Add globals cli flag (ref eslint#692)

Adds a --globals cli flag to define global variables. Takes a comma-
separated list of variable names to define. By default, all varibles
are read-only, but appending `:true` to the name will mark it as
writable.

Example:

    eslint --globals require,exports:true file.js

docs/command-line-interface/README.md

@@ -28,6 +28,7 @@ Options:
   -f, --format String        use a specific output format - default: stylish
   --reset                    set all default rules to off
   --eslintrc                 enable loading .eslintrc configuration. - default: true
+  --globals String           define global variables
   -v, --version              outputs the version number
 ```
 
@@ -93,6 +94,14 @@ Example
 
     eslint --env browser,node file.js
 
+### `--globals`
+
+This option defines one or more comma-separated global variables so that they will not be flagged as undefined by the `no-undef` rule. All are considered by default to be read-only, but appending `:true` to the name of the variable makes it writable.
+
+Example:
+
+    eslint --globals require,exports:true file.js
+
 ### `-v`, `--version`
 
 This option outputs the current ESLint version onto the console. All other options are ignored when present.

lib/config.js

@@ -130,6 +130,13 @@ function Config(options) {
     this.baseConfig.format = options.format;
     this.useEslintrc = (options.eslintrc !== false);
     this.env = options.env;
+    this.globals = (options.globals || "").split(",").reduce(function (globals, def) {
+
+        // Default "foo" to false and handle "foo:false" and "foo:true"
+        var parts = def.split(":");
+        globals[parts[0]] = (parts.length > 1 && parts[1] === "true");
+        return globals;
+    }, {});
     useConfig = options.config;
 
     if (useConfig) {
@@ -188,6 +195,7 @@ Config.prototype.getConfig = function (filePath) {
 
     config = this.mergeConfigs(Object.create(this.baseConfig), envConfig);
     config = this.mergeConfigs(config, userConfig);
+    config = this.mergeConfigs(config, { globals: this.globals });
 
     this.cache[directory] = config;
 

lib/options.js

@@ -57,5 +57,9 @@ module.exports = optionator({
     option: "env",
     type: "[String]",
     description: "Specify one or more comma-separated environments."
+  }, {
+    option: "globals",
+    type: "String",
+    description: "Define global variables."
   }]
 });

tests/fixtures/undef.js

@@ -0,0 +1,2 @@
+var bar = baz;
+bat = 42;

tests/lib/cli.js

@@ -277,4 +277,20 @@ describe("cli", function() {
             assert.equal(console.log.args[0][0].split("\n").length, 14);
         });
     });
+
+    describe("when executing with globals flag", function () {
+        it("should default defined variables to read-only", function () {
+            var exit = cli.execute("--globals baz,bat ./tests/fixtures/undef.js");
+
+            assert.isTrue(console.log.calledOnce);
+            assert.equal(exit, 1);
+        });
+
+        it("should allow defining writable global variables", function () {
+            var exit = cli.execute("--globals baz:false,bat:true ./tests/fixtures/undef.js");
+
+            assert.isTrue(console.log.notCalled);
+            assert.equal(exit, 0);
+        });
+    });
 });