jsx-max-props-per-line: Add when option

If `when` is set to `multiline`, then this rule skips checking entirely
if the jsx opening tag is a single line.

Defaults `when` to `always` which is the current behavior.

Author: kentor

docs/rules/jsx-max-props-per-line.md

@@ -35,7 +35,7 @@ The following patterns are not considered warnings:
 
 ```js
 ...
-"jsx-max-props-per-line": [<enabled>, { "maximum": <number> }]
+"jsx-max-props-per-line": [<enabled>, { "maximum": <number>, "when": <string> }]
 ...
 ```
 
@@ -60,6 +60,28 @@ The following patterns are not considered warnings:
 />;
 ```
 
+### `when`
+
+Possible values:
+- `always` (default) - Always check for max props per line.
+- `multiline` - Only check for max props per line when jsx tag spans multiple lines.
+
+The following patterns are considered warnings:
+```jsx
+// [1, {when: always}]
+<Hello firstName="John" lastName="Smith" />
+```
+
+The following patterns are not considered warnings:
+```jsx
+// [1, {when: multiline}]
+<Hello firstName="John" lastName="Smith" />
+<Hello 
+  firstName="John" 
+  lastName="Smith" 
+/>
+```
+
 ## When not to use
 
 If you are not using JSX then you can disable this rule.

lib/rules/jsx-max-props-per-line.js

@@ -23,6 +23,10 @@ module.exports = {
         maximum: {
           type: 'integer',
           minimum: 1
+        },
+        when: {
+          type: 'string',
+          enum: ['always', 'multiline']
         }
       }
     }]
@@ -33,6 +37,7 @@ module.exports = {
     var sourceCode = context.getSourceCode();
     var configuration = context.options[0] || {};
     var maximum = configuration.maximum || 1;
+    var when = configuration.when || 'always';
 
     function getPropName(propNode) {
       if (propNode.type === 'JSXSpreadAttribute') {
@@ -47,6 +52,10 @@ module.exports = {
           return;
         }
 
+        if (when === 'multiline' && node.loc.start.line === node.loc.end.line) {
+          return;
+        }
+
         var firstProp = node.attributes[0];
         var linePartitionedProps = [[firstProp]];
 

tests/lib/rules/jsx-max-props-per-line.js

@@ -25,12 +25,23 @@ var parserOptions = {
 var ruleTester = new RuleTester();
 ruleTester.run('jsx-max-props-per-line', rule, {
   valid: [{
+    code: '<App />',
+    parserOptions: parserOptions
+  }, {
     code: '<App foo />',
     parserOptions: parserOptions
   }, {
     code: '<App foo bar />',
     options: [{maximum: 2}],
     parserOptions: parserOptions
+  }, {
+    code: '<App foo bar />',
+    options: [{when: 'multiline'}],
+    parserOptions: parserOptions
+  }, {
+    code: '<App foo bar baz />',
+    options: [{maximum: 2, when: 'multiline'}],
+    parserOptions: parserOptions
   }, {
     code: '<App {...this.props} bar />',
     options: [{maximum: 2}],