chore(TypeScript): refactor objects into classes

fix: `Nil`(null, undefined) now renders as empty string
change: `parser.parseValue()` renamed to `parser.parseOutput`
change: registered tags/filters become static and shared across different liquid instances

test/unit/liquid.ts

@@ -0,0 +1,57 @@
+import Liquid from '../../src/liquid'
+import * as mock from 'mock-fs'
+import * as chai from 'chai'
+
+const expect = chai.expect
+
+describe('Liquid', function () {
+  describe('#constructor()', function () {
+    it('should throw on illegal root', function () {
+      expect(() => new (Liquid as any)({root: {}})).to.throw(/illegal root/)
+    })
+  })
+  describe('#plugin()', function () {
+    it('should call plugin on the instance', async function () {
+      const engine = new Liquid()
+      engine.plugin(function (Liquid) {
+        this.registerFilter('foo', x => `foo${x}foo`)
+      })
+      const html = await engine.parseAndRender('{{"bar"|foo}}')
+      expect(html).to.equal('foobarfoo')
+    })
+    it('should call plugin with Liquid', async function () {
+      const engine = new Liquid()
+      engine.plugin(function (Liquid) {
+        this.registerFilter('t', x => Liquid.isFalsy(x))
+      })
+      const html = await engine.parseAndRender('{{false|t}}')
+      expect(html).to.equal('true')
+    })
+  })
+  describe('#express()', function () {
+    const liquid = new Liquid({ root: '/root' })
+    const render = liquid.express()
+    before(function () {
+      mock({
+        '/root/foo': 'foo'
+      })
+    })
+    after(function () {
+      mock.restore()
+    })
+    it('should render single template', function (done) {
+      render.call({ root: '.' }, 'foo', null, (err, result) => {
+        if (err) return done(err)
+        expect(result).to.equal('foo')
+        done()
+      })
+    })
+    it('should render single template with Array-typed root', function (done) {
+      render.call({ root: ['.'] }, 'foo', null, (err, result) => {
+        if (err) return done(err)
+        expect(result).to.equal('foo')
+        done()
+      })
+    })
+  })
+})