Creating and initializing arrays
Declaring, creating, and initializing an array in JavaScript is as simple, as shown by the following:
var daysOfWeek = new Array(); //{1}
var daysOfWeek = new Array(7); //{2}
var daysOfWeek = new Array('Sunday', 'Monday', 'Tuesday', 'Wednes day', 'Thursday', 'Friday', 'Saturday'); //{3}
We can simply declare and instantiate a new array using the keyword new (line {1}). Also, using the keyword new, we can create a new array specifying the length of the array (line {2}). A third option would be passing the array elements directly to its constructor (line {3}).
However, using the new keyword is not best practice. If you want to create an array in JavaScript, we can assign empty brackets ([]), as in the following example:
var daysOfWeek = [];
We can also initialize the array with some elements, as follows:
var daysOfWeek = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday'];
If we want to know how many elements are in the array (its size), we can use the length property. The following code will give an output of 7:
console.log(daysOfWeek.length);
Accessing elements and iterating an array
To access a particular position of the array, we can also use brackets, passing the index of the position we would like to access. For example, let's say we want to output all the elements from the daysOfWeek array. To do so, we need to loop the array and print the elements, as follows:
for (var i=0; i<daysOfWeek.length; i++){
console.log(daysOfWeek[i]);
}
Let's take a look at another example. Let's say that we want to find out the first 20 numbers of the Fibonacci sequence. The first two numbers of the Fibonacci sequence are 1 and 2, and each subsequent number is the sum of the previous two numbers:
var fibonacci = []; //{1}
fibonacci[1] = 1; //{2}
fibonacci[2] = 1; //{3}
for(var i = 3; i < 20; i++){
fibonacci[i] = fibonacci[i-1] + fibonacci[i-2]; ////{4}
}
for(var i = 1; i<fibonacci.length; i++){ //{5}
console.log(fibonacci[i]); //{6}
}
- So, in line
{1}, we declared and created an array. - In lines
{2}and{3}, we assigned the first two numbers of the Fibonacci sequence to the second and third positions of the array (in JavaScript, the first position of the array is always referenced by 0, and as there is no 0 in the Fibonacci sequence, we will skip it). - Then, all we have to do is create the third to the twentieth number of the sequence (as we know the first two numbers already). To do so, we can use a loop and assign the sum of the previous two positions of the array to the current position (line
{4}, starting from index 3 of the array to the 19th index). - Then, to take a look at the output (line
{6}), we just need to loop the array from its first position to its length (line{5}).
If you would like to generate more than 20 numbers of the Fibonacci sequence, just change the number 20 to whatever number you like.