Neurological Phenomenon Multiplexer


    0.6.3 • Public • Published


    Contracts.js is a contract library for JavaScript that allows you to specify invariants between parts of your code and have them checked at runtime for violations. It's heavily inspired by the contract system found in Racket and tracks blame correctly for higher-order values.

    For example, you can specify the that following function takes two arguments, one that is an object with a string name field and the other that is an array filled with objects that have a loc number field, and returns a string.

    import @ from "contracts.js"
    ({name: Str}, [...{loc: Num}]) -> Str
    function calcAverageLoc(person, locArr) {
        var sum = locArr.reduce(function (l1, l2) {
            return l1.loc + l2.loc;
        return "Average lines of code for " +
      + " was " +
               sum / locArr.length;

    If you call the function with a bad argument:

    var typoPerson = {nam: "Bob"};
    calcAverageLoc(typoPerson, [{loc: 1000}, {loc: 789}, {loc: 9001}]);

    you will get a helpful error message pin pointing what went wrong:

    calcAverageLoc: contract violation
    expected: Str
    given: undefined
    in: the name property of
        the 1st argument of
        ({name: Str}, [....{loc: Num}]) -> Str
    function calcAverageLoc guarded at line: 4
    blaming: (calling context for calcAverageLoc)

    You can play around with this and other examples on the homepage.


    Uses sweet.js which you can install via npm:

    npm install -g sweet.js
    npm install contracts-js


    At the top of your file you will need to use some special syntax to import contracts.js:

    import @ from "contracts.js"
    // rest of your code goes here...

    This looks like ES6 modules but it's not really and will work with whatever module system you are using (if any). See here for details.

    Compile your JavaScript file with sweet.js using the contracts.js module:

    sjs --module contracts-js/macros -o output.js input.js

    Then run your output.js file in any JavaScript environment. Some features of contracts.js (eg. proxied objects and arrays) require ES6 features which not every JavaScript engine supports right now (any recent version of Firefox is fine along with node.js/V8 with the --harmony flag enabled).


    Contracts.js is documented here.

    Related Work

    An initial stab at adding good contract syntax via macros to JavaScript was done in sweet-contracts.

    rho-contracts is a contract library for vanilla JavaScript in the same style as contracts.js (in the sense that both project trace their design inspiration to the higher-order contracts in Racket). While contracts.js can work in vanilla JS (using the guard wrapper function), rho-contracts probably has better ergonomics when used as a library with no special syntax support.

    TreatJS is another contract library for JavaScript.


    npm i contracts-js

    DownloadsWeekly Downloads






    Last publish


    • disnet