javascript - object [] notation

Question

Started learning javascript and currently going thru object topic.

<!DOCTYPE html>
<html>
<body>
<h3>object prop access</h3>
<p id="demo"></p>
<p id="test"></p>
<script>

    var foo = {unique_prop: 1}, bar = {unique_prop: 2}, object = {};
    object[foo] = 'value1';

    //console.log(object[bar]);
    document.getElementById("demo").innerHTML = "Object property:"+object[bar];
    document.getElementById("test").innerHTML = "Object property object[foo]:"+object[foo];

    </script>
    </body>
    </html>

When I run this program it prints value "value1" , "value1" in separate lines. My question is there is no property name "bar" defined for "object" then why object[bar] prints value "value1" - which is assigned to object[foo].

--Divyesh

Ryan · Accepted Answer · 2015-11-22 05:25:01Z

6

Because an object property has a key and a value. The key is always a string. When you try to set a key for a value for which that key is a variable and it as an object, it will have the .toString method of the object/variable called which is this case is will return [object Object]

so bar and foo will be equivalent when converted to a string.

So, you can think of it as accessing the object like this object["[object Object]"]

bar.toString() // "[object Object]"

foo.toString() // "[object Object]"

answered Nov 22, 2015 at 5:25

Ryan

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2 Comments

user3069970 Over a year ago

Thanks for your reply. I tested this toString() method behaviour by printing object "Foo" and "Bar". However my question is code above sets property of object[foo] = "value1". There is no property "Bar" defined for object "Object". How can the object[Bar] = "value1" too?

user3069970 Over a year ago

I understood your point now.. Thanks a lot. I see that basically object[foo] or object[bar] is equivalent to object["[object Object]"]. So if object[foo] = "value1" so it object[bar].

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