Chai Iterator: Assertions for iterable objects

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Overview

Chai Iterator extends the Chai assertion library with methods for testing iterable objects. Introduced in the ES2015 specification, iterable objects have an @@iterator method, which allows us to iterate over them with a for...of loop. A number of built-in types are iterable by default, while custom iterable objects may also be defined. Chai Iterator makes it easy to test all such objects.

Basic usage

Here is a fairly exhaustive sample of the assertions we can make using Chai Iterator. While we could just as easily use expect or assert, we’ll use Chai’s should() assertion style, just to be different.

[2, 3, 5].should.be.iterable;

[2, 3, 5].should.iterate.over([2, 3, 5]);
[2, 3, 5].should.iterate.from([2, 3]);
[2, 3, 5].should.iterate.until([3, 5]);

[2, 3, 5].should.iterate.for.lengthOf(3);
[2, 3, 5].should.iterate.for.length.above(2);
[2, 3, 5].should.iterate.for.length.below(4);
[2, 3, 5].should.iterate.for.length.of.at.least(3);
[2, 3, 5].should.iterate.for.length.of.at.most(3);
[2, 3, 5].should.iterate.for.length.within(2, 4);

[2, 3, 5].should.not.iterate.over([1, 2, 3]);
[{n: 2}, {n: 3}].should.deep.iterate.from([{n: 2}]);

Let’s not limit ourselves to Arrays; we can test any iterable object.

'abcde'.should.iterate.until(['c', 'd', 'e']);

And we can pass any iterable as our expected values too.

'abcde'.should.iterate.until('cde');

User-defined iterable objects

That’s enough fiddling around with built-in objects whose iteration behavior is well-known. Chai Iterator is best used to test user-defined iterable objects, like the one constructed by the following class.

class Count {

  constructor(start=0, step=1) {
    this.start = start;
    this.step = step;
  }

  *[Symbol.iterator]() {
    for (let n = this.start; true; n += this.step) {
      yield n;
    }
  }
}

The sequence generated by Count.prototype[@@iterator]() is infinite; it continues to yield values indefinitely. Still, we can safely use the from() assertion with it, since it will terminate as soon as our expected iterable is done.

let tens = new Count(10, 10);

tens.should.be.iterable;
tens.should.iterate.from([10, 20, 30]);
tens.should.iterate.from([10, 20, 30, 40, 50]);

Just don’t go trying to use over() or until() on infinite sequences. The former will always fail; the latter will never stop.

Generators and iterators

Let’s generate the fibonacci sequence. A generator function is just a function that returns a Generator object — an iterator that is also iterable. We can test a Generator just as we would any other iterable.

function* fibonacci() {
  for (let [x, y] = [1, 1]; true; [x, y] = [y, x + y]) {
    yield x;
  }
}

fibonacci().should.iterate.from([1, 1, 2, 3, 5]);

Be careful though. Iterators can’t go back in time. Once a value has been yielded, it is lost forever. And so the following assertions pass.

let fiborator = fibonacci();

fiborator.should.iterate.from([1, 1, 2, 3, 5]);
fiborator.should.iterate.from([8, 13, 21, 34]);

It usually makes more sense to construct a new Generator for each assertion.

fibonacci().should.iterate.from([1, 1, 2, 3, 5]);
fibonacci().should.iterate.from([1, 1, 2, 3, 5, 8, 13]);

Compatibility

Chai Iterator requires that Symbol.iterator be available in the environment. In Node, this means the version must be v4.0 or greater. While the latest versions of most browsers are compatible, web-facing projects should almost certainly use a polyfill.

The Babel polyfill is one option for environments that do not natively support Symbol.iterator. More minimally, we can get away with just two sub-modules from the core-js library, like so.

require('core-js/es6/symbol');
require('core-js/fn/symbol/iterator');

Installation

Install Chai Iterator using npm. And be sure, of course, to install Chai.

npm install --save chai chai-iterator

Setup

Chai Iterator can be imported as a Node module, an AMD module, or included in an HTML <script> tag. For TypeScript users, declarations are installed with the package.

Node

To set up Chai Iterator for Node, make sure the version is v4.0 or higher, as prior versions lack support for the @@iterator method.

const chai = require('chai');
const chaiIterator = require('chai-iterator');

chai.use(chaiIterator);

AMD

Chai Iterator can be set up inside of an AMD module like so.

define((require, exports, module) => {
  let chai = require('chai');
  let chaiIterator = require('chai-iterator');

  chai.use(chaiIterator);
});

HTML script tag

Chai Iterator can be included via a <script> tag. If it is loaded after chai.js, Chai will use it automatically. The plugin function will also be available globally as chaiIterator.

