Adding Simple URL-based feature flags (#117)

* Fixes #66: As a developer, I want to limit the audience
that sees new features, so that we can control
the message and positioning of our tool.
Implements simple feature flagging via URL parameters.
Provide "?flags=x,y,z" to enable flags x, y, and z.
* Fixing type to use Location instead of URL
* Updating README with info on how to use feature flags

client/src/components/J40Header.tsx

@@ -4,13 +4,21 @@ import {GovBanner,
   Title,
   PrimaryNav,
   SiteAlert} from '@trussworks/react-uswds';
-import {useIntl} from 'gatsby-plugin-intl';
+import {useIntl, Link} from 'gatsby-plugin-intl';
 import {Helmet} from 'react-helmet';
-const headerLinks = [
-  <></>,
-];
+import {useFlags} from '../contexts/FlagContext';
+
+const headerLinks = (flags: string[] | undefined) => {
+  const timelineLink = <Link key="/timeline" to="/timeline"> Timeline </Link>;
+  const links = [];
+  if (flags && flags.includes('timeline')) {
+    links.push(timelineLink);
+  }
+  return links;
+};
 
 const J40Header = () => {
+  const flags = useFlags();
   const intl = useIntl();
   const title = intl.formatMessage({
     id: '71L0pp',
@@ -39,7 +47,7 @@ const J40Header = () => {
           <div className="usa-navbar">
             <Title className={'j40-title'}>{title}</Title>
           </div>
-          <PrimaryNav items={headerLinks}/>
+          <PrimaryNav items={headerLinks(flags)} />
         </div>
       </Header>
     </>

client/src/components/layout.tsx

@@ -2,17 +2,22 @@ import React, {ReactNode} from 'react';
 import * as styles from './layout.module.scss';
 import J40Header from './J40Header';
 import J40Footer from './J40Footer';
+import {URLFlagProvider} from '../contexts/FlagContext';
+
 interface ILayoutProps {
-  children: ReactNode
+  children: ReactNode,
+  location: Location
 }
 
-const Layout = ({children}: ILayoutProps) => {
+const Layout = ({children, location}: ILayoutProps) => {
   return (
-    <div className={styles.site}>
-      <J40Header />
-      <div className={styles.siteContent}>{children}</div>
-      <J40Footer />
-    </div>
+    <URLFlagProvider location={location}>
+      <div className={styles.site}>
+        <J40Header />
+        <div className={styles.siteContent}>{children}</div>
+        <J40Footer />
+      </div>
+    </URLFlagProvider>
   );
 };
 

client/src/contexts/FlagContext.test.tsx

@@ -0,0 +1,39 @@
+import * as React from 'react';
+import {render, screen} from '@testing-library/react';
+import {URLFlagProvider, useFlags} from './FlagContext';
+
+describe('URL params are parsed and passed to children', () => {
+  describe('when the URL has a "flags" parameter set', () => {
+    // We artificially set the URL to localhost?flags=1,2,3
+    beforeEach(() => {
+      window.history.pushState({}, 'Test Title', '/?flags=1,2,3');
+    });
+    describe('when using useFlags', () => {
+      beforeEach(() => {
+        const FlagConsumer = () => {
+          const flags = useFlags();
+          return (
+            <>
+              <div>{flags.includes('1') ? 'yes1' : 'no1'}</div>
+              <div>{flags.includes('2') ? 'yes2' : 'no2'}</div>
+              <div>{flags.includes('3') ? 'yes3' : 'no3'}</div>
+              <div>{flags.includes('4') ? 'yes4' : 'no4'}</div>
+            </>
+          );
+        };
+        render(
+            <URLFlagProvider location={location}>
+              <FlagConsumer />
+            </URLFlagProvider>,
+        );
+      });
+
+      it('gives child components the flag values', async () => {
+        expect(screen.queryByText('yes1')).toBeInTheDocument();
+        expect(screen.queryByText('yes2')).toBeInTheDocument();
+        expect(screen.queryByText('yes3')).toBeInTheDocument();
+        expect(screen.queryByText('yes4')).not.toBeInTheDocument();
+      });
+    });
+  });
+});

