diff --git a/src/helpers.spec.coffee b/src/helpers.spec.coffee
deleted file mode 100644
index 967625af5..000000000
--- a/src/helpers.spec.coffee
+++ /dev/null
@@ -1,244 +0,0 @@
-{expect: $expect} = require 'chai'
-
-$sinon = require 'sinon'
-
-#---------------------------------------------------------------------
-
-{$MappedCollection} = require './MappedCollection'
-
-$nonBindedHelpers = require './helpers'
-
-#=====================================================================
-
-describe 'Helper', ->
-
-  # Shared variables.
-  collection = $set = $sum = $val = null
-  beforeEach ->
-    # Creates the collection.
-    collection = new $MappedCollection()
-
-    # Dispatcher used for tests.
-    collection.dispatch = -> (@genkey.split '.')[0]
-
-    # Missing rules should be automatically created.
-    collection.missingRule = collection.rule
-
-    # # Monkey patch the collection to see all emitted events.
-    # emit = collection.emit
-    # collection.emit = (args...) ->
-    #   console.log args...
-    #   emit.call collection, args...
-
-    # Binds helpers to this collection.
-    {$set, $sum, $val} = do ->
-      helpers = {}
-      helpers[name] = fn.bind collection for name, fn of $nonBindedHelpers
-      helpers
-
-  #-------------------------------------------------------------------
-
-  # All helpers share the same logical code, we need only to test one
-  # extensively and test the others basically.
-  #
-  # $sum was chosen because it is the simplest helper to test.
-  describe '$sum', ->
-
-    it 'with single key', ->
-      collection.set foo: 1
-
-      collection.item sum: ->
-        @val = $sum {
-          key: 'foo'
-        }
-
-      $expect(collection.get 'sum').to.equal 1
-
-      collection.set foo:2
-
-      $expect(collection.get 'sum').to.equal 2
-
-      collection.remove 'foo'
-
-      $expect(collection.get 'sum').to.equal 0
-
-    it 'with multiple keys', ->
-      collection.set {
-        foo: 1
-        bar: 2
-      }
-
-      collection.item sum: ->
-        @val = $sum {
-          keys: ['foo', 'bar']
-        }
-
-      $expect(collection.get 'sum').to.equal 3
-
-      collection.set bar:3
-
-      $expect(collection.get 'sum').to.equal 4
-
-      collection.remove 'foo'
-
-      $expect(collection.get 'sum').to.equal 3
-
-    # FIXME: This test fails but this feature is not used.
-    it.skip 'with dynamic keys', ->
-      collection.set {
-        foo: 1
-        bar: 2
-      }
-
-      collection.rule sum: ->
-        @val = $sum {
-          key: -> (@key.split '.')[1]
-        }
-      collection.set {
-        'sum.foo': null
-        'sum.bar': null
-      }
-
-      $expect(collection.get 'sum.foo').to.equal 1
-      $expect(collection.get 'sum.bar').to.equal 2
-
-      collection.remove 'bar'
-
-      $expect(collection.get 'sum.foo').to.equal 1
-      $expect(collection.get 'sum.bar').to.equal 0
-
-    it 'with single rule', ->
-      collection.set {
-        'foo.1': 1
-        'foo.2': 2
-      }
-
-      collection.item sum: ->
-        @val = $sum {
-          rule: 'foo'
-        }
-
-      $expect(collection.get 'sum').to.equal 3
-
-      collection.set 'foo.2':3
-
-      $expect(collection.get 'sum').to.equal 4
-
-      collection.remove 'foo.1'
-
-      $expect(collection.get 'sum').to.equal 3
-
-    it 'with multiple rules', ->
-      collection.set {
-        'foo': 1
-        'bar.1': 2
-        'bar.2': 3
-      }
-
-      collection.item sum: ->
-        @val = $sum {
-          rules: ['foo', 'bar']
-        }
-
-      $expect(collection.get 'sum').to.equal 6
-
-      collection.set 'bar.1':3
-
-      $expect(collection.get 'sum').to.equal 7
-
-      collection.remove 'bar.2'
-
-      $expect(collection.get 'sum').to.equal 4
-
-    it 'with bind', ->
-      collection.set {
-        'foo': {
-          sum: 2 # This item will participate to `sum.2`.
-          val: 1
-        }
-        'bar': {
-          sum: 1 # This item will participate to `sum.1`.
