diff --git a/spec/gutter-container-spec.coffee b/spec/gutter-container-spec.coffee
deleted file mode 100644
index dc4af0b8c..000000000
--- a/spec/gutter-container-spec.coffee
+++ /dev/null
@@ -1,64 +0,0 @@
-Gutter = require '../src/gutter'
-GutterContainer = require '../src/gutter-container'
-
-describe 'GutterContainer', ->
-  gutterContainer = null
-  fakeTextEditor = {
-    scheduleComponentUpdate: ->
-  }
-
-  beforeEach ->
-    gutterContainer = new GutterContainer fakeTextEditor
-
-  describe 'when initialized', ->
-    it 'it has no gutters', ->
-      expect(gutterContainer.getGutters().length).toBe 0
-
-  describe '::addGutter', ->
-    it 'creates a new gutter', ->
-      newGutter = gutterContainer.addGutter {'test-gutter', priority: 1}
-      expect(gutterContainer.getGutters()).toEqual [newGutter]
-      expect(newGutter.priority).toBe 1
-
-    it 'throws an error if the provided gutter name is already in use', ->
-      name = 'test-gutter'
-      gutterContainer.addGutter {name}
-      expect(gutterContainer.addGutter.bind(null, {name})).toThrow()
-
-    it 'keeps added gutters sorted by ascending priority', ->
-      gutter1 = gutterContainer.addGutter {name: 'first', priority: 1}
-      gutter3 = gutterContainer.addGutter {name: 'third', priority: 3}
-      gutter2 = gutterContainer.addGutter {name: 'second', priority: 2}
-      expect(gutterContainer.getGutters()).toEqual [gutter1, gutter2, gutter3]
-
-  describe '::removeGutter', ->
-    removedGutters = null
-
-    beforeEach ->
-      gutterContainer = new GutterContainer fakeTextEditor
-      removedGutters = []
-      gutterContainer.onDidRemoveGutter (gutterName) ->
-        removedGutters.push gutterName
-
-    it 'removes the gutter if it is contained by this GutterContainer', ->
-      gutter = gutterContainer.addGutter {'test-gutter'}
-      expect(gutterContainer.getGutters()).toEqual [gutter]
-      gutterContainer.removeGutter gutter
-      expect(gutterContainer.getGutters().length).toBe 0
-      expect(removedGutters).toEqual [gutter.name]
-
-    it 'throws an error if the gutter is not within this GutterContainer', ->
-      fakeOtherTextEditor = {}
-      otherGutterContainer = new GutterContainer fakeOtherTextEditor
-      gutter = new Gutter 'gutter-name', otherGutterContainer
-      expect(gutterContainer.removeGutter.bind(null, gutter)).toThrow()
-
-  describe '::destroy', ->
-    it 'clears its array of gutters and destroys custom gutters', ->
-      newGutter = gutterContainer.addGutter {'test-gutter', priority: 1}
-      newGutterSpy = jasmine.createSpy()
-      newGutter.onDidDestroy(newGutterSpy)
-
-      gutterContainer.destroy()
-      expect(newGutterSpy).toHaveBeenCalled()
-      expect(gutterContainer.getGutters()).toEqual []
diff --git a/spec/gutter-container-spec.js b/spec/gutter-container-spec.js
new file mode 100644
index 000000000..f41f1d220
--- /dev/null
+++ b/spec/gutter-container-spec.js
@@ -0,0 +1,77 @@
+const Gutter = require('../src/gutter')
+const GutterContainer = require('../src/gutter-container')
+
+describe('GutterContainer', () => {
+  let gutterContainer = null
+  const fakeTextEditor = {
+    scheduleComponentUpdate () {}
+  }
+
+  beforeEach(() => {
+    gutterContainer = new GutterContainer(fakeTextEditor)
+  })
+
+  describe('when initialized', () =>
+    it('it has no gutters', () => {
+      expect(gutterContainer.getGutters().length).toBe(0)
+    })
+  )
+
+  describe('::addGutter', () => {
+    it('creates a new gutter', () => {
+      const newGutter = gutterContainer.addGutter({'test-gutter': 'test-gutter', priority: 1})
+      expect(gutterContainer.getGutters()).toEqual([newGutter])
+      expect(newGutter.priority).toBe(1)
+    })
+
+    it('throws an error if the provided gutter name is already in use', () => {
+      const name = 'test-gutter'
+      gutterContainer.addGutter({name})
+      expect(gutterContainer.addGutter.bind(null, {name})).toThrow()
+    })
+
+    it('keeps added gutters sorted by ascending priority', () => {
