Keys are strings

What is the output out of the following code? Explain your answer.

vara={},
b={key:'b'},
c={key:'c'};
a[b]=123;
a[c]=456;
console.log(a[b]);

一道有趣的题目，答案是 456。 我们知道，Javascript 中对象的 key 值，一定会是一个 string 值，如果不是，则会隐式地进行转换。当执行到 a[b]=123] 时，b 并不是一个 string 值，将 b 执行 toString() 方法转换（得到 “[object Object]“），a[c] 也是相同道理。所以代码其实可以看做这样执行：

vara={},
b={key:'b'},
c={key:'c'};
// a[b]=123;
a["[object Object]"]=123;
// a[c]=456;
a["[object Object]"]=456;
console.log(a["[object Object]"]);
这样就一目了然了。

delete

JavaScript also has a delete operator that "undefines" an object a property (thanks zproxy), it can be handy in certain situations, you can apply it to object properties and array members, variables declared with var cannot be deleted, but implicitly declared variables can be:

var obj = {
    val: 'Some string'
};
alert(obj.val); // displays 'Some string'
delete obj.val;
alert(obj.val); // displays 'undefined'

References:

1. 9 JavaScript Tips You May Not Know http://codetunnel.io/9-javascript-tips-you-may-not-know/