<script src="chai.js"></script>
<script src="chai-iterator.js"></script>

TypeScript

TypeScript declarations are included in the package. To use them, ensure Chai Iterator is installed with npm, then install the declarations and their dependencies via typings. And be sure to install the declarations for chai.

typings install --save-dev npm~chai npm:chai-iterator

In the compiler options, set "target" to "es6", or at least include a reference to lib.es6.d.ts. Now the following will just work.

import chai = require("chai");
import chaiIterator = require("chai-iterator");

chai.use(chaiIterator);

[2, 3, 5].should.iterate.over([2, 3, 5]);

Expect/Should API

Assertions

iterable

Asserts that the target is an iterable object, i.e., that it has an @@iterator method.

expect([2, 3, 5]).to.be.iterable;
expect('abcdefg').to.be.iterable;
expect(12345).not.to.be.iterable;

iterate.over(expected)

Asserts that the target iterates over a given sequence of values. Set the deep flag to use deep equality to compare values.

Param Type Description
expected object An iterable object.
expect([2, 3, 5]).to.iterate.over([2, 3, 5]);
expect('abcdefg').to.itetate.over('abcdefg');
expect([2, 3, 5]).not.to.iterate.over([2, 3]);
expect([{n: 2}, {n: 3}]).to.deep.iterate.over([{n: 2}, {n: 3}]);

iterate.from(expected)

Asserts that the target begins iterating over a given sequence of values. Set the deep flag to use deep equality to compare values.

Param Type Description
expected object An iterable object.
expect([2, 3, 5]).to.iterate.from([2, 3]);
expect('abcdefg').to.iterate.from('abc');
expect([2, 3, 5]).not.to.iterate.from([3, 5]);
expect([{n: 2}, {n: 3}]).to.deep.iterate.from([{n: 2}]);

iterate.until(expected)

Asserts that the target ends iteration with a given sequence of values. Set the deep flag to use deep equality to compare values.

Param Type Description
expected object An iterable object.
expect([2, 3, 5]).to.iterate.until([3, 5]);
expect('abcdefg').to.iterate.until('efg');
expect([2, 3, 5]).not.to.iterate.until([2, 3]);
expect([{n: 2}, {n: 3}]).to.deep.iterate.until([{n: 3}]);

iterate.for.lengthOf(n)

Asserts that the target yields exactly n values.

Param Type Description
n number A positive integer
expect([2, 3, 5]).to.iterate.for.lengthOf(3);
expect('abcdefg').to.iterate.for.lengthOf(7);
expect([2, 3, 5]).not.to.iterate.for.lengthOf(7);

iterate.for.length.above(n)

Asserts that the target yields more than n values.

Param Type Description
n number A positive integer
expect([2, 3, 5]).to.iterate.for.length.above(2);
expect('abcdefg').to.iterate.for.length.above(5);
expect([2, 3, 5]).not.to.iterate.for.length.above(3);

iterate.for.length.below(n)

Asserts that the target yields fewer than n values.

Param Type Description
n number A positive integer
expect([2, 3, 5]).to.iterate.for.length.below(4);
expect('abcdefg').to.iterate.for.length.below(10);
expect([2, 3, 5]).not.to.iterate.for.length.below(3);

iterate.for.length.of.at.least(n)

Asserts that the target yields at least n values.

Param Type Description
n number A positive integer
expect([2, 3, 5]).to.iterate.for.length.of.at.least(2);
expect([2, 3, 5]).to.iterate.for.length.of.at.least(3);
expect([2, 3, 5]).not.to.iterate.for.length.of.at.least(4);

iterate.for.length.of.at.most(n)

Asserts that the target yields at most n values.

Param Type Description
n number A positive integer
expect([2, 3, 5]).to.iterate.for.length.of.at.most(4);
expect([2, 3, 5]).to.iterate.for.length.of.at.most(3);
expect([2, 3, 5]).not.to.iterate.for.length.of.at.most(2);

iterate.for.length.within(min, max)

Asserts that the target yields between min and max values, inclusive.

Param Type Description
n number A positive integer
expect([2, 3, 5]).to.iterate.for.length.within(2, 4);
expect([2, 3, 5]).to.iterate.for.length.within(3, 3);
expect([2, 3, 5]).not.to.iterate.for.length.within(4, 7);

Assert API

Assertions

Parameters

The parameters for the assert methods are as follows.

