JavaScript syntax
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Native objects
The JavaScript language provides a handful of native objects. JavaScript native objects are considered part of the JavaScript specification. JavaScript environment notwithstanding, this set of objects should always be available.
Array
An Array is a JavaScript object prototyped from the Array constructor specifically designed to store data values indexed by integer keys. Arrays, unlike the basic Object type, are prototyped with methods and properties to aid the programmer in routine tasks (for example, join, slice, and push).
As in the C family, arrays use a zero-based indexing scheme: A value that is inserted into an empty array by means of the push method occupies the 0th index of the array.
const myArray = []; // Point the variable myArray to a newly ...
// ... created, empty Array
myArray.push("hello World"); // Fill the next empty index, in this case 0
console.log(myArray[0]); // Equivalent to console.log("hello World");
Arrays have a length property that is guaranteed to always be larger than the largest integer index used in the array. It is automatically updated, if one creates a property with an even larger index. Writing a smaller number to the length property will remove larger indices.
Elements of Arrays may be accessed using normal object property access notation:
myArray[1]; // the 2nd item in myArray
myArray["1"];
The above two are equivalent. It is not possible to use the "dot"-notation or strings with alternative representations of the number:
myArray.1; // syntax error
myArray["01"]; // not the same as myArray[1]
Declaration of an array can use either an Array literal or the Array constructor:
let myArray;
// Array literals
myArray = [1, 2]; // length of 2
myArray = [1, 2,]; // same array - Users can also have an extra comma at the end
// It is also possible to not fill in parts of the array
myArray = [0, 1, /* hole */, /* hole */, 4, 5]; // length of 6
myArray = [0, 1, /* hole */, /* hole */, 4, 5,]; // same array
myArray = [0, 1, /* hole */, /* hole */, 4, 5, /* hole */,]; // length of 7
// With the constructor
myArray = new Array(0, 1, 2, 3, 4, 5); // length of 6
myArray = new Array(365); // an empty array with length 365
Arrays are implemented so that only the defined elements use memory; they are "sparse arrays". Setting myArray[10] = 'someThing' and myArray[57] = 'somethingOther' only uses space for these two elements, just like any other object. The length of the array will still be reported as 58. The maximum length of an array is 4,294,967,295 which corresponds to 32-bit binary number (11111111111111111111111111111111)2.
One can use the object declaration literal to create objects that behave much like associative arrays in other languages:
const dog = {color: "brown", size: "large"};
dog["color"]; // results in "brown"
dog.color; // also results in "brown"
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