bvaughn

React recently introduced an experimental profiler API. This page gives instructions on how to use this API in a production release of your app.

Table of Contents

• Profiling in production
  • Create React App
  • Webpack 4

Profiling in production

React DOM automatically supports profiling in development mode for v16.5+, but since profiling adds some small additional overhead it is opt-in for production mode. This gist explains how to opt-in.

yelouafi

// We model the call stack using a linked list of Generators
// Each Generator has a _return field pointing back to its parent

function stepGen(gen, arg) {
    const {done, value} = gen.next(arg)
    if(done) {        
        if(gen._return) {
            stepGen(gen._return, value)
        }

bvaughn

This API was removed in React 17

---

Interaction tracing with React

React recently introduced an experimental profiler API. After discussing this API with several teams at Facebook, one common piece of feedback was that the performance information would be more useful if it could be associated with the events that caused the application to render (e.g. button click, XHR response). Tracing these events (or "interactions") would enable more powerful tooling to be built around the timing information, capable of answering questions like "What caused this really slow commit?" or "How long does it typically take for this interaction to update the DOM?".

With version 16.4.3, React added experimental support for this tracing by way of a new NPM package, scheduler. However the public API for this package is not yet finalized and will likely change with upcoming minor releases, so it should be used with caution.

bvaughn

const createLogger = (backgroundColor, color) => {
  const logger = (message, ...args) => {
    if (logger.enabled === false) {
      return;
    }

    console.groupCollapsed(
      `%c${message}`,
      `background-color: ${backgroundColor}; color: ${color}; padding: 2px 4px;`,
      ...args

getify

I received a question about this snippet of code:

function def(first="oldValue" , second=function(){
         return first;
}){
        var first="updatedValue";
        console.log('inside',first);
        console.log('function',second());
}

gaearon

Note:

When this guide is more complete, the plan is to move it into Prepack documentation.
For now I put it out as a gist to gather initial feedback.

A Gentle Introduction to Prepack (Part 1)

If you're building JavaScript apps, you might already be familiar with some tools that compile JavaScript code to equivalent JavaScript code:

• Babel lets you use newer JavaScript language features, and outputs equivalent code that targets older JavaScript engines.

rauschma

Strings

String.prototype.*

None of the string methods modify this – they always return fresh strings.

• charAt(pos: number): string ^ES1

  Returns the character at index pos, as a string (JavaScript does not have a datatype for characters). str[i] is equivalent to str.charAt(i) and more concise (caveat: may not work on old engines).

kentcdodds

javascript:(function(){if(window._activeElInterval){clearInterval(window._activeElInterval);delete window._activeElInterval;}else{var activeEl;window._activeElInterval=setInterval(function(){var currentActiveEl=document.activeElement;if(currentActiveEl!==activeEl){activeEl=currentActiveEl;console.log(activeEl);}},200);}})();

bramus

Bridging Native & React for iOS & Android

js -> native

• Simple Example
• Callbacks
• Promises
• Properties

native -> js

trueadm

// I'm suggesting we add a new "adopt X from <Y />" syntax to the JSX language
// it would de-sugar to render prop children, but look and read better than
// what we currently have. For example:

// 1.
// this sugar
function MyComponent(props) {
  adopt foo from <Bar />;
  return <div>{foo}</div>;
}