+      const gutter1 = gutterContainer.addGutter({name: 'first', priority: 1})
+      const gutter3 = gutterContainer.addGutter({name: 'third', priority: 3})
+      const gutter2 = gutterContainer.addGutter({name: 'second', priority: 2})
+      expect(gutterContainer.getGutters()).toEqual([gutter1, gutter2, gutter3])
+    })
+  })
+
+  describe('::removeGutter', () => {
+    let removedGutters
+
+    beforeEach(function () {
+      gutterContainer = new GutterContainer(fakeTextEditor)
+      removedGutters = []
+      gutterContainer.onDidRemoveGutter(gutterName => removedGutters.push(gutterName))
+    })
+
+    it('removes the gutter if it is contained by this GutterContainer', () => {
+      const gutter = gutterContainer.addGutter({'test-gutter': 'test-gutter'})
+      expect(gutterContainer.getGutters()).toEqual([gutter])
+      gutterContainer.removeGutter(gutter)
+      expect(gutterContainer.getGutters().length).toBe(0)
+      expect(removedGutters).toEqual([gutter.name])
+    })
+
+    it('throws an error if the gutter is not within this GutterContainer', () => {
+      const fakeOtherTextEditor = {}
+      const otherGutterContainer = new GutterContainer(fakeOtherTextEditor)
+      const gutter = new Gutter('gutter-name', otherGutterContainer)
+      expect(gutterContainer.removeGutter.bind(null, gutter)).toThrow()
+    })
+  })
+
+  describe('::destroy', () =>
+    it('clears its array of gutters and destroys custom gutters', () => {
+      const newGutter = gutterContainer.addGutter({'test-gutter': 'test-gutter', priority: 1})
+      const newGutterSpy = jasmine.createSpy()
+      newGutter.onDidDestroy(newGutterSpy)
+
+      gutterContainer.destroy()
+      expect(newGutterSpy).toHaveBeenCalled()
+      expect(gutterContainer.getGutters()).toEqual([])
+    })
+)
+})
diff --git a/src/gutter-container.coffee b/src/gutter-container.coffee
deleted file mode 100644
index 677fa4521..000000000
--- a/src/gutter-container.coffee
+++ /dev/null
@@ -1,87 +0,0 @@
-{Emitter} = require 'event-kit'
-Gutter = require './gutter'
-
-module.exports =
-class GutterContainer
-  constructor: (textEditor) ->
-    @gutters = []
-    @textEditor = textEditor
-    @emitter = new Emitter
-
-  scheduleComponentUpdate: ->
-    @textEditor.scheduleComponentUpdate()
-
-  destroy: ->
-    # Create a copy, because `Gutter::destroy` removes the gutter from
-    # GutterContainer's @gutters.
-    guttersToDestroy = @gutters.slice(0)
-    for gutter in guttersToDestroy
-      gutter.destroy() if gutter.name isnt 'line-number'
-    @gutters = []
-    @emitter.dispose()
-
-  addGutter: (options) ->
-    options = options ? {}
-    gutterName = options.name
-    if gutterName is null
-      throw new Error('A name is required to create a gutter.')
-    if @gutterWithName(gutterName)
-      throw new Error('Tried to create a gutter with a name that is already in use.')
-    newGutter = new Gutter(this, options)
-
-    inserted = false
-    # Insert the gutter into the gutters array, sorted in ascending order by 'priority'.
-    # This could be optimized, but there are unlikely to be many gutters.
-    for i in [0...@gutters.length]
-      if @gutters[i].priority >= newGutter.priority
-        @gutters.splice(i, 0, newGutter)
-        inserted = true
-        break
-    if not inserted
-      @gutters.push newGutter
-    @scheduleComponentUpdate()
-    @emitter.emit 'did-add-gutter', newGutter
-    return newGutter
-
-  getGutters: ->
-    @gutters.slice()
-
-  gutterWithName: (name) ->
-    for gutter in @gutters
-      if gutter.name is name then return gutter
-    null
-
-  observeGutters: (callback) ->
-    callback(gutter) for gutter in @getGutters()
-    @onDidAddGutter callback
-
-  onDidAddGutter: (callback) ->
-    @emitter.on 'did-add-gutter', callback
-
-  onDidRemoveGutter: (callback) ->
-    @emitter.on 'did-remove-gutter', callback
-
-  ###
-  Section: Private Methods
-  ###
-
-  # Processes the destruction of the gutter. Throws an error if this gutter is
-  # not within this gutterContainer.
