Move dateTickFormat to separate module

app/assets/javascripts/graphs/stat_graph_contributors_graph.js

 /* eslint-disable func-names, space-before-function-paren, no-var, prefer-rest-params, max-len, no-restricted-syntax, vars-on-top, no-use-before-define, no-param-reassign, new-cap, no-underscore-dangle, wrap-iife, comma-dangle, no-return-assign, prefer-arrow-callback, quotes, prefer-template, newline-per-chained-call, no-else-return, no-shadow */
 import _ from 'underscore';
 import d3 from 'd3';
-import { createDateTimeFormat } from '../locale';
+import { dateTickFormat } from '../lib/utils/tick_formats';
 
 const extend = function(child, parent) { for (var key in parent) { if (hasProp.call(parent, key)) child[key] = parent[key]; } function ctor() { this.constructor = child; } ctor.prototype = parent.prototype; child.prototype = new ctor(); child.__super__ = parent.prototype; return child; };
 const hasProp = {}.hasOwnProperty;
 
-const dayFormat = createDateTimeFormat({ month: 'short', day: 'numeric' });
-const monthFormat = createDateTimeFormat({ month: 'long' });
-const yearFormat = createDateTimeFormat({ year: 'numeric' });
-
-const xTickFormat = (date) => {
-  if (date.getDate() !== 1) {
-    return dayFormat.format(date);
-  } else if (date.getMonth() > 0) {
-    return monthFormat.format(date);
-  } else {
-    return yearFormat.format(date);
-  }
-};
-
 export const ContributorsGraph = (function() {
   function ContributorsGraph() {}
 
@@ -152,7 +138,7 @@ export const ContributorsMasterGraph = (function(superClass) {
     this.x_axis = d3.svg.axis()
       .scale(this.x)
       .orient('bottom')
-      .tickFormat(xTickFormat);
+      .tickFormat(dateTickFormat);
     return this.y_axis = d3.svg.axis().scale(this.y).orient("left").ticks(5);
   };
 
@@ -244,7 +230,7 @@ export const ContributorsAuthorGraph = (function(superClass) {
       .scale(this.x)
       .orient('bottom')
       .ticks(8)
-      .tickFormat(xTickFormat);
+      .tickFormat(dateTickFormat);
     return this.y_axis = d3.svg.axis().scale(this.y).orient("left").ticks(5);
   };
 

app/assets/javascripts/lib/utils/tick_formats.js

+import { createDateTimeFormat } from '../../locale';
+
+let dateTimeFormats;
+
+export const initDateFormats = () => {
+  const dayFormat = createDateTimeFormat({ month: 'short', day: 'numeric' });
+  const monthFormat = createDateTimeFormat({ month: 'long' });
+  const yearFormat = createDateTimeFormat({ year: 'numeric' });
+
+  dateTimeFormats = {
+    dayFormat,
+    monthFormat,
+    yearFormat,
+  };
+};
+
+initDateFormats();
+
+/**
+  Formats a localized date in way that it can be used for d3.js axis.tickFormat().
+
+  That is, it displays
+  - 4-digit for first of January
+  - full month name for first of every month
+  - day and abbreviated month otherwise
+
+  see also https://github.com/d3/d3-3.x-api-reference/blob/master/SVG-Axes.md#tickFormat
+  */
+export const dateTickFormat = (date) => {
+  if (date.getDate() !== 1) {
+    return dateTimeFormats.dayFormat.format(date);
+  }
+
+  if (date.getMonth() > 0) {
+    return dateTimeFormats.monthFormat.format(date);
+  }
+
+  return dateTimeFormats.yearFormat.format(date);
+};

spec/javascripts/helpers/locale_helper.js

+/* eslint-disable import/prefer-default-export */
+
+export const setLanguage = (languageCode) => {
+  const htmlElement = document.querySelector('html');
+
+  if (languageCode) {
+    htmlElement.setAttribute('lang', languageCode);
+  } else {
+    htmlElement.removeAttribute('lang');
+  }
+};

spec/javascripts/lib/utils/tick_formats_spec.js

+import { dateTickFormat, initDateFormats } from '~/lib/utils/tick_formats';
+
+import { setLanguage } from '../../helpers/locale_helper';
+
+describe('tick formats', () => {
+  describe('dateTickFormat', () => {
+    beforeAll(() => {
+      setLanguage('de');
+      initDateFormats();
+    });
+
+    afterAll(() => {
+      setLanguage(null);
+    });
+
+    it('returns year for first of January', () => {
+      const tick = dateTickFormat(new Date('2001-01-01'));
+
+      expect(tick).toBe('2001');
+    });
+
+    it('returns month for first of February', () => {
+      const tick = dateTickFormat(new Date('2001-02-01'));
+
+      expect(tick).toBe('Februar');
+    });
+
+    it('returns day and month for second of February', () => {
+      const tick = dateTickFormat(new Date('2001-02-02'));
+
+      expect(tick).toBe('2. Feb.');
+    });
+
+    it('ignores time', () => {
+      const tick = dateTickFormat(new Date('2001-02-02 12:34:56'));
+
+      expect(tick).toBe('2. Feb.');
+    });
+  });
+});

spec/javascripts/locale/index_spec.js

 import { createDateTimeFormat, languageCode } from '~/locale';
 
-const setLanguage = (languageCode) => {
-  const htmlElement = document.querySelector('html');
-
-  if (languageCode) {
-    htmlElement.setAttribute('lang', languageCode);
-  } else {
-    htmlElement.removeAttribute('lang');
-  }
-};
+import { setLanguage } from '../helpers/locale_helper';
 
 describe('locale', () => {
   afterEach(() => {