1. Introduction

The idea of nexl expressions was taken from Bash where you can perform simple operations with Bash variables like uppercasing characters:
MY_BASH_STR="Hello, World"
echo "${MY_BASH_STR^^}"
In this example Bash performs single uppercase operation on MY_BASH_STR variable.

nexl expression language allows to chain multiple operations in a single expression. Operations in nexl are "piped" like Bash does ( but without a pipe chain character ).

Let's say we have a fruits array and we need to sort this array and join its elements with comma.
The expression will be:
Let's take a closer look on each operation performed in this expression:

As we can see there are 3 operations performed in this expression:
 Resolution of a fruits variable
 Array sort
 Array elements concatenation with comma

The following operations are available in nexl:
JavaScript variables resolution
Default value operation
Type casting operation
Object property resolution operation
Inverted object property resolution operation
Object operations
Object key(s) resolution operation
Array operations
Joining array elements operation
"Add" operation
"Eliminate" operation
String operations
Slicing arrays and strings
Miscellaneous operations
Mandatory value operation
Functions call and stack operation
Operations separator
Assign variables operation
All those operations can be chained and combined in any order in nexl expressions depending on your needs.

Each operation represented by special character and value followed by that character ( in certain cases value is optional ).

Let's consider a sorting array operation from the last example:
The # character represents a group of different array operations and the S string followed by that character is a value of that operation.

Another example. The #LEN operation calculates array length ( see more about array operations here ).

Depending on operation type, operation value must either predefined or free styled string ( or even nexl expression ).

Returning again to the first example we have the following operation:
The & operation joins array element with a delimited provided as a string followed by the & character which is comma. But you can use any other string or nexl expression instead of comma.

If you are not interested in learning nexl expression language you can always write a JavaScript function and make all data aggregations inside:
function sortAndJoin() {
    // sorting array
    // joining array elements with comma
    return fruits.join(',');
And then to call that function from nexl expression:
But nexl expression language gives certain flexibility and adds extra features. Mixing both methods is better solution.