar
  • ProductTable
    • ProductCategoryRow
    • ProductRow

Encapsulated (heavy) Components 

Encapsulated but Heavy

• Fetching data
• Managing state
• Rendering UI

Smart vs Dumb Components 

Smart Components, AKA

• Container
• Controller
• Stateful

Dumb Components, AKA

• Presentational
• Stateless
• Pure

Stateful

• Example: App
• Children are stateful or pure components

Pure

• Example: Text
• Children are pure components

React Components

export class Hello extends React.Component {
  constructor(props) {
    super(props)

    state = {}
  }

  render() {
    return (
      <p>Hello { this.props.name }!</p>
    )
  }
}

React Class

const element = <Hello name="Amelia" />

ReactDOM.render(
  element,
  document.getElementById('root')
)

React Class

function Hello(props) {
  return (
    <p>Hello { props.name }!</p>
  )
}

React Function

https://facebook.github.io/react/docs/

components-and-props.html

Functional Benefits

Separation of

• stateful app components/code
• stateless, pure UI components

function Hello(props) {
  return (<p>Hello { props.name }!</p>)
}

describe('Hello', () => {
  it('should say hi', () => {
    const wrapper = shallow(<Hello name="Amelia" />)

    expect(wrapper.html())
           .toEqual('<p>Hello Amelia!</p>')
  })
})

Testable

function Hello(props) {
  return (
    <p>{ props.name }</p>
  )
}

function App() {
  return (
    <div>
      <Hello name="Amelia" />
      <Hello name="Diane" />
    </div>
  )
}

Composition