Add conditional preservation behavior

README.md

@@ -355,6 +355,7 @@ Possible values:
  - `false`: Removes `--var` declarations and replaces `var()` with their resolved/computed values.
  - `true`: Keeps `var()` declarations in the output and has the computed value as a fallback declaration. Also keeps computed `--var` declarations.
  - `'computed'`: Keeps computed `--var` declarations in the output. Handy to make them available to your JavaScript.
+ - `(declaration) => boolean|'computed'` : Handles preservation behavior based on the respective declaration.
 
 ### `variables` (default: `{}`)
 

index.js

@@ -183,12 +183,18 @@ module.exports = postcss.plugin('postcss-css-variables', function(options) {
 				});
 			});
 
+			let preserveDecl;
+			if (typeof opts.preserve === "function") {
+				preserveDecl = opts.preserve(decl);
+			} else {
+				preserveDecl = opts.preserve;
+			}
 			// Remove the variable declaration because they are pretty much useless after we resolve them
-			if(!opts.preserve) {
+			if(!preserveDecl) {
 				decl.remove();
 			}
 			// Or we can also just show the computed value used for that variable
-			else if(opts.preserve === 'computed') {
+			else if(preserveDecl === 'computed') {
 				decl.value = valueResults.value;
 			}
 			// Otherwise just keep them as var declarations

lib/resolve-decl.js

@@ -72,7 +72,7 @@ function eachMapItemDependencyOfDecl(variablesUsedList, map, decl, cb) {
 // Resolve the decl with the computed value
 // Also add in any media queries that change the value as necessary
 function resolveDecl(decl, map, /*optional*/shouldPreserve, /*optional*/preserveAtRulesOrder, /*optional*/logResolveValueResult) {
-	shouldPreserve = shouldPreserve || false;
+	shouldPreserve = (typeof shouldPreserve === "function" ? shouldPreserve(decl) : shouldPreserve) || false;
 	preserveAtRulesOrder = preserveAtRulesOrder || false;
 
 	// Make it chainable
@@ -101,7 +101,13 @@ function resolveDecl(decl, map, /*optional*/shouldPreserve, /*optional*/preserve
 		// Add the declaration to our new rule
 		ruleClone.append(declClone);
 
-		if(shouldPreserve === true) {
+		let preserveVariable;
+		if(typeof shouldPreserve === "function") {
+			preserveVariable = shouldPreserve(decl);
+		} else {
+			preserveVariable = shouldPreserve;
+		}
+		if(preserveVariable === true) {
 			declClone.cloneAfter();
 		}
 

test/fixtures/preserve-variables-conditionally.css

@@ -0,0 +1,21 @@
+:root {
+	--color-one: #0000ff;
+	--color-two: #00ff00;
+	--color-three: var(--color-two);
+}
+
+.before {
+	prop: before;
+	color: var(--color-one);
+}
+
+.after {
+	color: var(--color-two);
+	prop: after;
+}
+
+.before-and-after {
+	prop: before;
+	color: var(--missing, #ff0000);
+	otherprop: after;
+}

test/fixtures/preserve-variables-conditionally.expected.css

@@ -0,0 +1,22 @@
+:root {
+	--color-two: #00ff00;
+	--color-three: var(--color-two);
+}
+
+.before {
+	prop: before;
+	color: #0000ff;
+}
+
+.after {
+	color: #00ff00;
+	color: var(--color-two);
+	prop: after;
+}
+
+.before-and-after {
+	prop: before;
+	color: #ff0000;
+	color: var(--missing, #ff0000);
+	otherprop: after;
+}

test/test.js

@@ -227,6 +227,19 @@ describe('postcss-css-variables', function() {
 			'preserve-computed',
 			{ preserve: 'computed' }
 		);
+
+		test(
+			'preserves variables when `preserve` function applies',
+			'preserve-variables-conditionally',
+			{
+				preserve: function (declaration) {
+					return !(
+						declaration.value.includes("--color-one")
+						|| declaration.prop.includes("--color-one")
+					)
+				}
+			}
+		);
 	});