Param Type Description
value any Any value.
expected object An iterable object.
n number A positive integer.
message? string An optional message to display on error.

isIterable(value, [message])

Asserts that a value is an iterable object, i.e., that it is an object with an @@iterator method.

assert.isIterable([2, 3, 5]);
assert.isIterable('abcdefg');

isNotIterable(value, [message])

Asserts that a value is not an iterable object, i.e., that it lacks an @@iterator method.

assert.isNotIterable(235);
assert.isNotIterable(true);

iteratesOver(value, expected, [message])

Asserts that a value iterates exactly over a given sequence of values.

assert.iteratesOver([2, 3, 5], [2, 3, 5]);
assert.iteratesOver('abcdefg', 'abcdefg');

doesNotIterateOver(value, expected, [message])

Asserts that a value does not iterate exactly over a given sequence of values.

assert.doesNotIterateOver([2, 3, 5], [1, 2, 3]);
assert.doesNotIterateOver('abcdefg', 'abc');

deepIteratesOver(value, expected, [message])

Asserts that a value iterates exactly over a given sequence of values, using deep equality.

assert.deepIteratesOver([{n: 2}, {n: 3}], [{n: 2}, {n: 3}]);
assert.deepIteratesOver([[0, 2], [1, 3]], [[0, 2], [1, 3]]);

doesNotDeepIterateOver(value, expected, [message])

Asserts that a value does not iterate exactly over a given sequence of values, using deep equality.

assert.doesNotDeepIterateOver([{n: 2}, {n: 3}], [{n: 5}, {n: 7}]);
assert.doesNotDeepIterateOver([[0, 2], [1, 3]], [[1, 3], [0, 2]]);

iteratesFrom(value, expected, [message])

Asserts that a value begins iteration with a given sequence of values.

assert.iteratesFrom([2, 3, 5], [2, 3, 5]);
assert.iteratesFrom([2, 3, 5], [2, 3]);
assert.iteratesFrom('abcdefg', 'abc');
assert.iteratesFrom('abcdefg', '');

doesNotIterateFrom(value, expected, [message])

Asserts that a value does not begin iteration with a given sequence of values.

assert.doesNotIterateFrom([2, 3, 5], [3, 5]);
assert.doesNotIterateFrom('abcdefg', 'cdef');

deepIteratesFrom(value, expected, [message])

Asserts that a value begins iteration with a given sequence of values, using deep equality.

assert.deepIteratesFrom([{n: 2}, {n: 3}], [{n: 2}]);
assert.deepIteratesFrom([[0, 2], [1, 3]], [[0, 2]]);

doesNotDeepIterateFrom(value, expected, [message])

Asserts that a value does not begin iteration with a given sequence of values, using deep equality.

assert.doesNotDeepIterateFrom([{n: 2}, {n: 3}], [{n: 5}]);
assert.doesNotDeepIterateFrom([[0, 2], [1, 3]], [[1, 3]]);

iteratesUntil(value, expected, [message])

Asserts that a value ends iteration with a given sequence of values.

assert.iteratesUntil([2, 3, 5], [2, 3, 5]);
assert.iteratesUntil([2, 3, 5], [3, 5]);
assert.iteratesUntil('abcdefg', 'efg');
assert.iteratesUntil('abcdefg', '');

doesNotIterateUntil(value, expected, [message])

Asserts that a value does not end iteration with a given sequence of values.

assert.doesNotIterateUntil([2, 3, 5], [2, 3]);
assert.doesNotIterateUntil('abcdefg', 'cdef');

deepIteratesUntil(value, expected, [message])

Asserts that a value ends iteration with a given sequence of values, using deep equality.

assert.deepIteratesUntil([{n: 2}, {n: 3}], [{n: 3}]);
assert.deepIteratesUntil([[0, 2], [1, 3]], [[1, 3]]);

doesNotDeepIterateUntil(value, expected, [message])

Asserts that a value does not end iteration with a given sequence of values, using deep equality.

assert.doesNotDeepIterateUntil([{n: 2}, {n: 3}], [{n: 5}]);
assert.doesNotDeepIterateUntil([[0, 2], [1, 3]], [[0, 2]]);

lengthOf(value, n, [message])

Asserts that an iterable yields a given number of values. If value is not an iterable object, or if it has a 'length' property, Chai’s built-in assert.lengthOf() will be used.

function* range(min=0, max=Infinity, step=1) {
  for (let n = min; n < max; n += step) {
    yield n;
  }
}

assert.lengthOf(range(0, 10), 10);
assert.lengthOf(range(6, 42), 36);

License

Copyright © 2016 Akim McMath. Licensed under the MIT License.