Add opts.preserveInjectedVariables (#74)

Adds `opts.preserveInjectedVariables`, which when set to false, removes custom property declarations added via `opts.variables`.

Author: akdetrick

README.md

@@ -368,6 +368,13 @@ Can be a simple key-value pair or an object with a `value` property and an optio
 
 The object keys are automatically prefixed with `--` (according to CSS custom property syntax) if you do not provide it.
 
+### `preserveInjectedVariables` (default: `true`)
+
+Whether to preserve the custom property declarations inserted via the `variables` option from final output.
+
+A typical use case is [CSS Modules](https://github.com/css-modules/css-modules), where you would want to avoid
+repeating custom property definitions in every module passed through this plugin. Setting this option to `false`
+prevents JS-injected variables from appearing in output CSS.
 
 ```js
 var postcss = require('postcss');

index.js

@@ -62,7 +62,10 @@ var defaults = {
 	// Define variables via JS
 	// Simple key-value pair
 	// or an object with a `value` property and an optional `isImportant` bool property
-	variables: {}
+	variables: {},
+	// Preserve variables injected via JS with the `variables` option above
+	// before serializing to CSS (`false` will remove these variables from output)
+	preserveInjectedVariables: true
 };
 
 module.exports = postcss.plugin('postcss-css-variables', function(options) {
@@ -82,6 +85,10 @@ module.exports = postcss.plugin('postcss-css-variables', function(options) {
 		// We use this because we don't want to modify the AST when we still need to reference these later on
 		var nodesToRemoveAtEnd = [];
 
+		// Keep track of the injected from `opts.variables` to remove at the end
+		// if user passes `opts.preserveInjectedVariables = false`
+		var injectedDeclsToRemoveAtEnd = [];
+
 		// Map of variable names to a list of declarations
 		var map = {};
 
@@ -106,6 +113,11 @@ module.exports = postcss.plugin('postcss-css-variables', function(options) {
 				});
 				variableRootRule.append(varDecl);
 
+				// Collect JS-injected variables for removal if `opts.preserveInjectedVariables = false`
+				if (!opts.preserveInjectedVariables) {
+					injectedDeclsToRemoveAtEnd.push(varDecl);
+				}
+
 				// Add the entry to the map
 				prevVariableMap[variableName] = (prevVariableMap[variableName] || []).concat({
 					decl: varDecl,
@@ -249,6 +261,11 @@ module.exports = postcss.plugin('postcss-css-variables', function(options) {
 		// We clean up at the end because we don't want to modify the AST when we still need to reference these later on
 		nodesToRemoveAtEnd.forEach(cleanUpNode);
 
+		// Clean up JS-injected variables marked for removal
+		injectedDeclsToRemoveAtEnd.forEach(function(injectedDecl) {
+			injectedDecl.remove();
+		});
+
 
 		//console.log('map', map);
 

test/fixtures/js-defined-preserve-injected.css

@@ -0,0 +1,5 @@
+.box1 {
+	width: var(--js-defined1);
+	height: var(--js-defined2);
+	background: var(--js-defined-no-prefix);
+}

test/fixtures/js-defined-preserve-injected.expected.css

@@ -0,0 +1,8 @@
+.box1 {
+	width: 75px;
+	width: var(--js-defined1);
+	height: 80px;
+	height: var(--js-defined2);
+	background: #ff0000;
+	background: var(--js-defined-no-prefix);
+}

test/fixtures/js-defined-preserve.css

@@ -0,0 +1,5 @@
+.box1 {
+	width: var(--js-defined1);
+	height: var(--js-defined2);
+	background: var(--js-defined-no-prefix);
+}

test/fixtures/js-defined-preserve.expected.css

@@ -0,0 +1,18 @@
+:root {
+	--js-defined-no-prefix: #ff0000;
+}
+:root {
+	--js-defined2: 80px;
+}
+:root {
+	--js-defined1: 75px;
+}
+
+.box1 {
+	width: 75px;
+	width: var(--js-defined1);
+	height: 80px;
+	height: var(--js-defined2);
+	background: #ff0000;
+	background: var(--js-defined-no-prefix);
+}

test/test.js

@@ -12,6 +12,14 @@ var cssvariables = require('../');
 
 var CleanCSS = require('clean-css');
 
+var MOCK_JS_VARIABLES =  {
+	'--js-defined1': '75px',
+	'--js-defined2': {
+		value: '80px'
+	},
+	// Should be automatically prefixed with `--`
+	'js-defined-no-prefix': '#ff0000'
+};
 
 var testPlugin = function(filePath, expectedFilePath, options) {
 	options = options || {};
@@ -146,20 +154,27 @@ describe('postcss-css-variables', function() {
 		test(
 			'should work with JS defined variables',
 			'js-defined',
+			{ variables: MOCK_JS_VARIABLES }
+		);
+		test(
+			'should preserve -- declarations and var() values with `options.variables` AND `options.preserve`',
+			'js-defined-preserve',
+			{
+				variables: MOCK_JS_VARIABLES,
+				preserve: true
+			}
+		);
+		test(
+			'should preserve var() values and clean injected declarations with `options.variables` AND `options.preserve` AND `options.preserveInjectedVariables: false`',
+			'js-defined-preserve-injected',
 			{
-				variables: {
-					'--js-defined1': '75px',
-					'--js-defined2': {
-						value: '80px'
-					},
-					// Should be automatically prefixed with `--`
-					'js-defined-no-prefix': '#ff0000'
-				}
+				variables: MOCK_JS_VARIABLES,
+				preserve: true,
+				preserveInjectedVariables: false,
 			}
 		);
 	});
 
-
 	describe('with `options.preserve`', function() {
 		test(
 			'preserves variables when `preserve` is `true`',