topic:
var a = {
};
var b = {
key:'b'};
var c = {
key:'c'};
a[b] = 123;
a[c] = 456;
console.log(a[b]);
What will the above code output?
The answer is: 456
Why?
Here are a few knowledge points involved:
- Object property name type
- The difference between a.key and a[key]
- [object].toString() result
Object property name type
There can only be two types of attribute names of objects, namely String and Symbol. If the type of the newly added attribute is not one of the two types, then the toString() method will be called to convert it to string type
[object].toString() result
Given an object obj, the result of obj.toString() is[object,object]
The difference between a.key and a[key]
Suppose there is an object a
let a = {
key:'a'}
Executing a.key will actually get the attribute named in a key, here keyis a string, which is actually getting a['key']
And executing a[key] will look for keya variable named, soa[key] !== a[‘key’]
let a = {
key:'a'};
console.log(a.key);//'a'
console.log(a['key']);//'a'
console.log(a[key]);//error
After understanding these knowledge points, we can know the result of the code just now
-
Since both b and c are objects, calling a[b] or a[c] will execute the toString method of the object, a[b] is equivalent to a['[object, object]'], a[c], etc. Value in a['[object,object]']
So the above code is to reassign a['[object,object]']
will eventually output 456