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@doars/interpret

Interpret a subset JavaScript expression without using the eval function or Function constructor. Allowing it to be used in combination with a strict Content Security Policy that does not contain the unsafe-eval option.

The interpreter is written for the @doars/doars library, but can be used elsewhere as well. The features it support are meant to be simple and not allow for much complexity similar to what a formulae in a spreadsheet can do.

Even though the library does not use the eval function or Function constructor security is still an important concern when interpreting any code. Do not provide any functions via the context parameter that could cause harm, and you should not run any expression that might contain user input. So do take the accompanying risks into consideration before using this library.

Install

From NPM

Install the package from NPM, then import and use it.

npm i @doars/interpret

// Import library.
import { interpret, parse, run } from '@doars/interpret'

// Interpret expression.
const resultOne = interpret(
  '(hello == 3) ? "there" : general', // Expression.
  { hello: 4, general: 'kenobi' } // Context.
)
// resultOne = 'kenobi'

// Or interpret in separate steps.
// Parse the expression first.
const node = parse('(hello == 3) ? "there" : general')
// Then run the node.
const resultTwo = run(node, { hello: 4, general: 'kenobi' })
// resultTwo = 'kenobi'

API

Exported functions:

• interpret Interpret an expression.
  • @param {string} expression Expression to interpret.
  • @param {Object} context Context of the expression.
  • @returns {Array} results of the expression.
• parse Parse an expression.
  • @param {string} expression Expression to parse.
  • @returns {Object} The parsed expression.
• run Run a parsed expression.
  • @param {Object} node Parsed expression.
  • @param {Object} context Context of the expression.
  • @returns {Array} results of the expression.

The following node types are exported as variables: ARRAY, ASSIGN, BINARY, CALL, CONDITION, IDENTIFIER, LITERAL, MEMBER, OBJECT, PROPERTY, SEQUENCE, UNARY, UPDATE.

interpret is simply a short hand for run(parse(expression), context).

Supported features

The interpret does not support all JavaScript features. However any expression valid to be run by this library should also be valid JavaScript code. That being said the interpreter might ignore some syntax errors that are usually not allowed.

• Identifiers and member access: hello, hello.there, hello[there] and hello['there']. Any identifiers need te be given via the context parameter when running the expression.
• Function calls: hello(), hello(there) and hello('there', 'general', 'kenobi'). Any functions need te be given via the context parameter when running the expression.
• Multiple clauses: hello(); world(). The result of each expression is returned, hence the interpret and run functions always return an array.

As well as several value types and most operators. See an overview below for more information.

Value types

• Null: null.
• Undefined: undefined.
• Booleans: false and true.
• Strings: 'hello' and "there".
• Numbers: 1 and 12.3.
• Arrays: [], ['hello'] ['hello', 'there'].
• Objects: {}, { hello: 'there' }, { hello: 'there', general: 'kenobi' }, { [hello]: 'there' }, { hello } and { hello, there }.

Operators

• Arithmetic: 2 ** 3, as well as *, /, %, +, and -.
• Logical: false || true, as well as && and ??.
• Equality: true == false, as well as !=, ===, and !==.
• Relation: 1 > 0, as well as >, <=, and >=.
• Ternary: true ? 0 : 1.
• Unary: +1 as well as -1 and !false.
• Decrement and increment: --hello as well as hello--, ++hello and hello++.
• Assignment: hello = 'there'.
• Arithmetic assignment: hello **= 2 as well as *=, /=, %=, +=, and -=.
• Logical assignment: hello ||= 'there' as well as &&= and ??=.

Known issues

• Unable to define objects in objects: { hello: {} }.