Add factPriority to almanac

This brings getting the priority of a fact into line with getting the
value of a fact.

Author: Christopher Pardy

src/almanac.js

@@ -163,6 +163,18 @@ export default class Almanac {
     return factValuePromise
   }
 
+  /**
+   * Returns the priority of the fact if one exists.
+   * @param {string} factId - fact identifier
+   */
+  factPriority (factId) {
+    const fact = this._getFact(factId)
+    if (fact === undefined) {
+      return undefined
+    }
+    return fact.priority
+  }
+
   /**
    * Interprets value as either a primitive, or if a fact, retrieves the fact value
    */

test/almanac.test.js

@@ -1,6 +1,7 @@
 import { Fact } from '../src/index'
 import Almanac from '../src/almanac'
 import sinon from 'sinon'
+import { expect } from 'chai'
 
 describe('Almanac', () => {
   let almanac
@@ -189,4 +190,22 @@ describe('Almanac', () => {
       })
     })
   })
+
+  describe('factPriority()', () => {
+    function setup (priority) {
+      const fact = new Fact('foo', 5, { priority })
+      const factMap = new Map([[fact.id, fact]])
+      return new Almanac(factMap)
+    }
+
+    it('returns the priority if the fact exists', () => {
+      const almanac = setup(3)
+      expect(almanac.factPriority('foo')).to.equal(3)
+    })
+
+    it("returns undefined if the fact doesn't exist", () => {
+      const almanac = setup(6)
+      expect(almanac.factPriority('bar')).to.equal(undefined)
+    })
+  })
 })