## split()

The `split()` method in JavaScript allows you to split a string into an array of substrings based on a specified separator. The resulting array can then be used for a variety of purposes in your program.

Here's an example to illustrate how it works:

Let's say you have a string containing the names of three fruits separated by commas, like this:

``` javascript
let fruits = "apple,banana,orange";
```

To split this string into an array of substrings based on the commas, you can use the `split()` method:

``` javascript
let fruitsArray = fruits.split(",");
console.log(fruitsArray); // ["apple", "banana", "orange"]
```

This will create an array called `fruitArray` that contains three elements: `"apple"`, `"banana"`, and `"orange"`.

You can then use this array to perform various operations in your program, such as accessing individual elements or looping through all the elements.

It's worth noting that you can use any character as a separator in the `split()` method. If you don't specify a separator, the method will split the string into an array with a single element containing the original string.

``` javascript
let str = 'Hello World';
let strArray = str.split();
console.log(strArray); // ["Hello World"]
```

You can also use the `split()` method to split a string into an array of characters. You just need to specify an empty string as the separator.

``` javascript
let str = 'Hello World';
let strArray = str.split('');
console.log(strArray); // ["H", "e", "l", "l", "o", " ", "W", "o", "r", "l", "d"]
```

We can limit the number of elements in the resulting array by specifying a second parameter to the `split()` method. This parameter specifies the maximum number of elements to include in the array.

``` javascript
let str = 'Hello World';
let strArray = str.split('', 5);
console.log(strArray); // ["H", "e", "l", "l", "o"]
```


## join()

After we have used the `split` method to break a `string` into an `array` of substrings, we can use the `join` method to combine those substrings back into a string.

The `join` method concatenates all the elements of an `array` into a single `string`. It takes an optional parameter separator, which is used to separate the elements in the resulting `string`. If no separator is specified, a comma is used by default.

Here's an example of using the join method to combine an `array` of strings:

``` javascript
let fruits = ['apple', 'banana', 'orange'];
let result = fruits.join(', ');
console.log(result); // Output: "apple, banana, orange"
```

When we have a lot of strings and we need to combine them into a single string, the `join` method is a lot more efficient than using the `+` operator or the `concat` method. This is because the `join` method doesn't create a new string for each element in the array. Instead, it creates a new string only once, when the `join` method is called.

## Code Challenge

You will receive a string containing a list of names separated by commas.

Your task is to return a string containing all the names separated by `/`, that contains the letter "a" in it. It must be case-insensitive, so "a" and "A" are both valid.