Question

Crockford writes in http://javascript.crockford.com/survey.html:

“There are two ways to make a new array:

var myArray = [];
var myArray = new Array();”

So I’m confused by these two lines in some AJAX code I am reading:

var obj={};    // obj is an Object object (i.e. a hash table)
obj[4] = 'x';  // now obj is suddenly an Array object via an integer key?

In JavaScript are an object and an array really just the same thing, but with a variant on the key type?

In other words, is this the same as in php where we can use either a name (string) or an integer for a hash key?

I’ve Googled for an answer on this but can’t seem to nail down an article which discusses this issue.

One possibility that comes to mind is that perhaps the first line is syntactic lint because the 2nd line overwrites the previous definition of obj as it creates a new Array object.

Answer 1

As far as I know, no, an object can’t be coerced into an array. But, it can look and act like an array, and that’s what’s happening here. Numbers, and anything else that can be coerced to a string, are perfectly valid property names for Javascript objects, so

obj[4] = 1;
obj['spam'] = 2;

are both valid ways of setting a property on the object. That doesn’t make the object an array. An Array is a special class of object with specific methods (.slice(), .concat(), etc) and a length property that’s kept up to date with the number of items in the array.