Use assert and unassertify for enforcing internal invariants

CONTRIBUTING.md

@@ -0,0 +1,15 @@
+Hi, and thanks in advance for contributing to Mapbox GL. Here's how we work. Please follow these conventions when submitting an issue or pull request.
+
+## Issues
+
+When reporting a bug, if possible please include **executable code** that demonstrates the issue you have encountered. A good way to do so is to attach a link to an example on [JSFiddle](https://jsfiddle.net/).
+
+## Code conventions
+
+* Our code conventions are mostly enforced with eslint, which will be run as part of `npm test`.
+* In internal / private methods, we check preconditions with `assert`, helping us catch mistakes within the library. For performance, these checks are removed from the production build with [unassertify](https://www.npmjs.com/package/unassertify).
+* In external / public methods, we check preconditions where appropriate and emit an error. "Emit" can mean throwing an `Error`, passing an `Error` as a first callback argument, or emitting an `error` event, as appropriate for the context. These checks remain present in production builds, helping downstream authors avoid common mistakes.
+
+## Documentation conventions
+
+See [docs/README.md](https://github.com/mapbox/mapbox-gl-js/blob/master/docs/README.md).

js/source/tile_coord.js

@@ -1,13 +1,18 @@
 'use strict';
 
+var assert = require('assert');
+
 module.exports = TileCoord;
 
 function TileCoord(z, x, y, w) {
-    if (isNaN(w)) w = 0;
+    assert(!isNaN(z));
+    assert(!isNaN(x));
+    assert(!isNaN(y));
+    assert(z >= 0);
+    assert(x >= 0);
+    assert(y >= 0);
 
-    if (isNaN(z) || isNaN(x) || isNaN(y) || z < 0 || x < 0 || y < 0) {
-        throw new Error('Invalid TileCoord object: (' + z + ', ' + x + ', ' + y + ', ' + w + ')');
-    }
+    if (isNaN(w)) w = 0;
 
     this.z = +z;
     this.x = +x;

package.json

@@ -25,6 +25,7 @@
     "rbush": "^1.4.0",
     "request": "^2.39.0",
     "resolve-url": "^0.2.1",
+    "unassertify": "^1.0.2",
     "unitbezier": "^0.0.0",
     "vector-tile": "^1.1.3",
     "webworkify": "^1.0.2"
@@ -68,7 +69,7 @@
     "test": "npm run lint && prova test/js/*/*.js",
     "test-suite": "node test/render.test.js || true",
     "build": "browserify -d js/mapbox-gl.js --standalone mapboxgl > dist/mapbox-gl-dev.js && npm run docs",
-    "production": "browserify js/mapbox-gl.js -d -p [minifyify --map mapbox-gl.js.map --output dist/mapbox-gl.js.map] --standalone mapboxgl > dist/mapbox-gl.js",
+    "production": "browserify js/mapbox-gl.js -d -t unassertify -p [minifyify --map mapbox-gl.js.map --output dist/mapbox-gl.js.map] --standalone mapboxgl > dist/mapbox-gl.js",
     "prepublish": "npm run build && npm run production",
     "cov": "istanbul cover prova test/js/*/*.js test/render.test.js -x js/lib/debugtext.js",
     "docs": "node docs/_generate/generate.js"