Jexl 

Javascript Expression Language: Powerful context-based expression parser and evaluator

Quick start

Use it with promises or synchronously:

``javascript
const context = {
name: { first: 'Sterling', last: 'Archer' },
assoc: [
{ first: 'Lana', last: 'Kane' },
{ first: 'Cyril', last: 'Figgis' },
{ first: 'Pam', last: 'Poovey' }
],
age: 36
}

// Filter an array asynchronously...
await const res = jexl.eval('assoc[.first == "Lana"].last', context)
console.log(res) // Output: Kane

// Or synchronously!
console.log(jexl.evalSync('assoc[.first == "Lana"].last')) // Output: Kane

// Do math
await jexl.eval('age * (3 - 1)', context)
// 72

// Concatenate
await jexl.eval('name.first + " " + name["la" + "st"]', context)
// "Sterling Archer"

// Compound
await jexl.eval(
'assoc[.last == "Figgis"].first == "Cyril" && assoc[.last == "Poovey"].first == "Pam"',
context
)
// true

// Use array indexes
await jexl.eval('assoc[1]', context)
// { first: 'Cyril', last: 'Figgis' }

// Use conditional logic
await jexl.eval('age > 62 ? "retired" : "working"', context)
// "working"

// Transform
jexl.addTransform('upper', (val) => val.toUpperCase())
await jexl.eval('"duchess"|upper + " " + name.last|upper', context)
// "DUCHESS ARCHER"

// Transform asynchronously, with arguments
jexl.addTransform('getStat', async (val, stat) => dbSelectByLastName(val, stat))
try {
const res = await jexl.eval('name.last|getStat("weight")', context)
console.log(res) // Output: 184
} catch (e) {
console.log('Database Error', e.stack)
}

// Functions too, sync or async, args or no args
jexl.addFunction('getOldestAgent', () => db.getOldestAgent())
await jexl.eval('age == getOldestAgent().age', context)
// false

// Add your own (a)synchronous operators
// Here's a case-insensitive string equality
jexl.addBinaryOp(
'_=',
20,
(left, right) => left.toLowerCase() === right.toLowerCase()
)
await jexl.eval('"Guest" _= "gUeSt"')
// true

// Compile your expression once, evaluate many times!
const { expr } = jexl
const danger = expr"Danger " + place // Also: jexl.compile('"Danger " + place')
danger.evalSync({ place: 'zone' }) // Danger zone
danger.evalSync({ place: 'ZONE!!!' }) // Danger ZONE!!! (Doesn't recompile the expression!)
`

Play with it

- Jexl Playground - An interactive Jexl sandbox by Christian Zosel @czosel.
- Jexl on RunKit - JS sandbox with Jexl preloaded. Special thanks to Mike Cunneen @cunneen.

Installation

Jexl works on the backend, and on the frontend if bundled using a bundler like Parcel or Webpack.

Install from npm:

npm install jexl --save

or yarn:

yarn add jexl

and use it:

const jexl = require('jexl')

Async vs Sync: Which to use

There is little performance difference between eval and evalSync. The functional
difference is that, if eval is used, Jexl can be customized with asynchronous operators,
transforms, and even wait for unresolved promises in the context object with zero additional
overhead or handling on the programmer's part. evalSync eliminates those advantages,
exposing the expression to raw Promise objects if any are returned as the result of a
custom transform or operator. However, if your application doesn't require async methods,
the evalSync API can be simpler to use.

All the details

$3

| Operation | Symbol |
| --------- | :----: |
| Negate | ! |

$3

| Operation | Symbol |
| ---------------- | :----------: |
| Add, Concat | + |
| Subtract | - |
| Multiply | \* |
| Divide | / |
| Divide and floor | // |
| Modulus | % |
| Power of | ^ |
| Logical AND | && |
| Logical OR | || |

$3

| Comparison | Symbol |
| -------------------------- | :----: |
| Equal | == |
| Not equal | != |
| Greater than | > |
| Greater than or equal | >= |
| Less than | < |
| Less than or equal | <= |
| Element in array or string | in |

#### A note about in

The in operator can be used to check for a substring:
"Cad" in "Ron Cadillac", and it can be used to check for an array element:
"coarse" in ['fine', 'medium', 'coarse']. However, the == operator is used
behind-the-scenes to search arrays, so it should not be used with arrays of
objects. The following expression returns false: {a: 'b'} in [{a: 'b'}].

$3

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 |

$3

| Type | Examples |
| -------- | :----------------------------: |
| Booleans | true, false |
| Strings | "Hello \"user\"", 'Hey there!' |
| Numerics | 6, -7.2, 5, -3.14159 |
| Objects | {hello: "world!"} |
| Arrays | ['hello', 'world!'] |

$3

Parentheses work just how you'd expect them to:

| Expression | Result |
| ----------------------------------- | :----- |
| (83 + 1) / 2 | 42 |
| 1 < 3 && (4 > 2 || 2 > 4) | true |

$3

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 |

$3

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 |

$3

The power of Jexl is in transforming data, synchronously or asynchronously.
Transform functions take one or more arguments: The value to be transformed,
followed by anything else passed to it in the expression. They must return
either the transformed value, or a Promise that resolves with the transformed
value. Add them with jexl.addTransform(name, function).

