Javascript simultaneous object and array creation

Why does javascript allow me to do the following.

a = {two : 'World'};
a[1] = 'Hello';
console.log(a[1]);
console.log(a.two);

the output is

Hello
World

Shouldn't it complain that I am trying to use an object as an Array? This works with anything by the way, like so

b = new Date();
b[1] =开发者_如何学Go 'Wow';
console.log(b[1]);

the output is

wow

Is there a use for this? It seems to me like a bad programing practice.

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In Javascript, all arrays are objects. There is no hard-and-fast dividing line between the two. Arrays have certain properties and methods, but are implemented as objects.

The syntax [1] is one of the two equivalent Javascript member operators. These two are equivalent:

var foo = {};
foo.bar = 'foobar';
foo['bar'] = 'foobar';

However, with the dot notation (foo.bar), you can only access properties that are valid Javascript identifiers. This means:

a sequence of alphanumerical characters, also including the underscore ("_") and dollar sign ("$"), that cannot start with a number (source)

You can set the properties of any Javascript object -- array, object, Date object, string, number -- in this fashion, since they all derive from the same Object type.

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Shouldn't it complain that I am trying to use an object as an Array?

No. Numeric properties are allowed. The square bracket notation [] is used on Objects for keys that are not valid javascript identifiers. Not just on Arrays.

a[ "some-invalid***identifier" ] = 'some value';

This works with anything by the way, like so

Yes, same reason. new Date() returns an object to which properties can be assigned either via dot or square bracket notation.

---

You're not treating an object like an array - you're using numeric keys in an object. Just as these are both valid:

var o = { "test":1 };
o["test2"] = 2;

so are these:

var o = { 1: "test" };
o[2] = "test2";

Edit: As the comment below points out, the above syntax is actually misleading, as 1 gets converted to a string in both cases - so technically, this is exactly the same as

var o = { "1": "test" };

as you can see here:

var o = {1:"test"};
for (i in o) console.log(i, i===1, i==="1"); // 1 false true

---

Honestly, I think the console output of this can be pretty enlightening:

var a = {two: "World"};
a[1] = "Hello";
console.log(a);
//   1: "Hello"
//   two: "World"
// > __proto__: Object

var b = new Date();
b[1] = 'Wow';
console.dir(b[1]); // console.log just returns the timestamp
//   1: "Wow"
// v __proto__: Date
//   > constructor: function Date() { [native code] }
//   > getDate: ...

As you can see, these are both just objects, one inheriting from the generic Object and the other from Date. Just like any other object, they can be assigned a key/value pair, where your 1 is just being converted into a string.

It's definitely messy coding if you use it that way, but it's a pretty important illustration of JavaScript principles. There are plenty of useful ways to extend objects that are syntactically similar to what you're doing.

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Because the [key] notation to access the properties of an object will accept anything as a key, including a numeric. Other things you can use include: var a={}, b={}; a{b}=1; // object a's property is the object 'b'. Rather than bad programming practice, it provides a truly unified model for objects.

See comments.