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# [Android Components](../../../README.md) > Libraries > JEXL
Javascript Expression Language: Powerful context-based expression parser and evaluator.
This implementation is based on [Mozjexl](https://github.com/mozilla/mozjexl), a fork of Jexl (designed and created at TechnologyAdvice) for use at Mozilla, specifically as a part of SHIELD and Normandy.
Features not supported yet:
* JavaScript object properties (e.g. [String.length](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/length))
* Adding custom operators (binary/unary)
Other implementations:
* [JavaScript](https://github.com/mozilla/mozjexl)
* [Python](https://github.com/mozilla/pyjexl)
## Usage
### Setting up the dependency
Use Gradle to download the library from [maven.mozilla.org](https://maven.mozilla.org/) ([Setup repository](../../../README.md#maven-repository)):
```Groovy
implementation "org.mozilla.components:jexl:{latest-version}
```
### Evaluating expressions
```Kotlin
val jexl = Jexl()
val result = jexl.evaluate("75 > 42")
// evaluate() returns an object of type JexlValue. Calling toKotlin() converts this
// into a matching Kotlin type (in this case a Boolean).
println(result.value) // Prints "true"
```
Often expressions should return a `Boolean`value. In this case `evaluateBooleanExpression` is a helper that always returns a Kotlin `Boolean` and never throws an exception (Returns false).
```Kotlin
val jexl = Jexl()
// "result" has type Boolean and value "true"
val result = jexl.evaluateBooleanExpression("42 + 23 > 50", defaultValue = false)
```
### Unary Operators
| Operation | Symbol |
|-----------|:------:|
| Negate | ! |
### Binary Operators
| Operation | Symbol |
|------------------|:----------------:|
| Add, Concat | + |
| Subtract | - |
| Multiply | * |
| Divide | / |
| Divide and floor | // |
| Modulus | % |
| Power of | ^ |
| Logical AND | && |
| Logical OR | || |
### Comparison
| Comparison | Symbol |
|----------------------------|:------:|
| Equal | == |
| Not equal | != |
| Greater than | > |
| Greater than or equal | >= |
| Less than | < |
| Less than or equal | <= |
| Element in array or string | in |
### Ternary operator
Conditional expressions check to see if the first segment evaluates to a truthy
value. If so, the consequent segment is evaluated. Otherwise, the alternate
is. If the consequent section is missing, the test result itself will be used
instead.
| Expression | Result |
|-------------------------------------|--------|
| `"" ? "Full" : "Empty"` | Empty |
| `"foo" in "foobar" ? "Yes" : "No"` | Yes |
| `{agent: "Archer"}.agent ?: "Kane"` | Archer |
### Native Types
| Type | Examples |
|-----------|:------------------------------:|
| Booleans | `true`, `false` |
| Strings | "Hello \"user\"", 'Hey there!' |
| Integers | 6, -7, 5, -3 |
| Doubles | -7.2, -3.14159 |
| Objects | {hello: "world!"} |
| Arrays | ['hello', 'world!'] |
| Undefined | `undefined` |
The JavaScript implementation of Jexl uses a `Numeric` type. This implementation dynamically casts between `Integer` and `Double` as needed.
### Groups
Parentheses work just how you'd expect them to:
| Expression | Result |
|---------------------------------------|:-------|
| `(83 + 1) / 2` | 42 |
| `1 < 3 && (4 > 2 || 2 > 4)` | true |
### Identifiers
Access variables in the context object by just typing their name. Objects can
be traversed with dot notation, or by using brackets to traverse to a dynamic
property name.
Example context:
```javascript
{
name: {
first: "Malory",
last: "Archer"
},
exes: [
"Nikolai Jakov",
"Len Trexler",
"Burt Reynolds"
],
lastEx: 2
}
```
| Expression | Result |
|---------------------|---------------|
| `name.first` | Malory |
| `name['la' + 'st']` | Archer |
| `exes[2]` | Burt Reynolds |
| `exes[lastEx - 1]` | Len Trexler |
### Collections
Collections, or arrays of objects, can be filtered by including a filter
expression in brackets. Properties of each collection can be referenced by
prefixing them with a leading dot. The result will be an array of the objects
for which the filter expression resulted in a truthy value.
Example context:
```javascript
{
employees: [
{first: 'Sterling', last: 'Archer', age: 36},
{first: 'Malory', last: 'Archer', age: 75},
{first: 'Lana', last: 'Kane', age: 33},
{first: 'Cyril', last: 'Figgis', age: 45},
{first: 'Cheryl', last: 'Tunt', age: 28}
],
retireAge: 62
}
```
| Expression | Result |
|-------------------------------------------------|---------------------------------------------------------------------------------------|
| `employees[.first == 'Sterling']` | [{first: 'Sterling', last: 'Archer', age: 36}] |
| `employees[.last == 'Tu' + 'nt'].first` | Cheryl |
| `employees[.age >= 30 && .age < 40]` | [{first: 'Sterling', last: 'Archer', age: 36},{first: 'Lana', last: 'Kane', age: 33}] |
| `employees[.age >= 30 && .age < 40][.age < 35]` | [{first: 'Lana', last: 'Kane', age: 33}] |
| `employees[.age >= retireAge].first` | Malory |
### Transforms
The power of Jexl is in transforming data. Transform functions take one or more arguments: The value to be transformed, followed by anything else passed to it in the expression.
```Kotlin
val jexl = Jexl()
jexl.addTransform("split") { value, arguments ->
value.toString().split(arguments.first().toString()).toJexlArray()
}
jexl.addTransform("lower") { value, _ ->
value.toString().toLowerCase().toJexl()
}
jexl.addTransform("last") { value, _ ->
(value as JexlArray).values.last()
}
```
| Expression | Result |
|-------------------------------------------------|-----------------------|
| `"Pam Poovey"|lower|split(' ')|first` | poovey |
| `"password==guest"|split('=' + '=')` | ['password', 'guest'] |
### Context
Variable contexts are straightforward Objects that can be accessed
in the expression.
```Kotlin
val context = Context(
"employees" to JexlArray(
JexlObject(
"first" to "Sterling".toJexl(),
"last" to "Archer".toJexl(),
"age" to 36.toJexl()),
JexlObject(
"first" to "Malory".toJexl(),
"last" to "Archer".toJexl(),
"age" to 75.toJexl()),
JexlObject(
"first" to "Malory".toJexl(),
"last" to "Archer".toJexl(),
"age" to 33.toJexl())
)
)
```
| Expression | Result |
|-------------------------------------------------|------------------------------------------------------------------------------|
| `employees[.age >= 30 && .age < 40]` | [{first=Sterling, last=Archer, age=36}, {first=Malory, last=Archer, age=33}] |
| `employees[.age >= 30 && .age < 90][.age < 37]` | [{first=Malory, last=Archer, age=33}] |
## License
This Source Code Form is subject to the terms of the Mozilla Public
License, v. 2.0. If a copy of the MPL was not distributed with this
file, You can obtain one at http://mozilla.org/MPL/2.0/
|
