@jbrowse/jexl

@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
• Added lambdas and collection functions (any(xs, x => x > 1))
• Added host hooks for member and name resolution
• Added analyze(), which reports what an expression reads without running it
• Added check(), which checks an expression against a host's fields and functions, and print(), which renders a parsed expression back to text
• Added bcftools-style list operators, regex matching (~) and aggregates

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
Null	null
Strings	"Hello \"user\"", 'Hey there!'
Template Strings	`Hello ${name}`, `Total: ${price * qty}`
Numerics	6, -7.2, .5, 1e3, -5e-8
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: ! (logical negation), - (arithmetic negation)

Binary:

• Arithmetic: +, -, *, /, // (floor division), %, ^ (power)
• Comparison: ==, !=, >, >=, <, <=, in
• Regular expression match: ~, !~ (name ~ '^BRCA'; a leading (?i) ignores case)
• Logical: &&, ||, ?? (a ?? b is b only when a is null or undefined, so score ?? 0 keeps a real 0)
• Assignment: = (assigns a value to a bare variable name; a.b = 1 is not supported)

Ternary: condition ? consequent : alternate

Sequence: ; (separates multiple expressions)

From loosest to tightest:

Operators	Groups
;
=, lambdas	right to left
? :	right to left
||, ??	left to right
&&	left to right
==, !=, >, >=, <, <=, in, ~, !~	left to right
+, -	left to right
*, /, //	left to right
%, ^	^ from the right
prefix !, -
., [], calls	left to right

As in JavaScript, ?? does not mix with && or || without parentheses: a ?? b || c is a syntax error, (a ?? b) || c is not.

Lists

Operators read a list the way bcftools reads a multi-valued VCF tag.

A comparison holds when it holds for any value of the list, and != and !~ hold when no value matches, so each stays the negation of == and ~:

const context = {
  AF: [0.01, 0.2],
  FILTER: ['PASS'],
  CSQ: ['T|missense_variant|MODERATE', 'T|synonymous_variant|LOW']
}

jexl.eval('AF > 0.05', context) // true: the second allele's frequency is
jexl.eval("FILTER != 'PASS'", context) // false
jexl.eval("CSQ ~ 'missense_variant'", context) // true
jexl.eval("['DEL'] in ['DEL', 'INS']") // true

Arithmetic pairs lists value by value, and a single value, or a list of one, pairs with every value of the other side:

jexl.eval('AC / AN', { AC: [2, 4], AN: [10] }) // [0.2, 0.4]
jexl.eval('DP + 1', { DP: [25] }) // [26]

in finds a substring of a string, a member of a list or Set, or a key of an object or Map, so a host can hand an expression a gene list loaded from a file as a Set: name in genes.

sum, mean, median, min and max aggregate a list, reading each value through an optional lambda. They skip missing values, and count a boolean as 1 or 0, which makes mean of a test the fraction that pass:

jexl.eval('min(DV / DP) > 0.3', context) // bcftools MIN(DV/DP)>0.3
jexl.eval('count(samples, s => s.GQ > 90)', context) // bcftools N_PASS(GQ>90)
jexl.eval('mean(samples, s => s.GQ > 90)', context) // bcftools F_PASS(GQ>90)

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"
jexl.eval('exes.length', context) // 3

A name chained off an array reads the array itself, so exes.length is 3 and list.name is undefined; index the element you mean, list[0].name.

Functions

Call functions in expressions:

jexl.addFunction('log10', Math.log10)
jexl.addFunction('round', Math.round)

jexl.eval('round(log10(1234))')
// 3

jexl.eval('-log10(pvalue) > threshold', { pvalue: 1e-9, threshold: 8 })
// true

Functions live in one global pool and have no receiver, so a call written against a value passes that value as the first argument: a.b(x) means b(a, x). Since functions are conventionally written to take their subject first, the two spellings read differently but do the same thing:

jexl.addFunction('split', (str, sep) => String(str ?? '').split(sep))

jexl.eval("refName.split(' ')[0]", { refName: 'chr1 description' }) // "chr1"
jexl.eval("split(refName, ' ')[0]", { refName: 'chr1 description' }) // "chr1"

Note that this is a naming convention, not method dispatch: a.b(x) calls the function named b in the pool, never a method on the value of a.

Lambdas

x => body and (a, b) => body are functions a registered function can call. A parameter shadows the context variable of the same name; any other name reads the context. A lambda body cannot assign.

Jexl registers any, all, count, map, filter, find, sort and reduce, each taking a list and a lambda, and the aggregates above. A lone value counts as a list of one and a missing value as an empty list, which suits VCF INFO fields:

jexl.eval('any(feature.INFO.AF, af => af > 0.05)', context)
jexl.eval('map(xs, (x, i) => x * i)', { xs: [1, 2, 3] }) // [0, 2, 6]
jexl.eval('xs.filter(x => x > 1)', { xs: [1, 2, 3] }) // [2, 3]

A host function of the same name replaces the built-in one.

Variable Assignment

Assign values to variables using = (no let, var, or const needed). An assignment returns the assigned value, and the variable lasts for the rest of that evaluation. The context is never written to, so one context object can be reused across evaluations:

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

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

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

const context = { x: 1 }
jexl.eval('y = x * 2; x = y + 1; x', context)
// 3
// context is still { x: 1 }

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

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({
  abs: Math.abs,
  sqrt: Math.sqrt
})

// Use in expressions
jexl.eval('round(3.7)') // 4
jexl.eval('lower(name)', { name: 'HELLO' }) // "hello"
jexl.eval('sqrt(abs(-16))') // 4

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

Resolving Names Through the Host

When the values in a context keep their fields behind an accessor, a Jexl instance can resolve names itself instead of reading plain properties. Both hooks are fixed when the instance is constructed.

import { Jexl } from '@jbrowse/jexl'

const jexl = new Jexl({
  // `a.b` and `a[k]`, on any value that isn't null or undefined
  getMember: (subject, key) =>
    subject instanceof Feature ? subject.get(key) : subject[key],
  // a bare name, asked once per name as an expression compiles; returning
  // undefined keeps the plain `context[name]` read
  variableReader: (name) =>
    name === 'feature'
      ? undefined
      : (context) => context.feature.get(name) ?? context[name]
})

const expr = jexl.compile('feature.score > 10 && -log10(pvalue) > 2')
const context = { feature: undefined }
for (const feature of features) {
  context.feature = feature
  expr.eval(context)
}

A name the expression assigns reads the assignment once it has run, and goes through variableReader before then.

Analyzing Expressions

analyze reports what an expression reads, without evaluating it or needing its functions registered:

import { analyze } from '@jbrowse/jexl'

const ast = jexl.parse("get(feature, 'score') > 10 ? feature.name : 'n/a'")
analyze(ast, { accessors: { get: [] }, row: 'feature' })
// {
//   variables: ['feature'],
//   fields: [{ path: ['score'] }, { path: ['name'] }],
//   bare: false,
//   ...
// }

accessors names functions that read a path off their first argument, so get(feature, 'score') and feature.score both read the field score. An expression with no variables is a constant; one whose bare is true is only a path, which a host can read without jexl.

License

MIT License, same as TomFrost/Jexl

Publishing

Trusted publishing via GitHub Actions.

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