Object literals in [] notation in javascript

Question

I have two examples . In the first example :

a. 
    1. var object = {1 : "value"};

       alert(object[1]);

    2. var object = {1 : "value"};

       alert(object["1"]);

In both of the examples , the output is "value". I read in the books that object[1] will find a variable 1 and substitute the value with that. Since 1 cannot be declared as variable name in javascript (var 1="some var" //not allowed) , is it just alert(object[1]) tries to find the string declared in var object = {1 : "value"}; and alerts "value".

Because , there is no difference between 1. and 2. example alerts yield the same result.

b.
   1. 
    var object = {a : "value"};

    alert(object["a"]);

        The above example is pretty much clear that it is finding out string "a".

    2. 
    var object = {a : "value"};

    alert(object[a]);

The above example is an error , since we havent declared

var a = "some";

I am just curious to know the difference between a. 1 and a.2 and also if my understanding is correct wrt these examples?

In second example, interpreter is looking for variable a

Rayon
– Rayon

2016-09-06 10:14:07 +00:00
Commented Sep 6, 2016 at 10:14 — Rayon
– Rayon, Commented Sep 6, 2016 at 10:14

Quentin · Accepted Answer · 2016-09-06 10:14:55Z

In both of the examples , the output is "value". I read in the books that object[1] will find a variable 1 and substitute the value with that.

No. It takes a string.

If you pass it a number literal, it converts the number to a string.

If you pass it a string literal, it uses the string literal as a string.

If you pass it an variable then it gets the value of that variable and converts it to a string if it isn't already one.

1 is not a variable name. The grammar of JavaScript requires that it be treated as a number literal.

I am just curious to know the difference between a. 1 and 2

In case 1, you are passing a number literal. In case 2, you are passing an undeclared variable. You get a ReferenceError when you try to get a value from an undeclared variable.

You'd get the same effect in any other context where you were doing something with a value.

var foo = 1; // Assigns 1
var foo = bar; // Throws a reference error because bar is undeclared

RemcoGerlich · Accepted Answer · 2016-09-06 10:22:47Z

Two different things going on.

First, object keys are always strings, but you are allowed to write them without quotes in cases where that doesn't lead to syntax problems.

{1: "value"}
{a: "value"}

These are other ways of writing

{"1": "value"}
{"a": "value"}

Even if a happens to be a variable, when used like that in an object literal it justs means the string "a" and has nothing to do with the variable.

var key = "hmm";
var object = {key: "value"}; // object is now {"key": "value"}

If you want to have the value of the variable as property, you can use the following ES6 syntax in modern browsers:

object = {[key]: "value"}; // object is now {"hmm": "value"}

The other thing is what Quentin said; when you lookup a property's value with object[x], whatever x is will be converted to a string.

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