Added: strict option

Close #14

README.md

@@ -55,6 +55,12 @@ Checkout [tests](test) for more.
 
 ### Options
 
+#### `strict` (default: `true`)
+
+Per specifications, all fallbacks should be added since we can't verify if a
+computed value is valid or not.
+This option allows you to avoid adding too many fallback values in your CSS.
+
 #### `preserve` (default: `false`)
 
 Allows you to preserve custom properties & var() usage in output.

index.js

@@ -128,6 +128,7 @@ module.exports = function(options) {
         delete variables[name]
       }
     })
+    var strict = options.strict === undefined ? true : options.strict
     var appendVariables = options.appendVariables
     var preserve = options.preserve
     var map = {}
@@ -214,7 +215,11 @@ module.exports = function(options) {
       }
 
       helpers.try(function resolve() {
-        resolveValue(value, map, decl.source).forEach(function(resolvedValue) {
+        var resolved = resolveValue(value, map, decl.source)
+        if (!strict) {
+          resolved = [resolved.pop()]
+        }
+        resolved.forEach(function(resolvedValue) {
           var clone = decl.cloneBefore()
           clone.value = resolvedValue
         })

test/fixtures/substitution-strict.css

@@ -0,0 +1,10 @@
+:root {
+  --a: "a";
+  --b: var(--bUndef, bFallback);
+}
+
+div {
+  aProp: var(--a, aPropFallback);
+  bProp: var(--b, bPropFallback);
+  bProp: var(--cUndef, cPropFallback);
+}

test/fixtures/substitution-strict.expected.css

@@ -0,0 +1,5 @@
+div {
+  aProp: "a";
+  bProp: bFallback;
+  bProp: cPropFallback;
+}

test/index.js

@@ -136,3 +136,11 @@ test("append variables", function(t) {
   })
   t.end()
 })
+
+test("strict option", function(t) {
+  compareFixtures(t, "substitution-strict", {
+    strict: false,
+  })
+
+  t.end()
+})