Types

Unexpected comes with a type system that is used to explain how different types are compared, diffed, inspected and is also used to limit the scope of assertions.

The following types are provided by out of the box by Unexpected: any, arguments, array, array-like, binaryArray, boolean, Buffer, date, Error, function, Int8Array, Int16Array, Int32Array, NaN, null, number, object, regexp, string, Uint8Array, Uint16Array, Uint32Array, and undefined.

expect.addType(typeDefinition)

Unexpected can be extended with knowledge about new types by calling the addType method with a type definition. The type definition must implement the required parts of the following interface:

Required members:

• name: String - the name of the type.
• identify: boolean function(value) - a function deciding if the type should be used for the given value.

Note that your type has the option to take precedence over all the built-in types. Test subjects will be tested against the most recently defined type first, so identify functions should take care not to break with undefined, null and so on.

Optional members:

• base: String - the name of the base type. Defaults to any.
• equal: boolean function(a, b, equal) - a function capable of comparing two values of this type for equality. If not specified it is inherited from the base type.
• inspect: function(value, depth, output, inspect) - a function capable of inspecting a value of this type. If not specified it is inherited from the base type.
• diff: comparison function(actual, expected, output, diff, inspect) - a function producing a comparison between two values of this type. If not specified it is inherited from the base type.

Example

Adding new types to the system is best explained by an example. Let's say we wanted to add first class support for a Person type:

function Person(name, age) {
  this.name = name;
  this.age = age;
}

We start out by creating a basic type for handling Person instances. The name of the type should be Person and it should inherit from the built-in object type. Furthermore we add an identify method that will recognize Person instances.

expect.addType({
  name: 'Person',
  base: 'object',
  identify: function (value) {
    return value instanceof Person;
  },
});

When you specify a base type, you inherit the optional members you didn't implement. In this case we inherited the methods equal, inspect and diff from the object type.

Imagine that we make a failing expectation on a Person instance:

expect(new Person('John Doe', 42), 'to equal', new Person('Jane Doe', 24));

expected Person({ name: 'John Doe', age: 42 })
to equal Person({ name: 'Jane Doe', age: 24 })
 
Person({
  
name: 
'John Doe',
 
//
//
//
//
 
should equal 
'Jane Doe'
 
John Doe
Jane Doe
  
age: 
42
 
//
 
should equal 
24
})

That is already quite helpful, but it would be even nicer if the stringification of Person instances could read as valid calls to the constructor. We can fix that by implementing an inspect method on the type.

function Person(name, age) {
  this.name = name;
  this.age = age;
}
 
expect.addType({
  name: 'Person',
  base: 'object',
  identify: function (value) {
    return value instanceof Person;
  },
  inspect: function (person, depth, output, inspect) {
    output
      .text('new Person(')
      .append(inspect(person.name, depth))
      .text(', ')
      .append(inspect(person.age, depth))
      .text(')');
  },
});
 
expect(new Person('John Doe', 42), 'to equal', new Person('Jane Doe', 24));

Now we get the following output:

expected new Person('John Doe', 42) to equal new Person('Jane Doe', 24)
 
Person({
  
name: 
'John Doe',
 
//
//
//
//
 
should equal 
'Jane Doe'
 
John Doe
Jane Doe
  
age: 
42
 
//
 
should equal 
24
})

That is a bit better, let me explain how it works. The inspect method is called with the value to be inspected, the depth this type should be inspected with, an output the inspected value should be written to, and an inspect function that can be used to recursively inspect members. The output is an instance of magicpen extended with a number of styles.

We write new Person( without styling, then we append the inspected name, write a ,, inspect the age and finish with the closing parenthesis. When inspect is called without a depth parameter it defaults to depth-1. Values inspected with depth zero will be inspected as .... In this case we always want the name so we forward the same depth to the inspect function.

Let's say we wanted Person instances only to be compared by name and not by age. Then we need to override the equal method:

function Person(name, age) {
  this.name = name;
  this.age = age;
}
 
expect.addType({
  name: 'Person',
  base: 'object',
  identify: function (value) {
    return value instanceof Person;
  },
  inspect: function (person, depth, output, inspect) {
    output
      .text('new Person(')
      .append(inspect(person.name, depth))
      .text(', ')
      .append(inspect(person.age, depth))
      .text(')');
  },
  equal: function (a, b, equal) {
    return a === b || equal(a.name, b.name);
  },
});

This will produce the same output as above, but that means the diff if wrong. It states that the age should be changed. We can fix that the following way:

function Person(name, age) {
  this.name = name;
  this.age = age;
}
 
expect.addType({
  name: 'Person',
  base: 'object',
  identify: function (value) {
    return value instanceof Person;
  },
  inspect: function (person, depth, output, inspect) {
    output
      .text('new Person(')
      .append(inspect(person.name, depth))
      .text(', ')
      .append(inspect(person.age, depth))
      .text(')');
  },
  equal: function (a, b, equal) {
    return a === b || equal(a.name, b.name);
  },
  diff: function (actual, expected, output, diff, inspect) {
    return this.baseType.diff(
      { name: actual.name },
      { name: expected.name },
      output
    );
  },
});
 
expect(new Person('John Doe', 42), 'to equal', new Person('Jane Doe', 24));

expected new Person('John Doe', 42) to equal new Person('Jane Doe', 24)
 
{
  
name: 
'John Doe'
 
//
//
//
//
 
should equal 
'Jane Doe'
 
John Doe
Jane Doe
}

The above diff method just calls the diff method on the base type with objects that only contain the name. The object diff will take care of all the hard work. We could also have called the diff function we got as an argument, but that will go off detecting the types of the parameters, therefore it is faster to call diff method on the base directly when you know it is the one you need.

