test(proj): add tests and docs for proj module

Closes fgpv-vpgf#12

spec/projSpec.js

@@ -3,22 +3,50 @@
 const projBuilder = require('../src/proj.js');
 const proj4 = require('proj4');
 
+// describe declares a test suite
+// takes a suite name and a function, all tests within the suite are declared within the function
+// besides grouping tests together suites can also do a setup / teardown for each test or for the whole suite
 describe('Local projection', () => {
+
+    // make up some data for testing
     let fakeEsri = { EsriExtent: {} };
-    const sampleExtent = {
-        "xmin":-122.68,"ymin":45.53,"xmax":-122.45,"ymax":45.6,
-        "spatialReference":{"wkid":4326}
-    };
+    const sampleData = {x0:-95,y0:49,x1:-94.5,y1:49.5,sr:4326};
+    const sampleExtent = makeFakeEsriExtent(sampleData);
+    let proj;
+
+    function makeFakeEsriExtent(o) {
+        return {
+            "xmin":o.x0,"ymin":o.y0,"xmax":o.x1,"ymax":o.y1,
+            "spatialReference":{"wkid":o.sr}
+        };
+    }
 
     proj4.defs("EPSG:3978", "+proj=lcc +lat_1=49 +lat_2=77 +lat_0=49 +lon_0=-95 +x_0=0 +y_0=0 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs");
     proj4.defs("EPSG:3979", "+proj=lcc +lat_1=49 +lat_2=77 +lat_0=49 +lon_0=-95 +x_0=0 +y_0=0 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs");
     proj4.defs("EPSG:102100", proj4.defs('EPSG:3857'));
     proj4.defs("EPSG:54004", "+proj=merc +lon_0=0 +k=1 +x_0=0 +y_0=0 +ellps=WGS84 +datum=WGS84 +units=m +no_defs");
 
-    it('should reproject extents', () => {
-        const proj = projBuilder(fakeEsri);
+    // setup function running before each test
+    beforeEach(() => {
+        proj = projBuilder(fakeEsri);
+    });
+
+    // each test is also a name and a function
+    // tests use expect to generate testing assertions
+    // see the jasmine page for a full list of built in tests
+    it('should use the target WKID when reprojecting', () => {
         let res = proj.localProjectExtent(sampleExtent, 3978);
         expect(res.sr).toEqual(3978);
     });
 
+    it('should reproject from A -> B -> A without deviation within the same projection type', () => {
+        let res = proj.localProjectExtent(sampleExtent, 54004);
+        res = proj.localProjectExtent(makeFakeEsriExtent(res),4326);
+        'x0 x1 y0 y1'.split(' ').forEach(x => expect(res[x]).toBeCloseTo(sampleData[x],5));
+    });
+
+    it('should not fail silently if a projection is not set', () => {
+        expect(() => proj.localProjectExtent(sampleExtent, 111111)).toThrow();
+    });
+
 });