client/src/contexts/FlagContext.tsx

@@ -0,0 +1,56 @@
+import * as React from 'react';
+import * as queryString from 'query-string';
+
+/**
+ * FlagContext stores feature flags and passes them to consumers
+ */
+ interface IFlagContext {
+  /**
+   * Contains a list of all currently-active flags
+   */
+  flags: string[];
+}
+
+const FlagContext = React.createContext<IFlagContext>({flags: []});
+
+/**
+ * `useFlags` returns all feature flags.
+ *
+ * @return {Flags[]} flags All project feature flags
+ */
+const useFlags = () : string[] => {
+  const {flags} = React.useContext(FlagContext);
+  return flags;
+};
+
+interface IURLFlagProviderProps {
+  children: React.ReactNode,
+  location: Location
+}
+
+/**
+ * `URLFlagProvider` is a provider for FlagContext.
+ *   It is passed the current URL and parses the
+ *   "flags" parameter, assumed to be a comma-separated
+ *   list of currently-active flags.
+ * @param {URL} location : the current URL object
+ * @param {ReactNode} children : the children components
+ * @return {ReactNode} URLFlagProvider component
+ **/
+const URLFlagProvider = ({children, location}: IURLFlagProviderProps) => {
+  const flagString = queryString.parse(location.search).flags;
+  let flags: string[] = [];
+  if (flagString && typeof flagString === 'string') {
+    flags = (flagString as string).split(',');
+  }
+  console.log(JSON.stringify(location), JSON.stringify(flags));
+
+  return (
+    <FlagContext.Provider
+      value={{flags}}>
+      {children}
+    </FlagContext.Provider>
+  );
+};
+
+export {FlagContext, URLFlagProvider, useFlags};

client/src/pages/index.tsx

@@ -20,9 +20,12 @@ import pollutionIcon // @ts-ignore
 import washIcon from '/node_modules/uswds/dist/img/usa-icons/wash.svg';
 import J40Aside from '../components/J40Aside';
 
+interface IndexPageProps {
+  location: Location;
+};
 
 // markup
-const IndexPage = () => {
+const IndexPage = ({location}: IndexPageProps) => {
   const readMoreList: (any | string)[][] = [
     [ecoIcon, 'Clean energy and energy efficiency'],
     [busIcon, 'Clean transit'],
@@ -33,7 +36,7 @@ const IndexPage = () => {
   ];
 
   return (
-    <Layout>
+    <Layout location={location}>
       <main id="main-content" role="main">
         <div className={'grid-row grid-gap-2'}>
           <section className={'grid-container usa-section'}>

client/src/pages/timeline.tsx

@@ -5,8 +5,12 @@ import J40Aside from '../components/J40Aside';
 // @ts-ignore
 import renewIcon from '/node_modules/uswds/dist/img/usa-icons/autorenew.svg';
 
-const TimelinePage = () => {
-  return (<Layout>
+interface TimelinePageProps {
+  location: URL;
+};
+
+const TimelinePage = ({location}: TimelinePageProps) => {
+  return (<Layout location={location}>
     <main id="main-content" role="main">
       <div className={'grid-row grid-gap-2'}>
         <section className={'grid-container usa-section'}>

client/src/styles/global.scss

@@ -59,7 +59,7 @@
 
 .j40-address-readability {
   display: inline-block;
-  line-height: 1.5 !important;  // trussworks issue
+  line-height: 1.5 !important; // trussworks issue
   text-align: center;
 }
 
@@ -68,8 +68,8 @@
   &::before {
     color: white;
     background-color: #00a91c;
-    content: '✓'
+    content: "✓";
   }
 
-  border-left-color: #005ea2 !important;  // todo: fix
+  border-left-color: #005ea2 !important; // todo: fix
 }