TomFrostJexl 
Javascript Expression Language: Powerful context-based expression parser and evaluator
Quick start
Use it with promises or synchronously:
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 --saveor yarn:
yarn add jexland 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
Unary Operators
| Operation | Symbol |
|---|---|
| Negate | ! |
Binary Operators
| Operation | Symbol |
|---|---|
| Add, Concat | + |
| Subtract | - |
| Multiply | * |
| Divide | / |
| Divide and floor | // |
| Modulus | % |
| Power of | ^ |
| Logical AND | && |
| Logical OR | || |
Comparisons
| 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'}].
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!' |
| Numerics | 6, -7.2, 5, -3.14159 |
| Objects | {hello: "world!"} |
| Arrays | ['hello', 'world!'] |
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:
{
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:
{
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, 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).
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:
const xml2json = require('xml2json')
jexl.addTransform('xml', (val) => xml2json.toJson(val, { object: true }))
const context = {
xmlDoc: `
<Employees>
<Employee>
<FirstName>Cheryl</FirstName>
<LastName>Tunt</LastName>
</Employee>
<Employee>
<FirstName>Cyril</FirstName>
<LastName>Figgis</LastName>
</Employee>
</Employees>`
}
var expr = 'xmlDoc|xml.Employees.Employee[.LastName == "Figgis"].FirstName'
jexl.eval(expr, context).then(console.log) // Output: CyrilFunctions
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.
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 |
Context
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
Jexl
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
const someNumber = 10
const expression = jexl.expr`5 + ${someNumber}`
console.log(expression.evalSync()) // 15Note that expr will stay bound to its associated Jexl instance even if it's
pulled out of context:
const { expr } = jexl
jexl.addTransform('double', (val) => val * 2)
const expression = expr`2|double`
console.log(expression.evalSync()) // 4jexl.removeOp({string} operator)
Removes a binary or unary operator from the Jexl instance. For example, "^" can be passed to eliminate the "power of" operator.
Expression
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.