add Completion class

src/console/completion.js

@@ -0,0 +1,26 @@
+export default class Completion {
+  constructor(completions) {
+    if (typeof completions.length !== 'number') {
+      throw new TypeError('completions does not have a length in number');
+    }
+    this.completions = completions
+    this.index = 0;
+  }
+
+  prev() {
+    if (this.completions.length === 0) {
+      return null;
+    }
+    this.index = (this.index + this.completions.length - 1) % this.completions.length
+    return this.completions[this.index];
+  }
+
+  next() {
+    if (this.completions.length === 0) {
+      return null;
+    }
+    let item = this.completions[this.index];
+    this.index = (this.index + 1) % this.completions.length
+    return item;
+  }
+}

test/console/completion.test.js

@@ -0,0 +1,48 @@
+import { expect } from "chai";
+import Completion from '../../src/console/completion';
+
+describe('Completion class', () => {
+  describe('#constructor', () => {
+    it('creates new object by iterable items', () => {
+      new Completion([1,2,3,4,5]);
+      new Completion([]);
+      new Completion('hello');
+      new Completion('');
+    });
+
+    it('creates new object by iterable items', () => {
+      expect(() => new Completion({ key: 'value' })).to.throw(TypeError);
+      expect(() => new Completion(12345)).to.throw(TypeError);
+    });
+  });
+
+  describe('#next', () => {
+    it('complete next items', () => {
+      let completion = new Completion([3, 4, 5]);
+      expect(completion.next()).to.equal(3);
+      expect(completion.next()).to.equal(4);
+      expect(completion.next()).to.equal(5);
+      expect(completion.next()).to.equal(3);
+    });
+
+    it('returns null when empty completions', () => {
+      let completion = new Completion([]);
+      expect(completion.next()).to.be.null;
+    });
+  });
+
+  describe('#prev', () => {
+    it('complete prev items', () => {
+      let completion = new Completion([3, 4, 5]);
+      expect(completion.prev()).to.equal(5);
+      expect(completion.prev()).to.equal(4);
+      expect(completion.prev()).to.equal(3);
+      expect(completion.prev()).to.equal(5);
+    });
+
+    it('returns null when empty completions', () => {
+      let completion = new Completion([]);
+      expect(completion.prev()).to.be.null;
+    });
+  });
+});