TypeError: (0 , _react.useEffect) is not a function

It seem that microbundler does not tolerate to React. This one create bundle that attempt to use react from global scope, instead React that really exposed.

For the same reason your workaround with React.useEffect works as expected, just imagine that it looks like window.React.useEffect.

Here is an example of a primitive application:

import ReactDOM from 'react-dom';import React, { useEffect, useState } from 'react';/** * necessary workaround, microbundle use `h` pragma by default, * that undefined when use React * another option is to make build with option --jsx * @example microbundle --globals react=React --jsx React.createElement * yes, yet another workaround*/window.h = React.createElement;const X = () => {  const [A, B] = useState('world');  useEffect(() => {    B('MLyck');  }, [])  return `Hello ${A}`;}ReactDOM.render(<X />, document.querySelector('react-app'));

After bundling with just microbundle it completely broken, but when you try to bundle with

microbundle --globals react=React

as correctly suggest @Jee Mok, it will produce correct bundle.I hope comments will explain what happened.

!function (e, t) {  "object" == typeof exports && "undefined" != typeof module ?    t(require("react-dom"), require("react")) :    "function" == typeof define && define.amd ?      define(["react-dom", "react"], t) :      t(e.ReactDOM, e.React);  /*  String above is core of problem,  in case you try to bundle without options `--globals react=React`  it will looks like: `t(e.ReactDOM, e.react);`  Obviously `react` is not defined in `e` e.g. `this` e.g. `window`  due to react expose self as `React`   */}(this, function (e, t) {  e = e && e.hasOwnProperty("default") ? e.default : e, window.h = ("default" in t ? t.default : t).createElement, e.render(h(function () {    var e = t.useState("world"), n = e[0], r = e[1];    return t.useEffect(function () {      r("MLyck");    }, []), "Hello " + n;  }, null), document.querySelector("react-app"));});

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.13.1/umd/react.development.js"></script>    <script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.13.1/umd/react-dom.development.js"></script>    <react-app></react-app>