Improve code quality by increasing code coverage: lib/source-map-from…

…-file.js (bcoe#453)

refactor:  lib/source-map-from-file.js to improve code coverage (bcoe#453)
refactor: exposed source-map-from-file.js function to write test cases (bcoe#453)
test: Added two test case cases to cover error handling for improper formatting (bcoe#453)

lib/report.js

@@ -11,7 +11,7 @@ try {
 const { readdirSync, readFileSync, statSync } = require('fs')
 const { isAbsolute, resolve, extname } = require('path')
 const { pathToFileURL, fileURLToPath } = require('url')
-const getSourceMapFromFile = require('./source-map-from-file')
+const { getSourceMapFromFile } = require('./source-map-from-file')
 // TODO: switch back to @c88/v8-coverage once patch is landed.
 const v8toIstanbul = require('v8-to-istanbul')
 const util = require('util')

lib/source-map-from-file.js

@@ -82,8 +82,13 @@ function sourceMapFromDataUrl (url) {
   const { 0: format, 1: data } = url.split(',')
   const splitFormat = format.split(';')
   const contentType = splitFormat[0]
+  // always evaluates to true for our use cases.  Could possibly be false.
+  // Per https://github.com/bcoe/c8/pull/453#discussion_r1106306167
   const base64 = splitFormat[splitFormat.length - 1] === 'base64'
   if (contentType === 'application/json') {
+    // the data variable is never a json string in the test cases
+    // the upstream branch in node.js still has this logic
+    // https://github.com/nodejs/node/blob/v19.x/lib/internal/source_map/source_map_cache.js#L241
     const decodedData = base64 ? Buffer.from(data, 'base64').toString('utf8') : data
     try {
       return JSON.parse(decodedData)
@@ -97,4 +102,9 @@ function sourceMapFromDataUrl (url) {
   }
 }
 
-module.exports = getSourceMapFromFile
+module.exports = {
+  getSourceMapFromFile,
+  dataFromUrl,
+  sourceMapFromFile,
+  sourceMapFromDataUrl
+}