什么是装饰者模式

函数式例子：

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//Example 1
Function.prototype.before = function(beforeFn) {
    let self = this;
    return function() {
        beforeFn.apply(this, arguments);
        self.apply(this, arguments);
    }
}

Function.prototype.after = function(afterFn) {
    let self = this;
    return function() {
        self.apply(this, arguments);
        afterFn.apply(this, arguments);
    }
}


//Example 2
let a = function() {
    alert(1);
}

let _a = a;

a = function() {
    _a();
    alert(2);
}

//Example 3
window.onload = function() {
    alert(1);
}

let onload = window.onload;

window.onload = function() {
    onload();
    alert(2);
}