How to use styles in React?

The style attribute accepts a JavaScript object with camelCased properties rather than a CSS string. This is consistent with the DOM style JavaScript property, is more efficient, and prevents XSS security holes.

const divStyle = {
  color: 'blue',
  backgroundImage: 'url(' + imgUrl + ')'
};

function HelloWorldComponent() {
  return <div style={divStyle}>Hello World!</div>
}

Style keys are camelCased in order to be consistent with accessing the properties on DOM nodes in JavaScript (e.g. node.style.backgroundImage).

How events are different in React?

Handling events in React elements has some syntactic differences:

1. React event handlers are named using camelCase, rather than lowercase.

2. With JSX you pass a function as the event handler, rather than a string.

What will happen if you use setState() in constructor?

When you use setState(), then apart from assigning to the object state React also re-renders the component and all its children. You would get error like this: Can only update a mounted or mounting component. So we need to use this.state to initialize variables inside constructor.

What is the impact of indexes as keys?

Keys should be stable, predictable, and unique so that React can keep track of elements.

In the below code snippet each element's key will be based on ordering, rather than tied to the data that is being represented. This limits the optimizations that React can do.

{todos.map((todo, index) =>
  <Todo
    {...todo}
    key={index}
  />
)}

If you use element data for unique key, assuming todo.id is unique to this list and stable, React would be able to reorder elements without needing to reevaluate them as much.

{todos.map((todo) =>
  <Todo {...todo}
    key={todo.id} />
)}

Is it good to use setState() in componentWillMount() method?

Yes, it is safe to use setState() inside componentWillMount() method. But at the same it is recommended to avoid async initialization in componentWillMount() lifecycle method. componentWillMount() is invoked immediately before mounting occurs. It is called before render(), therefore setting state in this method will not trigger a re-render. Avoid introducing any side-effects or subscriptions in this method. We need to make sure async calls for component initialization happened in componentDidMount() instead of componentWillMount().

componentDidMount() {
  axios.get(`api/todos`)
    .then((result) => {
      this.setState({
        messages: [...result.data]
      })
    })
}

What will happen if you use props in initial state?

If the props on the component are changed without the component being refreshed, the new prop value will never be displayed because the constructor function will never update the current state of the component. The initialization of state from props only runs when the component is first created.

The below component won't display the updated input value:

class MyComponent extends React.Component {
  constructor(props) {
    super(props)

    this.state = {
      records: [],
      inputValue: this.props.inputValue
    };
  }

  render() {
    return <div>{this.state.inputValue}</div>
  }
}

Using props inside render method will update the value:

class MyComponent extends React.Component {
  constructor(props) {
    super(props)

    this.state = {
      record: []
    }
  }

  render() {
    return <div>{this.props.inputValue}</div>
  }
}

How do you conditionally render components?

In some cases you want to render different components depending on some state. JSX does not render false or undefined, so you can use conditional short-circuiting to render a given part of your component only if a certain condition is true.

const MyComponent = ({ name, address }) => (
  <div>
    <h2>{name}</h2>
    {address &&
      <p>{address}</p>
    }
  </div>
)

If you need an if-else condition then use ternary operator.

const MyComponent = ({ name, address }) => (
  <div>
    <h2>{name}</h2>
    {address
      ? <p>{address}</p>
      : <p>{'Address is not available'}</p>
    }
  </div>
)

Why we need to be careful when spreading props on DOM elements?

When we spread props we run into the risk of adding unknown HTML attributes, which is a bad practice. Instead we can use prop destructuring with ...rest operator, so it will add only required props.

For example,

const ComponentA = () =>
  <ComponentB isDisplay={true} className={'componentStyle'} />

const ComponentB = ({ isDisplay, ...domProps }) =>
  <div {...domProps}>{'ComponentB'}</div>

How you use decorators in React?

You can decorate your class components, which is the same as passing the component into a function. Decorators are flexible and readable way of modifying component functionality.

@setTitle('Profile')
class Profile extends React.Component {
    //....
}

/*
  title is a string that will be set as a document title
  WrappedComponent is what our decorator will receive when
  put directly above a component class as seen in the example above
*/
const setTitle = (title) => (WrappedComponent) => {
  return class extends React.Component {
    componentDidMount() {
      document.title = title
    }

    render() {
      return <WrappedComponent {...this.props} />
    }
  }
}

Note: Decorators are a feature that didn't make it into ES7, but are currently a stage 2 proposal.

How do you memoize a component?

There are memoize libraries available which can be used on function components.

For example moize library can memoize the component in another component.

import moize from 'moize'
import Component from './components/Component' // this module exports a non-memoized component

const MemoizedFoo = moize.react(Component)

const Consumer = () => {
  <div>
    {'I will memoize the following entry:'}
    <MemoizedFoo/>
  </div>
}

Update: Since React v16.6.0, we have a React.memo. It provides a higher order component which memoizes component unless the props change. To use it, simply wrap the component using React.memo before you use it.

  const MemoComponent = React.memo(function MemoComponent(props) {
    /* render using props */
  });
  OR
  export default React.memo(MyFunctionComponent);