复习一下
this
和声明提升
。
fe-star / exercise4 Goto Github PK
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课程练习 4:辨认所有this
试了半天全局变量global。。有个地方不太舒服,感觉怪怪的,代码如下:
function test() {
this.ttt = "11";
}
function test2(){
console.log(this.ttt) //11
this.ttt = "22";
}
console.log(global.ttt) //undefined
let ttt = "33";
console.log(global.ttt) //undefined
test();
console.log(global.ttt); //11
test2();
console.log(global.ttt); //22
第一个undefined好理解,第二个,我不是已经赋值了全局变量ttt了么,为何还是输出未定义
全局下的后两个console.log也好理解,但是如果函数内部的这个变量都挂在全局global上了,这不是造成变量污染了么?就像代码中的样子test2中的console.log
求解
this.should.equal(global)
在终端中断言正常执行,但是尝试使用karma进行测试时,在浏览器中提示global is not defined
。像这种测试用例中出现global的情况该如何使用karma进行测试呢?
问题记录:
(1)
it('bind undefined', function () {
var obj = {
say: function () {
function _say() {
// this 是什么?想想为什么?
this.should.equal(global)
}
return _say.bind(obj)
}()
}
obj.say()
})
在正在定义obj的时候,say就立即执行了,此时的 this
为 global, 此时的obj不存在
当obj.say()执行的时候,obj已经存在了,可是为什么 this
的指向还是 global?
同样的(后者可以理解),
it('bind normal', function () {
var obj = {}
obj.say = function () {
function _say() {
// this 是什么?想想为什么?
this.should.equal(obj)
}
return _say.bind(obj)
}()
obj.say()
})
在给obj赋值的时候,say立即执行了,此时的 this
指向 obj
当obj.say()执行的时候,obj一直存在,this
仍然指向 obj
(2)执行问题
测试用例里面的调用其实不是obj.say()函数,而是obj在定义时执行的函数?
然后obj.say()调用的是下面的紫色的函数吗?
老师 为什么43行先报错,第8行后报错
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