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 - You can also have an extra comma at the end // It's 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 and 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: dog = {color: "brown", size: "large"}; dog["color"]; // results in "brown" dog.color; // also results in "brown" One can use the object and array declaration literals to quickly create arrays that are associative, multidimensional, or both. (Technically, JavaScript does not support multidimensional arrays, but one can mimic them with arrays-of-arrays.) cats = [{color: "brown", size: "large"}, {color: "black", size: "small"}]; cats[0]["size"]; // results in "large" dogs = {rover: {color: "brown", size: "large"}, spot: {color: "black", size: "small"}}; dogs["spot"]["size"]; // results in "small" dogs.rover.color; // results in "brown" Date A Date object stores a signed millisecond count with zero representing 1970-01-01 00:00:00 UT and a range of ±108 days. There are several ways of providing arguments to the Date constructor. Note that months are zero-based. new Date(); // create a new Date instance representing the current time/date. new Date(2010, 2, 1); // create a new Date instance representing 2010-Mar-01 00:00:00 new Date(2010, 2, 1, 14, 25, 30); // create a new Date instance representing 2010-Mar-01 14:25:30 new Date("2010-3-1 14:25:30"); // create a new Date instance from a String. Methods to extract fields are