thisArg 指的是应该调用回调的上下文，基本上它就是 this 在回调中引用的内容.例如:

thisArg refers to context which callback should be called, basically it is what this refers to inside callback. For example:

var myObject = { name: 'myObject' };

[1,2].forEach(function(item) { 
 console.log(item); // 1, 2
 console.log(this === myObject); // true
}, myObject)