Instantly extend Chai.js and define custom assertions! We provide a simpler API to work with.

Chai Adhoc

Chai's API is designed to be used by plugins, and not optimzied for ad-hoc creation of custom assertions. This Chai plugin simplifies Chai's plugin API and makes it easier to create new assertions or modify existing ones in an ad-hoc way.

Do you find yourself creating functions to group tests and then use the function to test multiple subjects? Consider creating a custom assertion instead! By creating a custom assertion to accomplish tasks like this you gain the following benefits:

  1. Tests are uniform in style and easier to read.
  2. Since it's just an assertion, you can call it at the middle of a chain.

Why don't people do this? Because it's easier to create a function than defining a custom assertion. Not any more!

For plugin authors, the various helpers this plugin provides make plugin creation simpler and less error-prone, too. For example, Chai doesn't support overwriting chainable methods, but yet doesn't prevent you from doing it. The problem is, after overwriting a chainable method, the flag that is set by its property function will always be set even in newly created assertion objects, so Chai no longer works correctly (pure flag-setter properties like not don't have this problem). This plugin checks whether you are trying to overwrite a chainable method and doesn't allow you do it.


npm install chai-adhoc


Below are examples from Chai's plugin API documentation and their equivalents written using Chai Adhoc.

var chai = require('chai');
chai.use(function (chaiutils) {
    var Assertion = chai.Assertion;
    Assertion.addProperty('model', function () {
            this._obj instanceof Model
            , 'expected #{this} to be a Model'
            , 'expected #{this} to not be a Model'
    Assertion.addMethod('model', function (type) {
        var obj = this._obj;
        // first, our instanceof check, shortcut 
        new Assertion(this._obj);
        // second, our type check 
            obj._type === type
            , "expected #{this} to be of type #{exp} but got #{act}"
            , "expected #{this} to not be of type #{act}"
            , type        // expected 
            , obj._type   // actual 
    Assertion.overwriteProperty('ok', function (_super) {
        return function checkModel () {
            var obj = this._obj;
            if (obj && obj instanceof Model) {
                new Assertion(obj)''); // we always want this 
                var assertId = new Assertion(;
                utils.transferFlags(this, assertId, false); // false means don't transfer `object` flag 
            } else {
var chai = require('chai'),
    adhoc = require('chai-adhoc');
adhoc.addAssertion('model', function(ctx) {
        ctx.obj instanceof Model
        , 'expected #{this} to be a Model'
        , 'expected #{this} to not be a Model'
adhoc.addAssertion('model', function(ctxtype) {
    // first, our instanceof check, shortcut 
    // second, our type check 
        ctx.obj._type === type
        , "expected #{this} to be of type #{exp} but got #{act}"
        , "expected #{this} to not be of type #{act}"
        , type            // expected 
        , ctx.obj._type   // actual 
adhoc.extendAssertion('ok', function(ctx) {
    if (ctx.obj && ctx.obj instanceof Model) {
        return true;


This package contains 5 methods:

  1. addAssertion()

    Adds a new assertion, can be a property, a method, or a chainable method, depending on arguments passed to it.


    • name. Name of the new assertion.
    • func. Function to implement the assertion. If this function takes only one argument (the context object), a property is created; otherwise a method is created.
    • getter Optional. Function to be called when the assertion is accessed as a property. If set, a chainable method is created.

    More on func

    This function should take one or more arguments. The first argument ctx is an object containg context variables and methods to help you implement the assertion. Its members include:

    • obj. The subject of the assertion.
    • expect(). Creates an assertion object. The difference between it and chai.expect() is that its returned assertion object has an extra method flags(), which can be called to transfer all or specified flags from this to this new assertion object.
    • assert(): an alias of this.assert().
    • flag(): an alias of utils.flag(), with its first argument bound to this.

    Any additional arguments are considered params to the assertion function. For example, a property would take none, a method like above() would take 1, and a method like within() would take 2, etc.

    Inside func, this points to the assertion object on which this assertion is called, just like in Chai's plugin API.

    More on getter

    This function is used when defining a chainable methods, to be called whenever the property is accessed. Chai officially recommends only setting flags in this function, so it takes only one argument: flag, which is an alias of utils.flag(), with its first argument bound to this.

  2. addSimple()

    Adds a simple assertion, can be either a property or a method, cannot be a chainable method.

    One of the biggest annoyances when implementing assertions is having to deal with the 'negate' flag. This is the biggest obstacle when trying to use custom assertions in place of your test functions.

    The philosophy of this method is: if we will never use the assertion with a 'negate' flag anyway, why bother supporting it? Assertions defined using this method simply fail if the 'negate' flag is present.

    Otherwise it's the same as addAssertion(), other than the fact that it doesn't support creating chainable methods, and therefore only takes 2 arguments: name and func.

  3. extendAssertion()

    Extends an existing assertion, either a property or a method, cannot extend a chainable method, which is a restriction of Chai.

    It takes 2 arguments: name and func. name is the name of the assertion to extend, it must already exist.

    For func, see its description in addAssertion()'s section. There is one difference, though: its return value is important, very important, here. It must return truthy if the assertion has been evaluated for the subject, in which case work for this assertion is considered done; or falsy if it hasn't, in which case the original function for this assertion (_super()) is called. So you can extend an existing assertion and provide an alternative implementation only for some situations; in other situations, you return a falsy value and the default implementation of the assertion is used.

  4. wrapAssertion()

    Wraps an existing assertion, either a property or a method, cannot extend a chainable method, which is a restriction of Chai.

    Both this method and extendAssertion() calls Chai's overwriteProperty() and overwriteMethod() API methods to overwrite existing assertions. So what is the difference?

    The difference is that extendAssertion() is called before _super() is called, to allow you handle certain types of operands differently than _super(); in contrast, this method is called after _super() is called, to allow you handle _super()'s result. For example, if _super() throws an error, you can modify its message, or even suppress it if so desired.

    It takes 2 arguments: name and func. name is the name of the assertion to wrap, it must already exist.

    For func, it's again similar to that described in addAssertion()'s section, but this time there are two differences:

    • Its first argument is error, which is either an error object if the assertion failed (remember the assertion is evaluated before calling this function), or undefined if the assertion succeeded. After this it takes ctx and any parameters passed to the assertion function, just like before.
    • The return value is very important. If it returns falsy, and if the assertion failed, the error object will be rethrown. Return truthy if you want to suppress the error, but be careful not to accidentally return truthy and suppress the error!

    So if the assertion failed, you have three choices in func:

    • Throw error (after perhaps changing its properties), or throw a new Error object.
    • Return falsy (after perhaps changing error's properties).
    • Return truthy.

    In the first two cases, an error is thrown, the assertion fails; in the last case, the error is suppressed, and the assertion succeeds.

  5. format()

    Utility method to help you format error messages. The first argument is the string to format: it can contain special sequences #{1}, #{2}, etc, which will be replaced with the first, second, etc, argument passed to the method after the string, respectively.

    Note that util.inspect() is called on each argument before it's used to replace its corresponding placeholder in the string to format.


MIT License