显式绑定

function foo() {
  console.log(this)
}

foo.call(window); // window
foo.call({name: "codereasy"}); // {name: "codereasy"}
foo.call(666); // Object( empty )

function foo() {
  console.log(this);
}

var obj = {
  name: "codereasy"
}

var bar = foo.bind(obj);

bar(); // obj对象