Make next/link async safe (#4911)

Removes componentWillMount and uses memoize instead as recommended here: https://reactjs.org/blog/2018/06/07/you-probably-dont-need-derived-state.html

lib/link.js

@@ -2,7 +2,6 @@
 
 import { resolve, format, parse } from 'url'
 import React, { Component, Children } from 'react'
-import {polyfill} from 'react-lifecycles-compat'
 import PropTypes from 'prop-types'
 import exact from 'prop-types-exact'
 import Router, { _rewriteUrlForNextExport } from './router'
@@ -18,6 +17,23 @@ function isLocal (href) {
 
 const warnLink = execOnce(warn)
 
+function memoizedFormatUrl (formatUrl) {
+  let lastHref = null
+  let lastAs = null
+  let lastResult = null
+  return (href, as) => {
+    if (href === lastHref && as === lastAs) {
+      return lastResult
+    }
+
+    const result = formatUrl(href, as)
+    lastHref = href
+    lastAs = as
+    lastResult = result
+    return result
+  }
+}
+
 class Link extends Component {
   static propTypes = exact({
     href: PropTypes.oneOfType([PropTypes.string, PropTypes.object]).isRequired,
@@ -41,11 +57,6 @@ class Link extends Component {
     ]).isRequired
   })
 
-  constructor (props, ...rest) {
-    super(props, ...rest)
-    this.formatUrls(props)
-  }
-
   componentDidMount () {
     this.prefetch()
   }
@@ -56,21 +67,18 @@ class Link extends Component {
     }
   }
 
-  // eslint-disable-next-line camelcase
+  // The function is memoized so that no extra lifecycles are needed
-  UNSAFE_componentWillReceiveProps (nextProps) {
+  // as per https://reactjs.org/blog/2018/06/07/you-probably-dont-need-derived-state.html
-    this.formatUrls(nextProps)
+  formatUrls = memoizedFormatUrl((href, asHref) => {
-  }
+    return {
-
+      href: href && typeof href === 'object'
-  // We accept both 'href' and 'as' as objects which we can pass to `url.format`.
+        ? format(href)
-  // We'll handle it here.
+        : href,
-  formatUrls (props) {
+      as: asHref && typeof asHref === 'object'
-    this.href = props.href && typeof props.href === 'object'
+        ? format(asHref)
-      ? format(props.href)
+        : asHref
-      : props.href
+    }
-    this.as = props.as && typeof props.as === 'object'
+  })
-      ? format(props.as)
-      : props.as
-  }
 
   linkClicked = e => {
     const { nodeName, target } = e.currentTarget
@@ -80,8 +88,7 @@ class Link extends Component {
       return
     }
 
-    const { shallow } = this.props
+    let { href, as } = this.formatUrls(this.props.href, this.props.as)
-    let { href, as } = this
 
     if (!isLocal(href)) {
       // ignore click if it's outside our scope
@@ -105,7 +112,7 @@ class Link extends Component {
     const changeMethod = replace ? 'replace' : 'push'
 
     // straight up redirect
-    Router[changeMethod](href, as, { shallow })
+    Router[changeMethod](href, as, { shallow: this.props.shallow })
       .then((success) => {
         if (!success) return
         if (scroll) {
@@ -124,13 +131,14 @@ class Link extends Component {
 
     // Prefetch the JSON page if asked (only in the client)
     const { pathname } = window.location
-    const href = resolve(pathname, this.href)
+    const {href: parsedHref} = this.formatUrls(this.props.href, this.props.as)
+    const href = resolve(pathname, parsedHref)
     Router.prefetch(href)
   }
 
   render () {
     let { children } = this.props
-    let { href, as } = this
+    let { href, as } = this.formatUrls(this.props.href, this.props.as)
     // Deprecated. Warning shown by propType check. If the childen provided is a string (<Link>example</Link>) we wrap it in an <a> tag
     if (typeof children === 'string') {
       children = <a>{children}</a>
@@ -169,6 +177,4 @@ class Link extends Component {
   }
 }
 
-// Make UNSAFE_ compatible with version of React under 16.3
-polyfill(Link)
 export default Link