-          val: 2
-        }
-      }
-
-      collection.rule sum: ->
-        @val = $sum {
-          bind: ->
-            id = @val.sum
-            return unless id?
-            "sum.#{id}"
-          val: -> @val.val
-        }
-      collection.set {
-        'sum.1': null
-        'sum.2': null
-      }
-
-      $expect(collection.get 'sum.1').equal 2
-      $expect(collection.get 'sum.2').equal 1
-
-      collection.set {
-        'foo': {
-          sum: 1
-          val: 3
-        }
-      }
-
-      $expect(collection.get 'sum.1').equal 5
-      $expect(collection.get 'sum.2').equal 0
-
-      collection.remove 'bar'
-
-      $expect(collection.get 'sum.1').equal 3
-      $expect(collection.get 'sum.2').equal 0
-
-
-    it 'with predicate', ->
-      collection.set {
-        foo: 1
-        bar: 2
-        baz: 3
-      }
-
-      collection.item sum: ->
-        @val = $sum {
-          if: -> /^b/.test @rule
-        }
-
-      $expect(collection.get 'sum').equal 5
-
-      collection.set foo:4
-
-      $expect(collection.get 'sum').equal 5
-
-      collection.set bar:5
-
-      $expect(collection.get 'sum').equal 8
-
-      collection.remove 'baz'
-
-      $expect(collection.get 'sum').equal 5
-
-    it 'with initial value', ->
-      collection.set foo: 1
-
-      collection.item sum: ->
-        @val = $sum {
-          key: 'foo'
-          init: 2
-        }
-
-      $expect(collection.get 'sum').to.equal 3
-
-      collection.set foo:2
-
-      $expect(collection.get 'sum').to.equal 4
-
-      collection.remove 'foo'
-
-      $expect(collection.get 'sum').to.equal 2
-
-# TODO:
-# - dynamic keys
-# - dynamic rules
diff --git a/src/helpers.spec.js b/src/helpers.spec.js
new file mode 100644
index 000000000..1e83950f2
--- /dev/null
+++ b/src/helpers.spec.js
@@ -0,0 +1,270 @@
+/* eslint-env mocha */
+
+import $bind from 'lodash.bind'
+import $sinon from 'sinon'
+import {expect as $expect} from 'chai'
+
+import {$MappedCollection} from './MappedCollection'
+import * as $nonBindedHelpers from './helpers'
+
+// Enable source maps support for traces.
+import sourceMapSupport from 'source-map-support'
+sourceMapSupport.install()
+
+// ===================================================================
+
+describe('Helper', () => {
+  // Shared variables.
+  // let $set
+  // let $val
+  let $sum
+  let collection
+
+  beforeEach(() => {
+    // Creates the collection.
+    collection = new $MappedCollection()
+
+    // Dispatcher used for tests.
+    collection.dispatch = function () { return this.genkey.split('.')[0] }
+
+    // Missing rules should be automatically created.
+    collection.missingRule = collection.rule
+
+    // // Monkey patch the collection to see all emitted events.
+    // {
+    //   const {emit} = collection
+    //   collection.emit = (...args) => {
+    //     console.log(...args)
+    //     emit.call(collection, ...args)
+    //   }
+    // }
+
+    // Binds helpers to this collection.
+    // $set = $bind($nonBindedHelpers.$set, collection)
+    // $val = $bind($nonBindedHelpers.$val, collection)
+    $sum = $bind($nonBindedHelpers.$sum, collection)
+  })
+
+  // -----------------------------------------------------------------
+
+  // All helpers share the same logical code, we need only to test one
+  // extensively and test the others basically.
+  //
+  // $sum was chosen because it is the simplest helper to test.
+  describe('$sum', () => {
+    it('with single key', () => {
+      collection.set({ foo: 1 })
+
+      collection.item('sum', function () {
+        this.val = $sum({
+          key: 'foo'
+        })
+      })
+
+      $expect(collection.get('sum')).to.equal(1)
+
+      collection.set({ foo: 2 })
+
+      $expect(collection.get('sum')).to.equal(2)
+
+      collection.remove('foo')
+
+      $expect(collection.get('sum')).to.equal(0)
+    })
+
+    it('with multiple keys', () => {
+      collection.set({
+        foo: 1,
+        bar: 2
+      })
+
+      collection.item('sum', function () {
+        this.val = $sum({
+          keys: ['foo', 'bar']
+        })
+      })
+
+      $expect(collection.get('sum')).to.equal(3)
+
+      collection.set({ bar: 3 })
+
+      $expect(collection.get('sum')).to.equal(4)
+
+      collection.remove('foo')
+
+      $expect(collection.get('sum')).to.equal(3)
+    })
+
+    // FIXME: This test fails but this feature is not used.