`javascript
jexl.addTransform('split', (val, char) => val.split(char))
jexl.addTransform('lower', (val) => val.toLowerCase())
`

| Expression | Result |
| ------------------------------------------ | --------------------- |
| "Pam Poovey"|lower|split(' ')[1] | poovey |
| "password==guest"|split('=' + '=') | ['password', 'guest'] |

#### Advanced Transforms

Using Transforms, Jexl can support additional string formats like embedded
JSON, YAML, XML, and more. The following, with the help of the
xml2json module, allows XML to be
traversed just as easily as plain javascript objects:

`javascript
const xml2json = require('xml2json')

jexl.addTransform('xml', (val) => xml2json.toJson(val, { object: true }))

const context = {
xmlDoc:


Cheryl
Tunt


Cyril
Figgis


}

var expr = 'xmlDoc|xml.Employees.Employee[.LastName == "Figgis"].FirstName'

jexl.eval(expr, context).then(console.log) // Output: Cyril
`

$3

While Transforms are the preferred way to change one value into another value,
Jexl also allows top-level expression functions to be defined. Use these to
provide access to functions that either don't require an input, or require
multiple equally-important inputs. They can be added with
jexl.addFunction(name, function). Like transforms, functions can return a
value, or a Promise that resolves to the resulting value.

`javascript
jexl.addFunction('min', Math.min)
jexl.addFunction('expensiveQuery', async () => db.runExpensiveQuery())
`

| Expression | Result |
| --------------------------------------------- | ------------------------- |
| min(4, 2, 19) | 2 |
| counts.missions || expensiveQuery() | Query only runs if needed |

$3

Variable contexts are straightforward Javascript objects that can be accessed
in the expression, but they have a hidden feature: they can include a Promise
object, and when that property is used, Jexl will wait for the Promise to
resolve and use that value!

API

$3

#### jexl.Jexl

A reference to the Jexl constructor. To maintain separate instances of Jexl
with each maintaining its own set of transforms, simply re-instantiate with
new jexl.Jexl().

#### jexl.addBinaryOp(_{string} operator_, _{number} precedence_, _{function} fn_, _{boolean} [manualEval]_)

Adds a binary operator to the Jexl instance. A binary operator is one that
considers the values on both its left and right, such as "+" or "==", in order
to calculate a result. The precedence determines the operator's position in the
order of operations (please refer to lib/grammar.js to see the precedence of
existing operators). The provided function will be called with two arguments:
a left value and a right value. It should return either the resulting value,
or a Promise that resolves to the resulting value.

If manualEval is true, the left and right arguments will be wrapped in
objects with an eval function. Calling left.eval() or right.eval() will
return a promise that resolves to that operand's actual value. This is useful to
conditionally evaluate operands, and is how && and || work.

#### jexl.addUnaryOp(_{string} operator_, _{function} fn_)

Adds a unary operator to the Jexl instance. A unary operator is one that
considers only the value on its right, such as "!", in order to calculate a
result. The provided function will be called with one argument: the value to
the operator's right. It should return either the resulting value, or a Promise
that resolves to the resulting value.

#### jexl.addFunction(_{string} name_, \_{function} func)

Adds an expression function to this Jexl instance. See the Functions
section above for information on the structure of an expression function.

#### jexl.addFunctions(_{{}} map_)

Adds multiple functions from a supplied map of function name to expression
function.

#### jexl.addTransform(_{string} name_, _{function} transform_)

Adds a transform function to this Jexl instance. See the Transforms
section above for information on the structure of a transform function.

#### jexl.addTransforms(_{{}} map_)

Adds multiple transforms from a supplied map of transform name to transform
function.

#### jexl.compile(_{string} expression_)

Constructs an Expression object around the given Jexl expression string.
Expression objects allow a Jexl expression to be compiled only once but
evaluated many times. See the Expression API below. Note that the only
difference between this function and jexl.createExpression is that this
function will immediately compile the expression, and throw any errors
associated with invalid expression syntax.

#### jexl.createExpression(_{string} expression_)

Constructs an Expression object around the given Jexl expression string.
Expression objects allow a Jexl expression to be compiled only once but
evaluated many times. See the Expression API below.

#### jexl.getTransform(_{string} name_)

Returns {function|undefined}. Gets a previously set transform function,
or undefined if no function of that name exists.

#### jexl.eval(_{string} expression_, _{{}} [context]_)

Returns {Promise<*>}. Evaluates an expression. The context map is optional.

#### jexl.evalSync(_{string} expression_, _{{}} [context]_)

Returns {*}. Evaluates an expression and returns the result. The context map
is optional.

#### jexl.expr: _tagged template literal_

A convenient bit of syntactic sugar for jexl.createExpression

`javascript
const someNumber = 10
const expression = jexl.expr5 + ${someNumber}
console.log(expression.evalSync()) // 15
`

Note that expr will stay bound to its associated Jexl instance even if it's
pulled out of context:

`javascript
const { expr } = jexl
jexl.addTransform('double', (val) => val * 2)
const expression = expr2|double
console.log(expression.evalSync()) // 4
`

#### jexl.removeOp(_{string} operator_)

Removes a binary or unary operator from the Jexl instance. For example, "^" can
be passed to eliminate the "power of" operator.

$3

Expression objects are created via jexl.createExpression, jexl.compile, or
jexl.expr, and are a convenient way to ensure jexl expressions compile only
once, even if they're evaluated multiple times.

#### expression.compile()

Returns self {Expression}. Forces the expression to compile, even if it
was compiled before. Note that each compile will happen with the latest grammar
and transforms from the associated Jexl instance.

#### expression.eval(_{{}} [context]_)

Returns {Promise<*>}. Evaluates the expression. The context map is
optional.

#### expression.evalSync(_{{}} [context]_)

Returns {*}. Evaluates the expression and returns the result. The context
map is optional.

Other implementations

PyJEXL - A Python-based JEXL parser and evaluator.

License

Jexl is licensed under the MIT license. Please see LICENSE.txt` for full details.

Credits

Created by Tom Shawver in 2015 and contributed to by these great people.

Jexl was originally created at TechnologyAdvice in Nashville, TN.