Merge pull request #344 from rambleraptor/at-if-no-null

New rule: `at-if-no-null`

src/rules/at-if-no-null/README.md

@@ -0,0 +1,44 @@
+# at-if-no-null
+
+Check for equality to null is unnecessarily explicit since `null` is falsey in Sass.
+
+```scss
+a {
+    @if $x == null {}
+/**         ↑     ↑
+ * == or != null is unncessary */
+}
+```
+
+## Options
+
+true
+
+### `true`
+
+The following patterns are considered warnings:
+```scss
+a {
+    @if $x == null {}
+}
+```
+
+```scss
+a {
+    @if $x != null {}
+}
+```
+
+The following patterns are *not* considered warnings:
+
+```scss
+a {
+    @if $x {}
+}
+```
+
+```scss
+a {
+    @if not $x {}
+}
+```

src/rules/at-if-no-null/__tests__/index.js

@@ -0,0 +1,47 @@
+import rule, { ruleName, messages } from "..";
+
+testRule(rule, {
+  ruleName,
+  config: [true],
+  syntax: "scss",
+
+  accept: [
+    {
+      code: `a {
+      @if $x {}
+    }`,
+      description: "does not use the != null format"
+    },
+    {
+      code: `a {
+      @if not $x {}
+    }`,
+      description: "does not use the == null format"
+    },
+    {
+      code: `a {
+      @if $x != null and $x > 1 {}
+    }`,
+      description: "does not use the == null format"
+    }
+  ],
+
+  reject: [
+    {
+      code: `a {
+      @if $x == null {}
+    }`,
+      description: "uses the == null format",
+      message: messages.equals_null,
+      line: 2
+    },
+    {
+      code: `a {
+      @if $x != null {}
+    }`,
+      description: "uses the != null format",
+      message: messages.not_equals_null,
+      line: 2
+    }
+  ]
+});

src/rules/at-if-no-null/index.js

@@ -0,0 +1,49 @@
+import { namespace } from "../../utils";
+import { utils } from "stylelint";
+
+export const ruleName = namespace("at-if-no-null");
+
+export const messages = utils.ruleMessages(ruleName, {
+  equals_null: "Expected @if not statement rather than @if statement == null",
+  not_equals_null: "Expected @if statement rather than @if statement != null"
+});
+
+export default function(expectation) {
+  return (root, result) => {
+    const validOptions = utils.validateOptions(result, ruleName, {
+      actual: expectation
+    });
+
+    if (!validOptions) {
+      return;
+    }
+
+    root.walkAtRules(atrule => {
+      // Do nothing if it's not an @if
+      if (atrule.name !== "if") {
+        return;
+      }
+
+      // If rule != null and (expr), skip
+      if (atrule.params.match(/\(?[ \t]*.* != null and .*\)?/)) {
+        return;
+      }
+
+      if (atrule.params.match(/\(?[ \t]*.* == null[ \t]*\)?/)) {
+        utils.report({
+          message: messages.equals_null,
+          node: atrule,
+          result,
+          ruleName
+        });
+      } else if (atrule.params.match(/\(?[ \t]*.* != null[ \t]*\)?/)) {
+        utils.report({
+          message: messages.not_equals_null,
+          node: atrule,
+          result,
+          ruleName
+        });
+      }
+    });
+  };
+}

src/rules/index.js

@@ -8,6 +8,7 @@ import atFunctionParenthesesSpaceBefore from "./at-function-parentheses-space-be
 import atFunctionPattern from "./at-function-pattern";
 import atIfClosingBraceNewlineAfter from "./at-if-closing-brace-newline-after";
 import atIfClosingBraceSpaceAfter from "./at-if-closing-brace-space-after";
+import atIfNoNull from "./at-if-no-null";
 import atImportNoPartialLeadingUnderscore from "./at-import-no-partial-leading-underscore";
 import atImportPartialExtensionBlacklist from "./at-import-partial-extension-blacklist";
 import atImportPartialExtensionWhitelist from "./at-import-partial-extension-whitelist";
@@ -57,6 +58,7 @@ export default {
   "at-function-pattern": atFunctionPattern,
   "at-if-closing-brace-newline-after": atIfClosingBraceNewlineAfter,
   "at-if-closing-brace-space-after": atIfClosingBraceSpaceAfter,
+  "at-if-no-null": atIfNoNull,
   "at-import-no-partial-leading-underscore": atImportNoPartialLeadingUnderscore,
   "at-import-partial-extension-blacklist": atImportPartialExtensionBlacklist,
   "at-import-partial-extension-whitelist": atImportPartialExtensionWhitelist,