fix(json): strip BOM when reading JSON files (#8)

fs.readFile* don't strip the BOM header, even when specifying 'utf8' as
the encoding, and JSON.parse() doesn't handle it either. There are
technically a bunch of BOM indicators, but this is the one seen most
commonly and actually appears in a number of package.json files in the
wild.

See nodejs/node#6924, nodejs/node#3040 for background.

index.js

@@ -135,13 +135,18 @@ class Installer {
   }
 }
 module.exports = Installer
+module.exports._readJson = readJson
 
 function readJson (jsonPath, name, ignoreMissing) {
   return readFileAsync(path.join(jsonPath, name), 'utf8')
-  .then(str => JSON.parse(str))
+  .then(str => JSON.parse(stripBOM(str)))
   .catch({code: 'ENOENT'}, err => {
     if (!ignoreMissing) {
       throw err
     }
   })
 }
+
+function stripBOM (str) {
+  return str.replace(/^\uFEFF/, '')
+}

test/lib/package-json-with-bom.json

@@ -0,0 +1,6 @@
+{
+  "name": "strong-spawn-npm",
+  "version": "1.0.0",
+  "description": "Reliably spawn npm™ on any platform",
+  "homepage": "https://github.com/strongloop/strong-spawn-npm"
+}

test/specs/index.js

@@ -1,9 +1,10 @@
 'use strict'
 
+const fixtureHelper = require('../lib/fixtureHelper.js')
+const fs = require('fs')
 const path = require('path')
-const test = require('tap').test
 const requireInject = require('require-inject')
-const fixtureHelper = require('../lib/fixtureHelper.js')
+const test = require('tap').test
 
 let extract = () => {}
 const pkgName = 'hark-a-package'
@@ -298,3 +299,20 @@ test('skips lifecycle scripts with ignoreScripts is set', t => {
     t.end()
   })
 })
+
+test('handles JSON docs that contain a BOM', t => {
+  t.plan(2)
+  const Installer = requireInject('../../index.js', {/* just don't want to cache */})
+  const bomJSON = 'package-json-with-bom.json'
+  const bomJSONDir = path.resolve(__dirname, '../lib')
+  const actualJSON = {
+    name: 'strong-spawn-npm',
+    version: '1.0.0',
+    description: 'Reliably spawn npm™ on any platform',
+    homepage: 'https://github.com/strongloop/strong-spawn-npm'
+  }
+  // ensure that the file does indeed fail to be parsed by JSON.parse
+  t.throws(() => JSON.parse(fs.readFileSync(path.join(bomJSONDir, bomJSON), 'utf8')),
+           {message: 'Unexpected token \uFEFF'})
+  return Installer._readJson(bomJSONDir, bomJSON).then(obj => t.match(obj, actualJSON))
+})