expressions

Heximal Expressions

An expression syntax, parser, and evaluator for JavaScript-like expressions.

Part of the Heximal project.

Overview

Heximal Expressions is designed for libraries that evaluate user-written expressions, such as HTML templating engines. Heximal Expressions has a relatively rich syntax, supporting identifiers, operators, property access, method and function calls, and literals (including arrays and objects), function literals, assignments, and pipes.

Example:

(person.title + ' ' + person.getFullName()) | uppercase;

Usage

Installation

npm i @heximal/expressions

Usage

import {parse, EvalAstFactory} from '@heximal/expressions';

// An EvalAstFactory produces an AST that can be evaluated
const astFactory = new EvalAstFactory();

// parse() returns the AST
const expr = parse('(a + b([1, 2, 3]) * c)', astFactory);

// evaluate() with a scope object
const result = expr.evaluate({
  a: 42,
  b: (o: Array<number>) => o.length,
  c: 2,
});

console.log(result); // 48

Features

Fast, small parser

The Heximal Expressions parser is a hand-written, recursive descent, precedence-climbing parser. It's simple, fast and small.

Pluggable AST factories

parse() takes an AST factory so that different strategies can be used to produce ASTs. The default factory creates an AST as defined in lib/ast.js. lib/eval.js exports an EvalAstFactory that produces evaluatable ASTs.

Null-Safety

Expressions are generally null-safe. If a subexpression yields null or undefined, subsequent property access will return null, rather than throwing an exception. Property access, method invocation and operators are null-safe. Passing null to a function that doesn't handle null will not be null safe.

Syntax

Property access

Properties on the model and in the scope are looked up via simple property names, like foo. Property names are looked up first in the top-level variables, next in the model, then recursively in parent scopes. Properties on objects can be access with dot notation like foo.bar.

The keyword this always refers to the model if there is one, otherwise this is null. If you have model properties and top-level variables with the same name, you can use this to refer to the model property.

Literals

Heximal Expressions supports number, boolean, string, and map literals. Strings can use either single or double quotes.

• null and undefined
• Numbers: 1, 1.0
• Booleans: true, false
• Strings: 'abc', "xyz"
• Objects: { 'a': 1, 'b': 2 }
• Arrays: [1, 2, 3]

Function and method calls

If a property is a function in the scope, a method on the model, or a method on an object, it can be invoked with standard function syntax. Functions and Methods can take arguments. Arguments can be literals or variables.

Examples:

• Top-level function: myFunction()
• Top-level function with arguments: myFunction(a, b, 42)
• Model method: aMethod()
• Method on nested-property: a.b.anotherMethod()

Operators

Heximal Expressions supports the following binary and unary operators:

• Assignment: =
• Arithmetic operators: +, -, *, /, %, unary + and -
• Comparison operators: ==, !=, ===, !==, <=, <, >, >=
• Boolean operators: &&, ||, unary !
• Nullish coalescing: ??
• Pipeline operators: | (legacy) and |> (modern)

Expressions do not support bitwise operators such as &, |, << and >>, or increment/decrement operators (++ and --)

Assignment

The left-hand-side expression of the assignment operator (=) must be one of an ID, getter or setter, otherwise an exception is thrown.

Maps

Maps are sets of key/value pairs. The key can either be a quoted string, or an identifier:

Examples:

• {'a': 1, 'b': 2}
• {a: 1, b: 2}

Array and Object indexing

Arrays and objects can be accessed via the index operator: []

Examples:

• items[2]
• people['john']

Function expressions

Functions can be written with the arrow function syntax.

Examples:

• () => 3
• (a, b) => a + b

Filters and transformers

A filter is a function that transforms a value into another, used via the pipe syntax: value | filter Any function that takes exactly one argument can be used as a filter.

Example:

If person.name is "John", and a top-level function named uppercase has been registered, then person.name | uppercase will have the value "JOHN".

The pipe syntax is used rather than a regular function call so that we can support two-way bindings through transformers. A transformer is a filter that has an inverse function. Two-way transformers are not supported yet.

Acknowledgements

Heximal Expressions was moved and renamed from Jexpr, which itself was forked from polymer-expressions, which is a JavaScript port of a Dart library by the same name.

Both polymer-expressions implementations are no longer officially maintained by Google. Jexpr was moved to make it part of the Heximal project and monorepo.