+    it.skip('with dynamic keys', () => {
+      collection.set({
+        foo: 1,
+        bar: 2
+      })
+
+      collection.rule('sum', function () {
+        this.val = $sum({
+          key: function () { return this.genkey.split('.')[1] }
+        })
+      })
+      collection.set({
+        'sum.foo': null,
+        'sum.bar': null
+      })
+
+      $expect(collection.get('sum.foo')).to.equal(1)
+      $expect(collection.get('sum.bar')).to.equal(2)
+
+      collection.remove('bar')
+
+      $expect(collection.get('sum.foo')).to.equal(1)
+      $expect(collection.get('sum.bar')).to.equal(0)
+    })
+
+    it('with single rule', () => {
+      collection.set({
+        'foo.1': 1,
+        'foo.2': 2
+      })
+
+      collection.item('sum', function () {
+        this.val = $sum({
+          rule: 'foo'
+        })
+      })
+
+      $expect(collection.get('sum')).to.equal(3)
+
+      collection.set({ 'foo.2': 3 })
+
+      $expect(collection.get('sum')).to.equal(4)
+
+      collection.remove('foo.1')
+
+      $expect(collection.get('sum')).to.equal(3)
+    })
+
+    it('with multiple rules', () => {
+      collection.set({
+        'foo': 1,
+        'bar.1': 2,
+        'bar.2': 3
+      })
+
+      collection.item('sum', function () {
+        this.val = $sum({
+          rules: ['foo', 'bar']
+        })
+      })
+
+      $expect(collection.get('sum')).to.equal(6)
+
+      collection.set({ 'bar.1': 3 })
+
+      $expect(collection.get('sum')).to.equal(7)
+
+      collection.remove('bar.2')
+
+      $expect(collection.get('sum')).to.equal(4)
+    })
+
+    it('with bind', () => {
+      collection.set({
+        'foo': {
+          sum: 2, // This item will participate to `sum.2`.
+          val: 1
+        },
+        'bar': {
+          sum: 1, // This item will participate to `sum.1`.
+          val: 2
+        }
+      })
+
+      collection.rule('sum', function () {
+        this.val = $sum({
+          bind: function () {
+            const id = this.val.sum
+
+            return id && `sum.${id}`
+          },
+          val: function () { return this.val.val }
+        })
+      })
+      collection.set({
+        'sum.1': null,
+        'sum.2': null
+      })
+
+      $expect(collection.get('sum.1')).to.equal(2)
+      $expect(collection.get('sum.2')).to.equal(1)
+
+      collection.set({
+        'foo': {
+          sum: 1,
+          val: 3
+        }
+      })
+
+      $expect(collection.get('sum.1')).to.equal(5)
+      $expect(collection.get('sum.2')).to.equal(0)
+
+      collection.remove('bar')
+
+      $expect(collection.get('sum.1')).to.equal(3)
+      $expect(collection.get('sum.2')).to.equal(0)
+    })
+
+    it('with predicate', () => {
+      collection.set({
+        foo: 1,
+        bar: 2,
+        baz: 3
+      })
+
+      collection.item('sum', function () {
+        this.val = $sum({
+          if: function () { return /^b/.test(this.rule) }
+        })
+      })
+
+      $expect(collection.get('sum')).to.equal(5)
+
+      collection.set({ foo: 4 })
+
+      $expect(collection.get('sum')).to.equal(5)
+
+      collection.set({ bar: 5 })
+
+      $expect(collection.get('sum')).to.equal(8)
+
+      collection.remove('baz')
+
+      $expect(collection.get('sum')).to.equal(5)
+    })
+
+    it('with initial value', () => {
+      collection.set({ foo: 1 })
+
+      collection.item('sum', function () {
+        this.val = $sum({
+          key: 'foo',
+          init: 2
+        })
+      })
+
+      $expect(collection.get('sum')).to.equal(3)
+
+      collection.set({ foo: 2 })
+
+      $expect(collection.get('sum')).to.equal(4)
+
+      collection.remove('foo')
+
+      $expect(collection.get('sum')).to.equal(2)
+    })
+  })
+
+// TODO:
+// - dynamic keys
+// - dynamic rules
+})