Many programming languages support arrays with named indexes.
Arrays with named indexes are called associative arrays (or hashes).
JavaScript does not support arrays with named indexes.
In JavaScript, arrays always use numbered indexes.
Example
var person = [];
person[0] = "John";
person[1] = "Doe";
person[2] = 46;
var x = person.length; // person.length will return 3
var y = person[0]; // person[0] will return "John"
person[0] = "John";
person[1] = "Doe";
person[2] = 46;
var x = person.length; // person.length will return 3
var y = person[0]; // person[0] will return "John"
After that, some array methods and properties will produce incorrect results.
Example:
var person = [];
person["firstName"] = "John";
person["lastName"] = "Doe";
person["age"] = 46;
var x = person.length; // person.length will return 0
var y = person[0]; // person[0] will return undefined
person["firstName"] = "John";
person["lastName"] = "Doe";
person["age"] = 46;
var x = person.length; // person.length will return 0
var y = person[0]; // person[0] will return undefined
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