call/apply的第一個參數如果為null。this指向window

call/apply是用來改變函數的作用域的，第一次參數為this,第二個參數為傳輸的值，例如

    var a ="windowA"; var b = "windowB"; var str = "str"; var myObject = {a:"myA",b:"myB"}; function hello(s){ alert("a= "+this.a + ", b= "+this.b+" "+s); } hello.call(null,str);//a ="windowA" b = "windowB" str
    hello.call(myObject,str);//a="myA" b="myB" str

 

如果第一個參數為null,則this指向window(在node環境中則指向global)

hello.call(null)//this 指向window
hello.call(window)//this同樣指向window