SOURCE

var length = 1
function fn() {
    console.log(this.length)
}
var obj = {
    length: 100,
    action: function(callback) {
        // 输出1
        callback()
        // 输出2
        arguments[0]()
        // 输出3
        var foo = arguments[0]
        foo()
        // 输出4
        this.foo2 = arguments[0]
        this.foo2() 
    }
}

var arr1 = [1, 2, 3, 4]
obj.action(fn, ...arr1)

// 引题
var length = 1
function foo() {
    console.log(this.length)
}
var arr2 = [foo, 2, 3]
arr2[0]()
var f1 = arr2[0]
f1()