Merge pull request #8 from bem-sdk/issue-2

Id field impl

README.md

@@ -43,6 +43,7 @@ const cell = new BemCell({
 cell.entity; // ➜ BemEntityName { block: 'button', elem: 'text' }
 cell.tech;   // css
 cell.layer;  // common
+cell.id;     // button__text@common.css
 ```
 
 License

index.js

@@ -72,4 +72,36 @@ module.exports = class BemCell {
      * @returns {string} layer of cell.
      */
     get layer() { return this._layer; }
+
+    /**
+     * Returns the identifier of this cell.
+     *
+     * Important: should only be used to determine uniqueness of cell.
+     *
+     * @example
+     * const BemCell = require('@bem/cell');
+     * const BemEntityName = require('@bem/entity-name');
+     *
+     * const cell = new BemCell({
+     *     entity: new BemEntityName({ block: 'button', elem: 'text' }),
+     *     tech: 'css',
+     *     layer: 'desktop'
+     * });
+     *
+     * cell.id; // ➜ "button__text@desktop.css"
+     *
+     * @returns {string} identifier of cell.
+     */
+    get id() {
+        if (this._id) {
+            return this._id;
+        }
+
+        const layer = this._layer ? `@${this._layer}` : '';
+        const tech = this._tech ? `.${this._tech}` : '';
+
+        this._id = `${this._entity}${layer}${tech}`;
+
+        return this._id;
+    }
 };

test/id.test.js

@@ -0,0 +1,54 @@
+const test = require('ava');
+const BemEntityName = require('@bem/entity-name');
+
+const BemCell = require('../index');
+
+test('should provide `id` field', t => {
+    const cell = new BemCell({
+        entity: new BemEntityName({ block: 'block' }),
+        layer: 'desktop',
+        tech: 'css'
+    });
+
+    t.is(cell.id, 'block@desktop.css');
+});
+
+test('should provide `id` field for cell with entity `field` only', t => {
+    const cell = new BemCell({
+        entity: new BemEntityName({ block: 'block' })
+    });
+
+    t.is(cell.id, 'block');
+});
+
+test('should provide `id` field for cell with `tech` field', t => {
+    const cell = new BemCell({
+        entity: new BemEntityName({ block: 'block' }),
+        tech: 'css'
+    });
+
+    t.is(cell.id, 'block.css');
+});
+
+test('should provide `id` field for cell with `layer` field', t => {
+    const cell = new BemCell({
+        entity: new BemEntityName({ block: 'block' }),
+        layer: 'desktop',
+    });
+
+    t.is(cell.id, 'block@desktop');
+});
+
+test('should cache `id` field', t => {
+    const cell = new BemCell({
+        entity: new BemEntityName({ block: 'block' }),
+        layer: 'desktop',
+        tech: 'css'
+    });
+    const id = cell.id;
+
+    cell._tech = 'js';
+    cell._layer = 'common';
+
+    t.is(cell.id, id);
+});