Change Image component lazy=true to loading=lazy (#18138)

This PR updates the `<Image>` component to follow the same property naming as native `<img>`.

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/Img#attr-loading

This currently allows two values,`loading=lazy` and `loading=eager`, but there might be new values added in a future spec.

cc @atcastle 

packages/next/client/image.tsx

 import React, { ReactElement, useEffect, useRef } from 'react'
 import Head from '../next-server/lib/head'
 
+const VALID_LOADING_VALUES = ['lazy', 'eager', undefined] as const
+type LoadingValue = typeof VALID_LOADING_VALUES[number]
+
 const loaders = new Map<LoaderKey, (props: LoaderProps) => string>([
   ['imgix', imgixLoader],
   ['cloudinary', cloudinaryLoader],
@@ -18,12 +21,12 @@ type ImageData = {
 
 type ImageProps = Omit<
   JSX.IntrinsicElements['img'],
-  'src' | 'srcSet' | 'ref' | 'width' | 'height'
+  'src' | 'srcSet' | 'ref' | 'width' | 'height' | 'loading'
 > & {
   src: string
   quality?: string
   priority?: boolean
-  lazy?: boolean
+  loading?: LoadingValue
   unoptimized?: boolean
 } & (
     | { width: number; height: number; unsized?: false }
@@ -142,7 +145,7 @@ export default function Image({
   sizes,
   unoptimized = false,
   priority = false,
-  lazy,
+  loading,
   className,
   quality,
   width,
@@ -152,17 +155,23 @@ export default function Image({
 }: ImageProps) {
   const thisEl = useRef<HTMLImageElement>(null)
 
-  // Sanity Checks:
-  // If priority and lazy are present, log an error and use priority only.
-  if (priority && lazy) {
-    if (process.env.NODE_ENV !== 'production') {
+  if (process.env.NODE_ENV !== 'production') {
+    if (!VALID_LOADING_VALUES.includes(loading)) {
+      throw new Error(
+        `Image with src "${src}" has invalid "loading" property. Provided "${loading}" should be one of ${VALID_LOADING_VALUES.map(
+          String
+        ).join(',')}.`
+      )
+    }
+    if (priority && loading === 'lazy') {
       throw new Error(
-        `Image with src "${src}" has both "priority" and "lazy" properties. Only one should be used.`
+        `Image with src "${src}" has both "priority" and "loading=lazy" properties. Only one should be used.`
       )
     }
   }
 
-  if (!priority && typeof lazy === 'undefined') {
+  let lazy = loading === 'lazy'
+  if (!priority && typeof loading === 'undefined') {
     lazy = true
   }
 

test/integration/image-component/basic/pages/client-side.js

@@ -9,7 +9,7 @@ const ClientSide = () => {
       <Image
         id="basic-image"
         src="foo.jpg"
-        lazy={false}
+        loading="eager"
         width={300}
         height={400}
         quality={60}
@@ -18,7 +18,7 @@ const ClientSide = () => {
         id="attribute-test"
         data-demo="demo-value"
         src="bar.jpg"
-        lazy={false}
+        loading="eager"
         width={300}
         height={400}
       />
@@ -27,7 +27,7 @@ const ClientSide = () => {
         data-demo="demo-value"
         host="secondary"
         src="foo2.jpg"
-        lazy={false}
+        loading="eager"
         width={300}
         height={400}
       />
@@ -35,7 +35,7 @@ const ClientSide = () => {
         id="unoptimized-image"
         unoptimized
         src="https://arbitraryurl.com/foo.jpg"
-        lazy={false}
+        loading="eager"
         width={300}
         height={400}
       />

test/integration/image-component/basic/pages/index.js

@@ -9,7 +9,7 @@ const Page = () => {
       <Image
         id="basic-image"
         src="foo.jpg"
-        lazy={false}
+        loading="eager"
         width={300}
         height={400}
         quality={60}
@@ -18,7 +18,7 @@ const Page = () => {
         id="attribute-test"
         data-demo="demo-value"
         src="bar.jpg"
-        lazy={false}
+        loading="eager"
         width={300}
         height={400}
       />
@@ -27,7 +27,7 @@ const Page = () => {
         data-demo="demo-value"
         host="secondary"
         src="foo2.jpg"
-        lazy={false}
+        loading="eager"
         width={300}
         height={400}
       />
@@ -35,7 +35,7 @@ const Page = () => {
         id="unoptimized-image"
         unoptimized
         src="https://arbitraryurl.com/foo.jpg"
-        lazy={false}
+        loading="eager"
         width={300}
         height={400}
       />

test/integration/image-component/basic/pages/lazy.js

@@ -5,12 +5,18 @@ const Lazy = () => {
   return (
     <div>
       <p id="stubtext">This is a page with lazy-loaded images</p>
-      <Image id="lazy-top" src="foo1.jpg" height={400} width={300} lazy></Image>
+      <Image
+        id="lazy-top"
+        src="foo1.jpg"
+        height={400}
+        width={300}
+        loading="lazy"
+      ></Image>
       <div style={{ height: '2000px' }}></div>
       <Image
         id="lazy-mid"
         src="foo2.jpg"
-        lazy
+        loading="lazy"
         height={400}
         width={300}
         className="exampleclass"
@@ -22,7 +28,22 @@ const Lazy = () => {
         height={400}
         width={300}
         unoptimized
-        lazy
+        loading="lazy"
+      ></Image>
+      <div style={{ height: '2000px' }}></div>
+      <Image
+        id="lazy-without-attribute"
+        src="foo4.jpg"
+        height={400}
+        width={300}
+      ></Image>
+      <div style={{ height: '2000px' }}></div>
+      <Image
+        id="eager-loading"
+        src="foo5.jpg"
+        loading="eager"
+        height={400}
+        width={300}
       ></Image>
     </div>
   )

test/integration/image-component/basic/test/index.test.js

@@ -132,6 +132,38 @@ function lazyLoadingTests() {
       await browser.elementById('lazy-bottom').getAttribute('srcset')
     ).toBeFalsy()
   })
+  it('should load the fourth image lazily after scrolling down', async () => {
+    expect(
+      await browser.elementById('lazy-without-attribute').getAttribute('src')
+    ).toBeFalsy()
+    expect(
+      await browser.elementById('lazy-without-attribute').getAttribute('srcset')
+    ).toBeFalsy()
+    let viewportHeight = await browser.eval(`window.innerHeight`)
+    let topOfBottomImage = await browser.eval(
+      `document.getElementById('lazy-without-attribute').parentElement.offsetTop`
+    )
+    let buffer = 150
+    await browser.eval(
+      `window.scrollTo(0, ${topOfBottomImage - (viewportHeight + buffer)})`
+    )
+    await waitFor(200)
+    expect(
+      await browser.elementById('lazy-without-attribute').getAttribute('src')
+    ).toBe('https://example.com/myaccount/foo4.jpg?auto=format')
+    expect(
+      await browser.elementById('lazy-without-attribute').getAttribute('srcset')
+    ).toBeTruthy()
+  })
+
+  it('should load the fifth image eagerly, without scrolling', async () => {
+    expect(await browser.elementById('eager-loading').getAttribute('src')).toBe(
+      'https://example.com/myaccount/foo5.jpg?auto=format'
+    )
+    expect(
+      await browser.elementById('eager-loading').getAttribute('srcset')
+    ).toBeTruthy()
+  })
 }
 
 async function hasPreloadLinkMatchingUrl(url) {