Tag: JavaScript

Webpack: Managing Tree Shaking

Posted on by Choon-Chern Lim

PROBLEM

Sometimes, Webpack’s tree-shaking may accidentally eliminate imported code from import statements.

For example, we may have a root JS file that imports a CSS file:-

import React from 'react';
import ReactDOM from 'react-dom';
import '../css/index.css';

...

… and for some reason, the CSS file will never appear in the final bundle even if the Webpack config contains proper rules to handle CSS files.

SOLUTION

To prevent Webpack from removing any “unreferenced” code (ex: global CSS files, JS polyfills, etc), list these side effects under package.json, for example:-

{
  "name": "front-end-stack",
  "sideEffects": [
    "*.css"
  ]
}

React: Debugging Layout Thrashing

Posted on by Choon-Chern Lim

PROBLEM

When the React app grows larger over time, it is highly likely to run into situations where the component keeps re-rendering for no apparent reason.

There are many reasons why this is happening… to name a few…. parent component re-renders, this.props or this.state has changed, etc.

The key is to quickly find out what causes the component to re-render constantly.

SOLUTION

The simplest solution, in my opinion, is to paste the following block into the troublesome component:-

componentDidUpdate(prevProps, prevState) {
  const debug = (label, currObject = {}, prevObject = {}) => {
    Object.entries(currObject).forEach(([key, currValue]) => {
      if (prevObject[key] !== currValue) {
        console.log(`[DEBUG] ${label} has changed: `, key);
        console.log('[DEBUG] - BEFORE : ', prevObject[key]);
        console.log('[DEBUG] -  AFTER : ', currValue);
      }
    });
  };

  debug('Prop', this.props, prevProps);
  debug('State', this.state, prevState);
}

Adding this componentDidUpdate(..) lifecycle allows us to quickly find out which property or state has changed.

When running the app, the console may display something like this:-

Console Log

The key is to look for identical object or array displayed in both “BEFORE” and “AFTER” statements. This shows that while the values look similar, they fail on strict equality check (‘===’), which causes the component to re-render.

React + Recompose: Calling Multiple HOC Wrappers

Posted on by Choon-Chern Lim

PROBLEM

Sometimes, wrapping a React component with multiple High Order Components (HOC) can get rather unwieldy and unreadable.

For example:-

import React from 'react';
import { withRouter } from 'react-router-dom';
import { withStyles } from 'material-ui/styles';
import withWidth from 'material-ui/utils/withWidth';

class MyComponent extends React.PureComponent {
	// ...
}

export default withRouter(withStyles(styles)(withWidth()(MyComponent)));

SOLUTION

To fix this, we can leverage recompose library.

Now, we can rewrite the above example like this:-

import React from 'react';
import { withRouter } from 'react-router-dom';
import { withStyles } from 'material-ui/styles';
import withWidth from 'material-ui/utils/withWidth';
import compose from 'recompose/compose';

class MyComponent extends React.PureComponent {
	// ...
}

export default compose(
  withRouter,
  withStyles(styles),
  withWidth(),
)(MyComponent);

Keep in mind, the HOC order defined in compose(..) is important.