How to spy on a recursive function in JavaScript

Note: I’ve seen variations of this question asked in different ways and in reference to different testing tools. I thought it would useful to have the issue and solution clearly described. My tests are written using Sinon spies for readability and will run using Jest or Jasmine (and need only minor changes to run using Mocha and Chai), but the behavior described can be seen using any testing framework and with any spy implementation.

ISSUE

I can create tests that verify that a recursive function returns the correct value, but I can’t spy on the recursive calls.

EXAMPLE

Given this recursive function:

const fibonacci = (n) => {
  if (n < 0) throw new Error('must be 0 or greater');
  if (n === 0) return 0;
  if (n === 1) return 1;
  return fibonacci(n - 1) + fibonacci(n - 2);
}

…I can test that it returns the correct values by doing this:

describe('fibonacci', () => {
  it('should calculate Fibonacci numbers', () => {
    expect(fibonacci(5)).toBe(5);
    expect(fibonacci(10)).toBe(55);
    expect(fibonacci(15)).toBe(610);
  });
});

…but if I add a spy to the function it reports that the function is only called once:

describe('fibonacci', () => {
  it('should calculate Fibonacci numbers', () => {
    expect(fibonacci(5)).toBe(5);
    expect(fibonacci(10)).toBe(55);
    expect(fibonacci(15)).toBe(610);
  });
  it('should call itself recursively', () => {
    const spy = sinon.spy(fibonacci);
    spy(10);
    expect(spy.callCount).toBe(177); // FAILS: call count is 1
  });
});

Answer

ISSUE

Spies work by creating a wrapper function around the original function that tracks the calls and returned values. A spy can only record the calls that pass through it.

If a recursive function calls itself directly then there is no way to wrap that call in a spy.

SOLUTION

The recursive function must call itself in the same way that it is called from outside itself. Then, when the function is wrapped in a spy, the recursive calls are wrapped in the same spy.

Example 1: Class Method

Recursive class methods call themselves using this which refers to their class instance. When the instance method is replaced by a spy, the recursive calls automatically call the same spy:

class MyClass {
  fibonacci(n) {
    if (n < 0) throw new Error('must be 0 or greater');
    if (n === 0) return 0;
    if (n === 1) return 1;
    return this.fibonacci(n - 1) + this.fibonacci(n - 2);
  }
}

describe('fibonacci', () => {

  const instance = new MyClass();

  it('should calculate Fibonacci numbers', () => {
    expect(instance.fibonacci(5)).toBe(5);
    expect(instance.fibonacci(10)).toBe(55);
  });
  it('can be spied on', () => {
    const spy = sinon.spy(instance, 'fibonacci');
    instance.fibonacci(10);
    expect(spy.callCount).toBe(177); // PASSES
    spy.restore();
  });
});

Note: the class method uses this so in order to invoke the spied function using spy(10); instead of instance.fibonacci(10); the function would either need to be converted to an arrow function or explicitly bound to the instance with this.fibonacci = this.fibonacci.bind(this); in the class constructor.

Example 2: Modules

A recursive function within a module becomes spy-able if it calls itself using the module. When the module function is replaced by a spy, the recursive calls automatically call the same spy:

ES6

// ---- lib.js ----
import * as lib from './lib';

export const fibonacci = (n) => {
  if (n < 0) throw new Error('must be 0 or greater');
  if (n === 0) return 0;
  if (n === 1) return 1;
  // call fibonacci using lib
  return lib.fibonacci(n - 1) + lib.fibonacci(n - 2);
};


// ---- lib.test.js ----
import * as sinon from 'sinon';
import * as lib from './lib';

describe('fibonacci', () => {
  it('should calculate Fibonacci numbers', () => {
    expect(lib.fibonacci(5)).toBe(5);
    expect(lib.fibonacci(10)).toBe(55);
  });
  it('should call itself recursively', () => {
    const spy = sinon.spy(lib, 'fibonacci');
    spy(10);
    expect(spy.callCount).toBe(177); // PASSES
    spy.restore();
  });
});

Common.js

// ---- lib.js ----
exports.fibonacci = (n) => {
  if (n < 0) throw new Error('must be 0 or greater');
  if (n === 0) return 0;
  if (n === 1) return 1;
  // call fibonacci using exports
  return exports.fibonacci(n - 1) + exports.fibonacci(n - 2);
}


// ---- lib.test.js ----
const sinon = require('sinon');
const lib = require('./lib');

describe('fibonacci', () => {
  it('should calculate Fibonacci numbers', () => {
    expect(lib.fibonacci(5)).toBe(5);
    expect(lib.fibonacci(10)).toBe(55);
  });
  it('should call itself recursively', () => {
    const spy = sinon.spy(lib, 'fibonacci');
    spy(10);
    expect(spy.callCount).toBe(177); // PASSES
    spy.restore();
  });
});

Example 3: Object Wrapper

A stand-alone recursive function that is not part of a module can become spy-able if it is placed in a wrapping object and calls itself using the object. When the function within the object is replaced by a spy the recursive calls automatically call the same spy:

const wrapper = {
  fibonacci: (n) => {
    if (n < 0) throw new Error('must be 0 or greater');
    if (n === 0) return 0;
    if (n === 1) return 1;
    // call fibonacci using the wrapper
    return wrapper.fibonacci(n - 1) + wrapper.fibonacci(n - 2);
  }
};

describe('fibonacci', () => {
  it('should calculate Fibonacci numbers', () => {
    expect(wrapper.fibonacci(5)).toBe(5);
    expect(wrapper.fibonacci(10)).toBe(55);
    expect(wrapper.fibonacci(15)).toBe(610);
  });
  it('should call itself recursively', () => {
    const spy = sinon.spy(wrapper, 'fibonacci');
    spy(10);
    expect(spy.callCount).toBe(177); // PASSES
    spy.restore();
  });
});