fix(diff-snapshot): JSON.stringify failed due to circular referneces (#…

…72)

* fix(diff-snapshot): Fix stringify failure due to circular referneces.

* fix(diff-snapshot): Apply tslint --fix

* Add node v9 to CI build

* DO NOT SUBMIT: Fix pngjs.

* Remove comment.

* Upgrade pngjs to 3.3.3

.travis.yml

@@ -1,4 +1,5 @@
 language: node_js
 node_js:
-  - "8"
   - "6"
+  - "8"
+  - "9"

package.json

@@ -55,7 +55,7 @@
     "lodash": "^4.17.4",
     "mkdirp": "^0.5.1",
     "pixelmatch": "^4.0.2",
-    "pngjs": "^3.3.0",
+    "pngjs": "^3.3.3",
     "rimraf": "^2.6.2"
   },
   "peerDependencies": {

src/diff-snapshot.js

@@ -156,9 +156,12 @@ function diffImageToSnapshot(options) {
       );
 
       const input = { imagePath: diffOutputPath, image: compositeResultImage };
+      // image._packer property contains a circular reference since node9, causing JSON.stringify to
+      // fail. Might as well discard all the hidden properties.
+      const serializedInput = JSON.stringify(input, (name, val) => (name[0] === '_' ? undefined : val));
 
       // writing diff in separate process to avoid perf issues associated with Math in Jest VM (https://github.com/facebook/jest/issues/5163)
-      const writeDiffProcess = childProcess.spawnSync('node', [`${__dirname}/write-result-diff-image.js`], { input: Buffer.from(JSON.stringify(input)) });
+      const writeDiffProcess = childProcess.spawnSync('node', [`${__dirname}/write-result-diff-image.js`], { input: Buffer.from(serializedInput) });
       // in case of error print to console
       if (writeDiffProcess.stderr.toString()) { console.log(writeDiffProcess.stderr.toString()); } // eslint-disable-line no-console, max-len
     }