You could also do something really custom as seen below:

const inlineDiff = true; // used to change inlining in a later example
 
expect.addType({
  name: 'Person',
  base: 'object',
  identify: function (value) {
    return value instanceof Person;
  },
  inspect: function (person, depth, output, inspect) {
    output
      .text('new Person(')
      .append(inspect(person.name, depth))
      .text(', ')
      .append(inspect(person.age, depth))
      .text(')');
  },
  equal: function (a, b, equal) {
    return a === b || equal(a.name, b.name);
  },
  diff: function (actual, expected, output, diff, inspect) {
    output.inline = inlineDiff;
    const nameDiff = diff(actual.name, expected.name);
 
    output.text('new Person(').nl().indentLines();
 
    if (nameDiff && nameDiff.inline) {
      output.append(nameDiff);
    } else {
      output
        .i()
        .append(inspect(actual.name))
        .text(',')
        .sp()
        .annotationBlock(function () {
          this.error('should be ').append(inspect(expected.name));
          if (nameDiff) {
            this.nl().append(nameDiff);
          }
        })
        .nl();
    }
 
    output.i().append(inspect(actual.age)).outdentLines().nl().text(')');
 
    return output;
  },
});
 
expect(new Person('John Doe', 42), 'to equal', new Person('Jane Doe', 24));

That produces the following output:

expected new Person('John Doe', 42) to equal new Person('Jane Doe', 24)
 
new Person(
  'John Doe', 
//
//
//
 
should be 'Jane Doe'
John Doe
Jane Doe
  42
)

This is a rather complicated example and I won't go though the details, but I would like to comment on the inline flag. When we diff objects against each other, the values of the keys will be diffed against each other. That means diffs are inserted into the containing structure. You can control this behavior using the inline flag. If the child diff is inline, it means that it will be appended directly into the parent; otherwise the diff will be inserted in an annotation block. The outputs below shows the contrast between setting the Person diff to inline or not.

inlineDiff = true;
expect(
  {
    JohnDoe: new Person('John Doe', 42),
    JaneDoe: new Person('Janie Doe', 24),
  },
  'to equal',
  {
    JohnDoe: new Person('John Doe', 42),
    JaneDoe: new Person('Jane Doe', 24),
  }
);

expected
{
  
JohnDoe: new Person('John Doe', 42),
JaneDoe: new Person('Janie Doe', 24)
}
to equal
{
  
JohnDoe: new Person('John Doe', 42),
JaneDoe: new Person('Jane Doe', 24)
}
 
{
  
JohnDoe: new Person('John Doe', 42),
  
JaneDoe: new Person(
  'Janie Doe', 
//
//
//
 
should be 'Jane Doe'
Janie Doe
Jane Doe
  24
)
}

inlineDiff = false;
expect(
  {
    JohnDoe: new Person('John Doe', 42),
    JaneDoe: new Person('Janie Doe', 24),
  },
  'to equal',
  {
    JohnDoe: new Person('John Doe', 42),
    JaneDoe: new Person('Jane Doe', 24),
  }
);

expected
{
  
JohnDoe: new Person('John Doe', 42),
JaneDoe: new Person('Janie Doe', 24)
}
to equal
{
  
JohnDoe: new Person('John Doe', 42),
JaneDoe: new Person('Jane Doe', 24)
}
 
{
  
JohnDoe: new Person('John Doe', 42),
  
JaneDoe: 
new Person('Janie Doe', 24)
 
//
//
//
//
//
//
//
//
 
should equal 
new Person('Jane Doe', 24)
 
new Person(
  'Janie Doe', 
//
//
//
 
should be 'Jane Doe'
Janie Doe
Jane Doe
  24
)
}

Now that we have implemented a type, we can start adding assertions to it. These assertions will only work on this type or types inheriting from the type.

expect.addAssertion(
  '<Person> to be above legal age',
  function (expect, subject) {
    expect(subject.age, 'to be greater than or equal to', 18);
  }
);
 
expect(new Person('Jane Doe', 24), 'to be above legal age');

Because Person inherits from object you can use all assertion defined for object or any of its ancestors. Here is an example:

expect(new Person('Jane Doe', 24), 'to have keys', 'name', 'age');
expect(new Person('Jane Doe', 24), 'to satisfy', {
  name: expect.it('to be a string').and('not to be empty'),
  age: expect.it('to be a number').and('not to be negative'),
});

The best resource for learning more about custom types is to look at how the predefined types are built:

lib/types.js