chore(gatsby-image): Add note about why !! is used

polarathene · polarathene · commit 32e17ef99bd5 · 2019-05-20T19:16:46.000+12:00
As explained at: gatsbyjs#13287 (comment)
diff --git a/packages/gatsby-image/src/index.js b/packages/gatsby-image/src/index.js
@@ -256,6 +256,8 @@ class Image extends React.Component {
         this.setState({ isVisible: true }, () =>
           this.setState({
             imgLoaded: imageInCache,
+            // `currentSrc` should be a string, but can be `undefined` in IE,
+            // !! operator validates the value is not undefined/null/""
             imgCached: !!this.imageRef.current.currentSrc,
           })
         )

Original file line number	Diff line number	Diff line change
`@@ -256,6 +256,8 @@ class Image extends React.Component {`
`256`	`256`	`this.setState({ isVisible: true }, () =>`
`257`	`257`	`this.setState({`
`258`	`258`	`imgLoaded: imageInCache,`
	`259`	+ // `currentSrc` should be a string, but can be `undefined` in IE,
	`260`	`+ // !! operator validates the value is not undefined/null/""`
`259`	`261`	`imgCached: !!this.imageRef.current.currentSrc,`
`260`	`262`	`})`
`261`	`263`	`)`