@jbrowse/jexl

A fork of the TomFrost/jexl minimal expression lang for jbrowse

Major changes include

• Removing async
• Remove 'transforms' (use functions instead)
• Remove array filtering expressions
• Added template strings
• Added multiple expression evaluation

Quick Examples

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

// Template strings with interpolation
jexl.eval('`Hello ${name.first} ${name.last}`', context)
// "Hello Sterling Archer"

jexl.eval('`Age in 5 years: ${age + 5}`', context)
// "Age in 5 years: 41"

// Math operations
jexl.eval('age * (3 - 1)', context)
// 72

// String concatenation
jexl.eval('name.first + " " + name.last', context)
// "Sterling Archer"

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

// Array indexes
jexl.eval('assoc[1].first', context)
// "Cyril"

Language Reference

Native Types

Type	Examples
Booleans	true, false
Strings	"Hello \"user\"", 'Hey there!'
Template Strings	`Hello ${name}`, `Total: ${price * qty}`
Numerics	6, -7.2, 5, -3.14159
Objects	{hello: "world!"}
Arrays	['hello', 'world!']

Template Strings

Template strings use backticks and support expression interpolation with ${}:

const context = { name: 'World', price: 10, qty: 3 }

jexl.eval('`Hello ${name}!`', context)
// "Hello World!"

jexl.eval('`Total: $${price * qty}`', context)
// "Total: $30"

// Escape backticks and dollar signs with backslash
jexl.eval('`Code: \\`example\\``')
// "Code: `example`"

jexl.eval('`Price: \\$100`')
// "Price: $100"

Operators

Unary: ! (negate)

Binary:

• Arithmetic: +, -, *, /, // (floor division), %, ^ (power)
• Comparison: ==, !=, >, >=, <, <=, in
• Logical: &&, ||
• Assignment: = (assigns value to variable)

Ternary: condition ? consequent : alternate

Sequence: ; (separates multiple expressions)

Identifiers

Access context variables by name. Use dot notation or brackets for nested properties:

const context = {
  name: { first: 'Malory', last: 'Archer' },
  exes: ['Nikolai', 'Len', 'Burt'],
  lastEx: 2
}

jexl.eval('name.first', context) // "Malory"
jexl.eval('name["last"]', context) // "Archer"
jexl.eval('exes[2]', context) // "Burt"
jexl.eval('exes[lastEx - 1]', context) // "Len"

Functions

Call functions in expressions:

jexl.addFunction('min', Math.min)
jexl.addFunction('max', Math.max)

jexl.eval('min(5, 2, 9)')
// 2

jexl.eval('max(temperature, threshold)')
// evaluates with context

Multiple Expressions

Separate multiple expressions with semicolons. The result is the value of the last expression:

jexl.eval('5; 10; 15')
// 15

jexl.eval('1 + 1; 2 + 2; 3 + 3')
// 6

Variable Assignment

Assign values to variables using = (no let, var, or const needed). Assignments mutate the context and return the assigned value:

jexl.eval('x = 5')
// 5

jexl.eval('x = 5; x * 2')
// 10

jexl.eval('x = 5; y = 10; x + y')
// 15

const context = {}
jexl.eval('x = 5; y = x * 2; y', context)
// 10
// context is now { x: 5, y: 10 }

API

Evaluation

import jexl from '@jbrowse/jexl'

// Evaluate an expression
const result = jexl.eval('expression', context)

// Compile once, evaluate many times
const expr = jexl.compile('name.first + " " + name.last')
const result = expr.eval({ name: { first: 'John', last: 'Doe' } })

Adding Custom Functions

// Add a single function
jexl.addFunction('round', Math.round)
jexl.addFunction('lower', (str) => str.toLowerCase())

// Add multiple functions
jexl.addFunctions({
  min: Math.min,
  max: Math.max,
  abs: Math.abs
})

// Use in expressions
jexl.eval('round(3.7)') // 4
jexl.eval('lower(name)', { name: 'HELLO' }) // "hello"
jexl.eval('max(1, 5, 3)') // 5

Adding Custom Operators

// Add a binary operator
jexl.addBinaryOp(
  '~=',
  20,
  (left, right) => left.toLowerCase() === right.toLowerCase()
)

jexl.eval('"Hello" ~= "hello"') // true

// Add a unary operator
jexl.addUnaryOp('~', (right) => Math.floor(right))

jexl.eval('~3.7') // 3

License

MIT License, same as TomFrost/Jexl