-  removeGutter: (gutter) ->
-    index = @gutters.indexOf(gutter)
-    if index > -1
-      @gutters.splice(index, 1)
-      @scheduleComponentUpdate()
-      @emitter.emit 'did-remove-gutter', gutter.name
-    else
-      throw new Error 'The given gutter cannot be removed because it is not ' +
-          'within this GutterContainer.'
-
-  # The public interface is Gutter::decorateMarker or TextEditor::decorateMarker.
-  addGutterDecoration: (gutter, marker, options) ->
-    if gutter.name is 'line-number'
-      options.type = 'line-number'
-    else
-      options.type = 'gutter'
-    options.gutterName = gutter.name
-    @textEditor.decorateMarker(marker, options)
diff --git a/src/gutter-container.js b/src/gutter-container.js
new file mode 100644
index 000000000..3faece073
--- /dev/null
+++ b/src/gutter-container.js
@@ -0,0 +1,108 @@
+const {Emitter} = require('event-kit')
+const Gutter = require('./gutter')
+
+module.exports = class GutterContainer {
+  constructor (textEditor) {
+    this.gutters = []
+    this.textEditor = textEditor
+    this.emitter = new Emitter()
+  }
+
+  scheduleComponentUpdate () {
+    this.textEditor.scheduleComponentUpdate()
+  }
+
+  destroy () {
+    // Create a copy, because `Gutter::destroy` removes the gutter from
+    // GutterContainer's @gutters.
+    const guttersToDestroy = this.gutters.slice(0)
+    for (let gutter of guttersToDestroy) {
+      if (gutter.name !== 'line-number') { gutter.destroy() }
+    }
+    this.gutters = []
+    this.emitter.dispose()
+  }
+
+  addGutter (options) {
+    options = options || {}
+    const gutterName = options.name
+    if (gutterName === null) {
+      throw new Error('A name is required to create a gutter.')
+    }
+    if (this.gutterWithName(gutterName)) {
+      throw new Error('Tried to create a gutter with a name that is already in use.')
+    }
+    const newGutter = new Gutter(this, options)
+
+    let inserted = false
+    // Insert the gutter into the gutters array, sorted in ascending order by 'priority'.
+    // This could be optimized, but there are unlikely to be many gutters.
+    for (let i = 0; i < this.gutters.length; i++) {
+      if (this.gutters[i].priority >= newGutter.priority) {
+        this.gutters.splice(i, 0, newGutter)
+        inserted = true
+        break
+      }
+    }
+    if (!inserted) {
+      this.gutters.push(newGutter)
+    }
+    this.scheduleComponentUpdate()
+    this.emitter.emit('did-add-gutter', newGutter)
+    return newGutter
+  }
+
+  getGutters () {
+    return this.gutters.slice()
+  }
+
+  gutterWithName (name) {
+    for (let gutter of this.gutters) {
+      if (gutter.name === name) { return gutter }
+    }
+    return null
+  }
+
+  observeGutters (callback) {
+    for (let gutter of this.getGutters()) { callback(gutter) }
+    return this.onDidAddGutter(callback)
+  }
+
+  onDidAddGutter (callback) {
+    return this.emitter.on('did-add-gutter', callback)
+  }
+
+  onDidRemoveGutter (callback) {
+    return this.emitter.on('did-remove-gutter', callback)
+  }
+
+  /*
+  Section: Private Methods
+  */
+
+  // Processes the destruction of the gutter. Throws an error if this gutter is
+  // not within this gutterContainer.
+  removeGutter (gutter) {
+    const index = this.gutters.indexOf(gutter)
+    if (index > -1) {
+      this.gutters.splice(index, 1)
+      this.scheduleComponentUpdate()
+      this.emitter.emit('did-remove-gutter', gutter.name)
+    } else {
+      throw new Error('The given gutter cannot be removed because it is not ' +
+          'within this GutterContainer.'
+      )
+    }
+  }
+
+  // The public interface is Gutter::decorateMarker or TextEditor::decorateMarker.
+  addGutterDecoration (gutter, marker, options) {
+    if (gutter.name === 'line-number') {
+      options.type = 'line-number'
+    } else {
+      options.type = 'gutter'
+    }
+    options.gutterName = gutter.name
+    return this.textEditor.decorateMarker(marker, options)
+  }
+}