Are Javascript Arrays Truly Dense?

Are Javascript Arrays Truly Dense?

The question arises: Are Javascript arrays truly implemented as dense data structures? If one attempts to access an array element using a large integer index, as exemplified by the code fragment below:

array[Date.getTime()] = value;

Will the interpreter consequently instantiate all array elements from index 0 to the current time? Moreover, are there variations in behavior across different browsers?

Delving into the Implementation

To shed light on this issue, it's essential to understand the internal workings of Javascript arrays. Contrary to the conventional impression of arrays as dense structures, Javascript arrays are implemented as hash tables, employing key-value pairs for data storage. This design choice allows for flexibility in indexing, enabling the use of not only large integers but also strings, floating-point numbers, and even arbitrary objects as valid indices.

The caveat, however, lies in the automatic conversion of all keys to strings using the toString() method before insertion into the hash. To demonstrate this behavior, consider the following code:

var array = [];
array[0] = "zero";
array[new Date().getTime()] = "now";
array[3.14] = "pi";

for (var i in array) {
  alert("array["+i+"] = " + array[i] + ", typeof("+i+") == " + typeof(i));
}

The output of this code reveals the surprising nature of Javascript arrays:

array[0] = zero, typeof(0) == string
array[1254503972355] = now, typeof(1254503972355) == string
array[3.14] = pi, typeof(3.14) == string

Notice the conversion of all indices (0, the current time, and 3.14) to strings before storing them in the hash table. This behavior emphasizes the fundamental distinction between Javascript arrays and traditional dense data structures, where element access is constrained by sequential indices.

Implications for Array Iteration

It's crucial to be aware of these indexing characteristics when iterating over Javascript arrays. While the common practice is to use the for (var i = 0; i < array.length; i) syntax, this approach can lead to issues when dealing with non-standard array indices. For instance, in the example code above, the array.length property holds a value of 0, leading to an empty iteration. To remedy this situation, it's advisable to adopt the for...in syntax, which only enumerates indices that are explicitly defined in the array.

The above is the detailed content of Are Javascript Arrays Truly Dense?. For more information, please follow other related articles on the PHP Chinese website!