Design-by-Contract and Test-by-Contract for JavaScript

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Test-by-contract. Pre- and postcondition verification for Javascript.
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The web, nor node, are ready for ES6 modules at 2017-03-06.
This code is written for Nodejs. Browsers use is supported via browserify. Run
> npm run browserify
to create a browser distribution.


  • on Mac, during development, with Node 8
  • on Linux, with, via Travis Build Status
(https://travis-ci.org/Toryt/contracts), with - Node 8 - Node 10 - Node 12 - on the 'latest' versions of Chrome, Firefox, Safari, and Edge, on
- Windows 10
- macOS High Sierra
- on the 'latest' versions of the default browser, on
- the 'latest' version of iOS
- the 'latest' version of Android


Some workarounds were made to accommodate different Safari. This only concerns using Error stack traces to report where a contract has failed.
Safari support for traces is bad. First of all, Safari haphazardly skips frames in its stack trace. Second, there is a difference in the stack traces generated when using Safari 'live', and via Web Driver. In the latter case, no lines or columns are reported, and the stack contains a lot of empty lines. As a result, it is impossible to return a sensible location of contract failure consistently with Safari. A best effort will need to suffice.
Furthermore, Safari on iOS is very weird in adding a default prototype object to a non-arrow function. That should always be there, but experience shows it is sometimes 'late'. It seems to be added 'lazily'.
Environment detection is used in tests to exclude some conditions in some environments.
More extensive, and sadly fragile, browser detection is used in tests for Safari.


  • I
- I/1.0 : First release, minimally functional, Mac - node - I/2.0 : Use it in web projects, Mac - I/2.1 : Use it with node on linux (Travis) - I/2.2 : Now also tests with Intern on node, on Mac and Linux. Intern is preferred over Mocha. This version only
changes private code, test code, and build setup.
- I/2.3 : Cleanup, renaming, administration, license, fixing warnings - I/2.4 : Now also tests with Intern on Mac on Chrome, Safari, Firefox (3 failures, waiting for april version 53) - I/2.5 : Fix in behavior and test of util.browserModuleLocation - I/1.2.5 : Relax a test to support use of '(
  • II
  • - II/1.0 : First release on npm. The code is functional on node, and there is no need to wait for browser validation
    to start using it on node.
    - II/1.1 : Use scope in package name - II/1.2 : Cosmetic changes - II/2.1.3 : Relax a test to support use of '(
  • III
  • - III/3.0.0 : Now also supports contracts for (classic) constructors. In general, the prototype of an implementation
    is "forwarded" to the contract function. The ECMAScript2015 "class" construct is not yet supported (a "class"
    generated constructor cannot be `call`ed or `apply`ed). This is an incompatible change, since we now require more of
    a general contract function. Also, from now on, we will use versioning differently. Since npm insists on semantic
    versioning, the major version number will be the arabic numeral of the roman mark version. We now use `yarn`, but
    that is optional.
    - III/3.0.1 : Relax a test to support use of '(
  • IV
  • - IV/4.0.0:
    - CI is realised completely, with automated tests for LTS Node versions and all major browsers on all major
      platforms. We no longer use Bitbucket or Bitbucket Pipelines. Everything is done with GitHub, [travis], and
    - dropped direct support for older JS and browsers - now geared toward Node 6 and higher; use `browserify` to
      support browsers
    - fixed display name
    - support arrow functions
    - By default, only preconditions are tested. Postcondition testing must be enabled, for all contracts, a particular
      contract, or 1 implementation, by setting `contract.verifyPostconditions = true`. All testing can be disabled by
      setting `contract.verify = false`.
    - Support `Promise` and `async` functions through `PromiseContract` (with `Contract.Promise` as alias)
    - IV/4.0.1:
    - fix issue with stack trace of condition errors in Chrome ≥ v73
    - IV/4.0.2:
    - fix bug with circular structures in this (and other arguments) with util.inspect (It actually turns out there are
      2 separate, separate bugs in IntelliJ and mocha-reporter)
    - IV/4.0.3:
    - fix bug in determination of Contract Violation location on Windows. It turns out that Node (8, at least) in
      Windows uses \n as EOL in stack traces, and not os.EOL.
    - IV/4.0.4:
    - replace `must` with `should` in tests
    - IV/4.0.5:
    - fix missing EOLs - these where lost in the 4.0.4 rebase
    - IV/4.0.6:
    - test also on Node 12 and 13 (use 12 as dev env)
    - upgrade dependencies (dev)
    - fix test issue with async functions in Node 12 (a similar issue as with 4.0.1) (there is still a test issue with
      Node 12 and later when running tests in IntelliJ)
    - IV/4.0.7:
    - fix test issue with Node 12 and later when running tests in IntelliJ
    - IV/4.0.8:
    - fix bug in `isImplementedBy`: `cf.contract.isImplementedBy(cf)` did not work
    - IV/4.1.0:
    - support Symbols as arguments
    - contract function `name` is now a true property, with the same settings as the implementation function name, and
      no longer a derived property (in response to https://github.com/sinonjs/sinon/issues/2203)
    - IV/4.1.1:
    - ignore unnecessary files in distribution
    - IV/4.1.2:
    - fix bug in dealing with `arguments`, when they contain a Symbol


    • Upgrade to Mocha 8, but then we can no longer support Node 8

    • IV
    - IV/4.2.0: jsdoc or d.ts - IV/4.2.1: Documentation - IV/4.3.0: extend function contracts - specialization / generalization - IV/4.4.0: type conditions ('interfaces') - IV/4.5.0: support class
    • V
    - V/5.0.0: change to Joi-like conditions (conditions-per-argument) - V/5.1.0: old-support - V/5.2.0: test support Mocha