this一般运用场景：
1.位于函数中，谁调用指向谁
var make = "Mclaren";
var model = "720s"
function fullName() {
console.log(this.make + " " + this.model);
}
var car = {
make: "Lamborghini",
model: "Huracán",
fullName: function () {
console.log(this.make + " " + this.model);
}
}
car.fullName(); // Lmborghini Huracán
window.fullName(); // Mclaren 720S
fullName(); // Mclaren 720S
2.事件调用，指向调用元素
<button onclick="this.style.display='none'">
Remove Me!
</button>
3.方法中调用
var car = {
make: "Lamborghini",
model: "Huracán",
fullName: function () {
console.log(this.make + " " + this.model);
console.log(car.make + " " + car.model);
}
}
car.fullName();
4.直接使用this,则默认全局