Add context awareness to your apps and frameworks by safely evaluating user-defined conditional expressions. Useful for evaluating expressions in config files, prompts, key bindings, completions, templates, and many other user cases.
npm install whence> Add context awareness to your apps and frameworks by safely evaluating user-defined conditional expressions. Useful for evaluating expressions in config files, prompts, key bindings, completions, templates, and many other user cases.
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Install with npm:
``sh`
$ npm install --save whence
This libarary doest returneth true if thine 'when' clause doest matcheth the granted context object.
Seriously though, what does this library do?
Whence uses eval-estree-expression to _safely evaluate_ user-defined conditional expressions, sometimes referred to as "when" clauses.
Why do I need this?
Add _context awareness_ to your apps and frameworks.
Conditional expressions are useful in config files, creating prompts, determining key bindings, filtering suggestions and completions, variables in templates and snippets, and many other user cases.
It's even more useful when those conditional expressions can be evaluated safely.
Example: configuration files
For example, when authoring configuration files for workflows, pipelines, builds, and so on, it's common for developers to define expressions with conditionals to determine if or when a job, task, or step should run based on environment variables, etc. These configurations are typically defined using YAML, JSON or a similar data format, which means that conditional expressions must be written as strings, booleans, or numbers. Whence makes it safe and easy to evaluate these expressions.
Other use cases
* Templates and snippets - Use whence to conditionally render files, sections, or variables
* Completions and suggestions - Use whence to filter completions and suggestions in your text editor or prompt system
* Key bindings - VS Code and other text editors use when clauses or something similar to determine the keybindings to use when a key is pressed.
How safe is it?
No assignment operators, functions, or function calls are allowed by default to make it as safe as possible to evaluate user-defined expressions. To accomplish this, whence uses the eval-estree-expression library, which takes an estree expression from [@babel/parser][], esprima, acorn, or any similar library that parses and returns a valid estree expression.
Why another "eval" library?
What we found
Every other eval library I found had one of the following shortcomings:
* Uses eval or Node's vm or something similar to evaluate code. This is to risky, or too heavy for our use cases.
* Functions are either the primary use case or are supported by default. We don't want users to be able to define functions in their config files.
* Naive attempts to sanitize code before evaluating it
* Brittle, incomplete, hand-rolled parsers
What whence does differently
* Whence takes a valid [estree][] AST for an _expression_, not statements, functions, etc.
* Although functions are not supported by default, you can enable support if you really need it (see the eval-estree-expression docs for more details)
* Special care was taken in eval-estree-expression to disallow assignment operators, functions, or other potentially malicious code, like setting __proto__, constructor, prototype, or undefined as property names on nested properties.
`js
const whence = require('whence');
// async usage
console.log(await whence('name =~ /^d.*b$/', { name: 'doowb' })); //=> true
console.log(await whence('amount > 100', { amount: 101 })); //=> true
console.log(await whence('a < b && c > d', { a: 0, b: 1, c: 3, d: 2 })); //=> true
console.log(await whence('platform === "darwin"', { platform: process.platform })); //=> true if macOS
console.log(await whence('platform === "darwin"', { platform: 'win32' })); //=> false
// sync usage
console.log(whence.sync('name =~ /^d.*b$/', { name: 'doowb' })); //=> true
console.log(whence.sync('amount > 100', { amount: 101 })); //=> true
console.log(whence.sync('a < b && c > d', { a: 0, b: 1, c: 3, d: 2 })); //=> true
console.log(whence.sync('platform === "darwin"', { platform: process.platform })); //=> true if macOS
console.log(whence.sync('platform === "darwin"', { platform: 'win32' })); //=> false
`
See eval-estree-expression and that project's unit tests for many more examples of the types of expressions that are supported.
