A contract library for JavaScript

npm install contracts-js
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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 is used in the CoffeeScript dialect but can also be used directly in normal JavaScript programs if you don't want to or can't use CoffeeScript.

This library is possible because of and requires Proxies which is a new feature of JavaScript that is currently only implemented in Firefox 4+ and chrome/V8 with the experimental javascript flag enabled (in about:flags or use the --harmony flag on the command line).


To use include these files:

  • src/stacktrace.js
  • lib/contracts.js

This adds a Contracts object to the global scope that has two properties Contracts.contracts (which contains some prebuilt contracts to use) and Contracts.combinators (which contains utility functions to build new contracts).

We can now wrap a function in a contract like so:

var C = contracts,
    id = C.guard(
, C.Num),
          function(x) { return x; });

id("foo"); // contract violation!

If you would like to load all of the combinators into the global scope, just run contracts.autoload().

More documentation and rational can be found at the sister project


Guards a value with a contract

Contracts.guard :: (Contract, Any, Str?, Str?) -> { use: () -> Any }
Contracts.guard(contract, value [, server[, client]])
  • contract the contract to apply to the value
  • value value to be wrapped in a contract
  • server optional name of the server "module"
  • client optional name of the client "module"


Creates a contract that checks first-order values (i.e. not functions or objects).

Contracts.check :: ((Any) -> Bool, Str) -> Contract
Contracts.check(predicate, name)
  • predicate function that takes a value and return true if the contract should pass or false otherwise
  • name name of the contract. Displayed in contract violation messages.

This is used to build contracts that get applied to values via the guard function. The guard function handles calling the predicate supplied to check at the appropriate time.

An example of a contract to check for numbers:

Contracts.check(function(x) { 
    return typeof(x) === 'number'; 
}, 'Number') :: (Contract or [...Contract], 
                                 ((Any) -> Contract) or Contract,
                                     callOnly: Bool
                                     newOnly: Bool
                                     pre: (Any) -> Bool
                                     post: (Any) -> Bool
                                     this: {...}
                                 }) -> Contract, range, options)
  • domain Either a single contract or an array of contracts for each argument to the function
  • range Either a single contract for the function's result or a function that returns a contract.
  • options An options object:
    • callOnly Signal a contract violation if new is used with the function
    • newOnly Signal a contract violation if new is not used with the function
    • pre A predicate to run before the function is run
    • post A predicate to run after the function is run
    • this An object contract to guard the this object

Dependent function contracts (where the result depends on the argument values) are handled by using a function as the range. When the function returns its argument values are first passed to the range function which should return a contract. This contract is then used to check the original function's result.

As a contrived example:, function(x) { 
    if(x === 42) {
        return Contracts.Num;
    } else {
        return Contracts.Str;

If the function contracted is called with 42 then its result must be a Num otherwise it must be a Str.

Note that arguments are potentially mutable (they might be one value at the beginning of the function and different when the function returns) so keep that in mind when using dependent contracts.


Contracts.object :: ({ ... }, 
                                        extensible: Bool
                                        sealed: Bool
                                        frozen: Bool
                                        invariant: (Any) -> Bool
                                    }) -> Contract
Contracts.object(object, options)
  • object An object with properties mapping to contracts that should be present in the contracted object
  • options An objects object:
    • extensible Object should be extensible
    • sealed Object should be sealed
    • frozen Object should be frozen
    • invariant Predicate to run each time the contracted object changes

Object contracts are built with an object that maps properties to objects. Example:

    foo: Str,
    bar: Num

In this case the contracted object must have both the foo and bar properties (if missing, a contract violation is thrown at contract application time) and these properties must abide by their respective contracts (which are checked each time the property is changed).

Object invariants can be checked with the invariant option. Whenever any property is changed the invariant function is called with a reference to the object. If the invariant returns false a contract violation is thrown.

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