JavaScript 中的关联数组与对象

Question

In my script there is a need to create a hash table, and I searched in google for this. Most of the folks are recommending JavaScript object for this purpose. The The problem is some of the keys in the hash table have a "." in them. I am able to create these keys easily with the associative arrays.

I don't understand why associative arrays are bad. The first thing that is mentioned on the sites that I looked at is the length property.

I am coming from the Perl background, where I used hashes. Most common uses were to get the value from a key, check if a key exists, delete a key-value pair, and add a key-value pair. If these are my common uses, can I safely use an associative array?

Answer 1

In JavaScript, objects are associative arrays...there aren't separate concepts for them. You are also able to safely use '.' in a key name, but you can only access the value using the bracket notation:

var foo = {}
foo['bar'] = 'test';
foo['baz.bin'] = 'value';

alert(foo.bar); // Shows 'test'
alert(foo['baz.bin']); // Shows 'value'

If you're using them already and they work, you're safe.

Answer 2

In a JavaScript, an object and array are pretty much the same thing, with an array having a bit of magical functionality (autoupdating the length property and such) and prototype methods suitable for arrays. It is also much easier to construct an object than using an associative array:

var obj = {"my.key": "myValue"};

vs.

var obj = [];
obj["my.key"] = "myValue";

Therefore never use the array object for this, but just the regular object.

Some functionality:

var obj = {}; // Initialized empty object

Delete a key-value pair:

delete obj[key];

Check if a key exists:

key in obj;

Get the key value:

obj[key];

Add a key-value pair:

obj[key] = value;

Answer 3

Because there is no such thing as built-in associative arrays in JavaScript. That's why it's bad.

In fact, when you use something like:

theArray["a"] = "Hello, World!";

It simply creates a property called "a" and set its value to "Hello, World!". This is why the length is always 0, and why the output of alert(theArray) is empty.

Answer 4

Actually, an "associative array" is pretty much the same as an "array-like object" in ECMAScript. Even arrays are objects in ECMAScript, just with the exception to have numeric keys (which are still strings in the background) and a .length property, along with some inherited methods from Array.prototype .

So, a Perl hash and an ECMAScript object behave similarly. You might not know that you can access object properties not only via a dot, but also with brackets and strings, like

var myObj = { foo: 42 };

myObj.foo; // 42
myObj['foo']; // 42

Knowing that, you can also use keys with .

var myObj = { };
myObj['hello.foo.world'] = 42;

Of course, you can access that key only with the bracket notation.

Answer 5

You can use . in key names on JavaScript objects (AKA associative arrays) if you'd like; they're accepted without issue. The minor drawback is you can't use shortcut notations with the dotted keys, eg

var x = {};
x['hello'] = 'there';
alert(x.hello);

is perfectly acceptable and will pop up an alert with 'there' in it. But if you use a dotted name:

var x = {};
x['this.is'] = 'sparta';
alert(x.this.is);

will fail, as JavaScript will look for an attribute named this in the x object, which does not exist. There is only the this.is attribute.

Answer 6

There isn't an associative array. It's just an object.

foo.bar;    // Equivalent to...
foo["bar"]; // Looks like associative array.

Answer 7

For the sake of convenience of using data, there should be no difference between an object and an array. You can think of it as an object or you can think of it as an associative array. At the end, you can just think of everything as data .

• For PHP , [ ] accepts 0, 1, or more items(array), and it is called an associative array . It is JSON in PHP's coat:

  $data = ["message"=>[ "id"=>405, "description"=>"Method not allowed.", "detail"=>[]], "object" => []];

• For JavaScript , { } accepts 0, 1, or more items(array), and it is called an object . This data format is JSON:

  data = {"message": { "id":405, "description":"Method not allowed.", "detail" : {}}, "object" : {}};

I just call them data . The simplest way to describe data is JSON , or its variants.