Whence's default behavior (and purpose) is to return a boolean. Most implementors will be interested in this library for that reason. However, if you need the evaluated result and do not want values to be cast to booleans, you should probably use eval-estree-expression directly. For example:
`js
// whence behavior
console.log(whence.sync('1 + 9')); //=> true
// eval-estree-expression behavior
console.log(whence.sync('1 + 9')); //=> 10
`
Returns true if the given value is truthy, or the value ("left") iscontext
equal to or contained within the ("right") value. This method iswhence()
used by the function (the main export), but you can use this
method directly if you don't want the values to be evaluated.
Params
* value {any}: The value to test.context
* {Object}: The value to compare against.parent
* {type}returns
* {Boolean}: Returns true or false.
Parses the given expression string with [@babel/parser][] and returns and AST. You may also an [estree][]-compatible expression AST.
Params
* source {String}: Expression string or an [estree][]-compatible expression AST.options
* {Object}returns
* {Object}
Example
`js
const { parse } = require('whence');
console.log(parse('platform === "darwin"'));
// Resuls in something like this:
// Node {
// type: 'BinaryExpression',
// value: Node { type: 'Identifier', name: 'platform' },
// operator: '===',
// context: Node {
// type: 'StringLiteral',
// extra: { rawValue: 'darwin', raw: '"darwin"' },
// value: 'darwin'
// }
// }
`
Asynchronously evaluates the given expression and returns a boolean.
Params
* source {String|Object}: Expression string or an [estree][]-compatible expression AST.context
* {Object}options
* {Object}returns
* {Boolean}
Example
`js
const whence = require('whence');
console.log(await whence('10 < 20')); //=> true
console.log(whence.sync('10 < 20')); //=> true
`
Synchronous version of whence. Aliased as whence.sync().
Params
* source {String|Object}: Expression string or an [estree][]-compatible expression AST.context
* {Object}options
* {Object}returns
* {Boolean}
Example
`js
const { whenceSync } = require('whence');
console.log(whenceSync('10 < 20')); //=> true
`
Compiles the given expression and returns an async function.
Params
* source {String|Object}: Expression string or an [estree][]-compatible expression AST.options
* {Object}returns
* {Function}: Returns a function that takes a context object.
Example
`js
const { compile } = require('whence');
const fn = compile('type === "foo"');
console.log(await fn({ type: 'foo' })); //=> true
console.log(await fn({ type: 'bar' })); //=> false
`
Synchronous version of compile. This method is also alias as .compile.sync().
Params
* source {String|Object}: Expression string or an [estree][]-compatible expression AST.options
* {Object}returns
* {Function}: Returns a function that takes a context object.
Example
`js
const { compile } = require('whence');
const fn = compile.sync('type === "foo"');
console.log(fn({ type: 'foo' })); //=> true
console.log(fn({ type: 'bar' })); //=> false
`
Supports all options from eval-estree-expression.
Although whence doesn't like functions...
`js`
console.log(whence.sync('/[a-c]+/.test(foo)', { foo: 'bbb' })); //=> throws an error
You can talk whence into evaluating them by setting the functions option to true.
`js`
console.log(whence.sync('/[a-c]+/.test(foo)', { foo: 'bbb' }, { functions: true })); //=> true
console.log(whence.sync('/[a-c]+/.test(foo)', { foo: 'zzz' }, { functions: true })); //=> false
Contributing
Pull requests and stars are always welcome. For bugs and feature requests, please create an issue.
Running Tests
Running and reviewing unit tests is a great way to get familiarized with a library and its API. You can install dependencies and run tests with the following command:
`sh`
$ npm install && npm test
Building docs
_(This project's readme.md is generated by verb, please don't edit the readme directly. Any changes to the readme must be made in the .verb.md readme template.)_
To generate the readme, run the following command:
`sh``
$ npm install -g verbose/verb#dev verb-generate-readme && verb
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Copyright © 2025, Jon Schlinkert.
Released under the MIT License.
*
_This file was generated by verb-generate-readme, v0.8.0, on February 07, 2025._