How to use index to access elements in array object in js

In the previous article, we learned what an array is and how to create an array. Please see "How to create an array object for js". This time we will learn about the method of using index to access elements in an array object. You can refer to it if necessary.

After we create the array, what do we need to do? Should the elements in the created array be output, and how should they be output? Let’s talk about it today.

Let’s look at a small example first.

<script>
var arr = new Array(3); 
arr[0] = "one";
arr[1] = "two";
arr[2] = "three";
var first = arr[0];
console.log(first);
var last = arr[arr.length - 1];
console.log(last);
</script>

The output result of this small example on the console is

You can see that the first element and the last element of this array object are output on the console an element. So how is this done? Let's take a look together.

In this example, you can see that it uses "arr[0]" or "arr[arr.length - 1]", see If we want to access the elements in the array object, we need to use "array object [subscript]" to achieve it.

There is one thing we need to pay attention to: [0] is the first element of the array. [1] is the second element of the array.

If we access an element that does not exist in the array, then we will return "undefined".

For example, if we access not "arr[arr.length - 1]" but "arr[arr.length]", then we "undefined" will be returned. why? We all know that the word length means length. In this example, we are accessing the length of this array. When we access the element of the length of this array, we can know that we are accessing arr[3]. In an array, we use [0] to represent the first element of the array, that means, we will use "length - 1" to represent the last element of the array. In this case, arr[3] will not exist and "undefined" will be returned.

[Recommended learning: javascript advanced tutorial]

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