fix: write to disk incrementally (#40)

@SanterreJo thank you for the contribution 👍 I'll make an effort to get a release out soon, if I get buried in other work, please feel free to contact me here:

http://devtoolscommunity.herokuapp.com/

lib/puppeteer-to-istanbul.js

@@ -16,7 +16,14 @@ class PuppeteerToIstanbul {
   }
 
   writeIstanbulFormat () {
-    var fullJson = {}
+    mkdirp.sync('./.nyc_output')
+
+    const outFilePath = './.nyc_output/out.json'
+
+    fs.writeFileSync(outFilePath, '')
+
+    const fd = fs.openSync(outFilePath, 'a')
+    fs.writeSync(fd, '{')
 
     this.puppeteerToV8Info.forEach(jsFile => {
       const script = v8toIstanbul(jsFile.url)
@@ -25,12 +32,18 @@ class PuppeteerToIstanbul {
       let istanbulCoverage = script.toIstanbul()
       let keys = Object.keys(istanbulCoverage)
 
-      fullJson[keys[0]] = istanbulCoverage[keys[0]]
-      fullJson[keys[0]].originalUrl = jsFile.originalUrl
+      let jsonPart = {}
+      jsonPart[keys[0]] = istanbulCoverage[keys[0]]
+      jsonPart[keys[0]].originalUrl = jsFile.originalUrl
+
+      let jsonStr = JSON.stringify(jsonPart)
+        .replace(/^{/, '')
+        .replace(/}$/, '')
+      fs.writeSync(fd, jsonStr)
     })
 
-    mkdirp.sync('./.nyc_output')
-    fs.writeFileSync('./.nyc_output/out.json', JSON.stringify(fullJson), 'utf8')
+    fs.writeSync(fd, '}')
+    fs.closeSync(fd)
   }
 }