JEXL Syntax Overview

JEXL (JavaScript Expression Language) uses a clean, readable syntax that combines the best features of JavaScript expressions with powerful data transformation capabilities.

Basic Structure

A JEXL expression consists of:

Identifiers - Variable and property names
Literals - Fixed values (strings, numbers, booleans, etc.)
Operators - Symbols that perform operations
Functions - Callable procedures that return values
Transforms - Functions applied via the pipe operator (|)

Identifiers and Property Access

Simple Identifiers

name          // Access 'name' property
age           // Access 'age' property
isActive      // Access 'isActive' property

Nested Property Access

user.name            // Access nested property
user.profile.email   // Access deeply nested property
company.address.city // Multiple levels of nesting

Array Access

users[0]        // Access first element
users[0].name   // Access property of array element
scores[-1]      // Access last element (negative indexing)

Dynamic Property Access

user[propertyName]     // Use variable as property name
data["complex-key"]    // Access property with special characters
settings[configKey]    // Dynamic configuration access

Literals

String Literals

"hello world"     // Double quotes
'hello world'     // Single quotes
"It's working"    // Escaping with different quotes
'She said "hi"'   // Escaping with different quotes
"Line 1\nLine 2"  // Escape sequences

Number Literals

42        // Integer
3.14159   // Decimal
-17       // Negative
1.23e-4   // Scientific notation
0xFF      // Hexadecimal

Boolean Literals

true      // Boolean true
false     // Boolean false

Array Literals

[]                    // Empty array
[1, 2, 3]            // Number array
["a", "b", "c"]      // String array
[true, false, null]  // Mixed types
[user.name, user.age] // Expressions as elements

Object Literals

{}                           // Empty object
{name: "John", age: 30}     // Simple object
{x: 1, y: 2, z: x + y}     // Expressions as values
{"complex-key": value}      // Quoted keys
{[dynamicKey]: value}       // Computed keys

Null Literal

null      // Null value

Operators

Arithmetic Operators

a + b     // Addition
a - b     // Subtraction
a * b     // Multiplication
a / b     // Division
a % b     // Modulus
a ^ b     // Exponentiation

Comparison Operators

a == b    // Equality
a != b    // Inequality
a < b     // Less than
a <= b    // Less than or equal
a > b     // Greater than
a >= b    // Greater than or equal

Logical Operators

a && b    // Logical AND
a || b    // Logical OR
!a        // Logical NOT

Other Operators

a in b         // Membership test
a ? b : c      // Ternary conditional
a | transform  // Transform (pipe operator)

Functions

Functions are called with parentheses and can take multiple arguments:

length(array)              // Single argument
max([1, 2, 3, 4, 5])      // Array argument
substring(text, 0, 5)     // Multiple arguments
contains(text, "search")  // String arguments

Transforms

Transforms use the pipe operator (|) to apply functions to values:

// Single transform
"hello" | uppercase        // "HELLO"

// Chained transforms
"  hello world  " | trim | uppercase | split(" ")
// ["HELLO", "WORLD"]

// Transform with arguments
array | filter("value > 10") | map("value * 2")
// Filter then transform each element

Comments

JEXL supports both line and block comments:

// This is a line comment
name | uppercase  // Comment at end of line

/*
 * This is a block comment
 * spanning multiple lines
 */

Expression Evaluation

Left-to-Right Evaluation

a + b * c         // Evaluates as: a + (b * c)
a | b | c         // Evaluates as: (a | b) | c

Operator Precedence

From highest to lowest precedence:

Property access (., [])
Function calls (())
Unary operators (!, -)
Exponentiation (^)
Multiplication, Division, Modulus (*, /, %)
Addition, Subtraction (+, -)
Comparison (<, <=, >, >=)
Equality (==, !=)
Membership (in)
Logical AND (&&)
Logical OR (||)
Ternary (? :)
Transforms (|)

Parentheses for Grouping

(a + b) * c       // Force addition before multiplication
a && (b || c)     // Group logical operations
(user | profile).name  // Transform then access property

Context Variables

JEXL expressions are evaluated against a context object:

// Context: { name: "John", age: 30, users: [...] }

name                    // "John"
age > 25               // true
users | length         // Number of users

Advanced Syntax

Nested Expressions

users | filter("value.age > " + minAge) | map("value.name")
// Uses variable in filter expression

Complex Object Construction

{
  fullName: firstName + " " + lastName,
  isAdult: age >= 18,
  summary: name + " is " + age + " years old"
}

Conditional Chains

score >= 90 ? "A" :
score >= 80 ? "B" :
score >= 70 ? "C" :
score >= 60 ? "D" : "F"

Best Practices

1. Use Meaningful Names

// Good
user.profile.displayName

// Less clear
u.p.dn

2. Chain Transforms Logically

// Good - logical flow
data | filter("value.active") | sort("value.name") | map("value.email")

// Harder to read
data | map("value.email") | sort | filter("value")

3. Use Comments for Complex Logic

// Calculate weighted average score
(homework * 0.3 + midterm * 0.3 + final * 0.4) | round(2)

// Group calculations
score = (quiz1 + quiz2 + quiz3) / 3;
grade = score >= 90 ? "A" : score >= 80 ? "B" : "C"

Common Patterns

Data Filtering and Mapping

// Get active users' names
users | filter("value.status == 'active'") | map("value.name")

Aggregations

// Calculate total and average
items | sum("value.price")
items | average("value.rating")

String Processing

// Clean and format text
title | trim | lowercase | replace(" ", "-")

Conditional Logic

// Status based on conditions
status = errors > 0 ? "error" : warnings > 0 ? "warning" : "success"

Next: Learn about Data Types in JEXL expressions.