fix: do not break calc with single var (#69)

src/__tests__/index.js

@@ -583,3 +583,24 @@ test(
   'calc(unknown(#fff) * other-unknown(200px))',
   'calc(unknown(#fff)*other-unknown(200px))',
 );
+
+test(
+  'should not strip calc with single CSS custom variable',
+  testValue,
+  'calc(var(--foo))',
+  'calc(var(--foo))',
+);
+
+test(
+  'should strip unnecessary calc with single CSS custom variable',
+  testValue,
+  'calc(calc(var(--foo)))',
+  'calc(var(--foo))',
+);
+
+test(
+  'should not strip calc with single CSS custom variables and value',
+  testValue,
+  'calc(var(--foo) + 10px)',
+  'calc(var(--foo) + 10px)',
+);

src/lib/stringifier.js

@@ -1,69 +1,73 @@
-const order = {
-  "*": 0,
-  "/": 0,
-  "+": 1,
-  "-": 1,
-};
-
-function round(value, prec) {
-  if (prec !== false) {
-    const precision = Math.pow(10, prec);
-    return Math.round(value * precision) / precision;
-  }
-  return value;
-}
-
-function stringify(node, prec) {
-  switch (node.type) {
-    case "MathExpression": {
-      const {left, right, operator: op} = node;
-      let str = "";
-
-      if (left.type === 'MathExpression' && order[op] < order[left.operator])
-        str += `(${stringify(left, prec)})`;
-      else
-        str += stringify(left, prec);
-
-      str += order[op] ? ` ${node.operator} ` : node.operator;
-
-      if (right.type === 'MathExpression' && order[op] < order[right.operator])
-        str += `(${stringify(right, prec)})`;
-      else
-        str += stringify(right, prec);
-
-      return str;
-    }
-    case "Value":
-      return round(node.value, prec);
-    case 'Function':
-      return node.value;
-    default:
-      return round(node.value, prec) + node.unit;
-  }
-}
-
-export default function (
-    calc,
-    node,
-    originalValue,
-    options,
-    result,
-    item
-  ) {
-  let str = stringify(node, options.precision);
-
-  if (node.type === "MathExpression") {
-    // if calc expression couldn't be resolved to a single value, re-wrap it as
-    // a calc()
-    str = `${calc}(${str})`;
-
-    // if the warnWhenCannotResolve option is on, inform the user that the calc
-    // expression could not be resolved to a single value
-    if (options.warnWhenCannotResolve) {
-      result.warn(
-        "Could not reduce expression: " + originalValue,
-        { plugin: 'postcss-calc', node: item });
-    }
-  }
-  return str;
-}
+const order = {
+  "*": 0,
+  "/": 0,
+  "+": 1,
+  "-": 1,
+};
+
+function round(value, prec) {
+  if (prec !== false) {
+    const precision = Math.pow(10, prec);
+    return Math.round(value * precision) / precision;
+  }
+  return value;
+}
+
+function stringify(node, prec) {
+  switch (node.type) {
+    case "MathExpression": {
+      const {left, right, operator: op} = node;
+      let str = "";
+
+      if (left.type === 'MathExpression' && order[op] < order[left.operator])
+        str += `(${stringify(left, prec)})`;
+      else
+        str += stringify(left, prec);
+
+      str += order[op] ? ` ${node.operator} ` : node.operator;
+
+      if (right.type === 'MathExpression' && order[op] < order[right.operator])
+        str += `(${stringify(right, prec)})`;
+      else
+        str += stringify(right, prec);
+
+      return str;
+    }
+    case "Value":
+      return round(node.value, prec);
+    case 'Function':
+      return node.value;
+    default:
+      return round(node.value, prec) + node.unit;
+  }
+}
+
+export default function (
+    calc,
+    node,
+    originalValue,
+    options,
+    result,
+    item
+  ) {
+  let str = stringify(node, options.precision);
+
+  const shouldPrintCalc =
+    node.type === "MathExpression" ||
+    (node.type === "Function" && node.value.toLowerCase().slice(0, 4) === "var(");
+
+  if (shouldPrintCalc) {
+    // if calc expression couldn't be resolved to a single value, re-wrap it as
+    // a calc()
+    str = `${calc}(${str})`;
+
+    // if the warnWhenCannotResolve option is on, inform the user that the calc
+    // expression could not be resolved to a single value
+    if (options.warnWhenCannotResolve) {
+      result.warn(
+        "Could not reduce expression: " + originalValue,
+        { plugin: 'postcss-calc', node: item });
+    }
+  }
+  return str;
+}

src/lib/transform.js

@@ -12,8 +12,9 @@ const MATCH_CALC = /((?:-(moz|webkit)-)?calc)/i;
 function transformValue(value, options, result, item) {
   return valueParser(value).walk(node => {
     // skip anything which isn't a calc() function
-    if (node.type !== 'function' || !MATCH_CALC.test(node.value))
+    if (node.type !== 'function' || !MATCH_CALC.test(node.value)) {
       return node;
+    }
 
     // stringify calc expression and produce an AST
     const contents = valueParser.stringify(node.nodes);