make this respect a target if one is used

Otherwise when invoked "globally", `this` is set to the
function instance

README.md

@@ -99,16 +99,23 @@ var fn = createFunction('name', 0, FunctionSubclass, args);
 ### invoke → Symbol
 
 You use the `invoke` Symbol to define the function to execute when a created
-function instance is invoked. Note that `this` in the invoke function is bound to
-the created function instance.
+function instance is invoked.
+
+Note that `this` in the invoke function is bound to the created function instance
+when invoked without a receiver (i.e. "globally"). Otherwise, `this` respects the
+left-hand receiver, or target when used with `.call()`/`.apply()`.
 
 ``` js
 var invoke = require('function-class/invoke');
 
 fn[invoke] = function () {
   console.log('fn has been invoked!');
+  return this;
 };
 
 fn();
 // fn has been invoked!
+
+assert.equal(fn(), fn);
+assert.equal(fn.call(global), global);
 ```

index.js

@@ -12,7 +12,12 @@ function createFunctionInstance (name, length, constructor, args) {
 
   // create function instance
   fn = functionNameArity(name, length, function () {
-    return fn[invoke].apply(fn, arguments);
+    "use strict";
+    var invokeFn = fn[invoke];
+    if (typeof invokeFn !== 'function')
+      throw new Error('you must define the `[function-class/invoke]` function on this instance');
+    var thisArg = typeof this === 'undefined' ? fn : this;
+    return invokeFn.apply(thisArg, arguments);
   });
 
   if (constructor) {

test.js

@@ -86,3 +86,15 @@ assert.ok(oneOffCalled);
 var snowman = createFunction('⌛', 30);
 assert.equal(snowman.name, '⌛');
 assert.equal(snowman.length, 30);
+
+
+// `this` arg
+var thisArg = createFunction();
+thisArg[invoke] = function () {
+  return this;
+};
+assert.equal(thisArg(), thisArg);
+assert.equal(thisArg()()(), thisArg());
+assert.equal(thisArg.call(global), global);
+assert.equal(thisArg.call(1).valueOf(), 1);
+assert.equal(thisArg.call(true).valueOf(), true);