spy.js

/**
 * Simple spying utility.
 *
 * Simple utility to spy on object methods. By not bothering with error checking
 * (e.g.: Does the object exist? Does the method exist?), and keeping
 * interactions with the spy to a minimum, we can make this very small and
 * simple.
 *
 * The function returns a spy object, which tracks calls made to the method, and
 * can be used to restore the original method implementation.
 *
 * No mocking or pattern matching is supported; it's up to the caller to
 * implement those.
 *
 * @module jsutils/wunit/spy
 */

function spyOn(obj, method) {
  let calls = [];

  const {
    configurable,
    enumerable,
    get,
    set,
    writable,
    value,
  } = Object.getOwnPropertyDescriptor(obj, method);

  const isGetterSetter = get && set;

  if (isGetterSetter) {
    Object.defineProperty(obj, method, {
      configurable,
      enumerable,
      get: () => {
        const returnValue = get.apply(obj);
        calls.push({ returnValue, args: [] });
        return returnValue;
      },
      set: (...args) => {
        set.apply(obj, args);
        calls.push({ returnValue: undefined, args });
      },
    });
  } else {
    const realMethod = obj[method];
    Object.defineProperty(obj, method, {
      configurable,
      enumerable,
      writable: false,
      value: (...args) => {
        const returnValue = realMethod.apply(obj, args);
        calls.push({ returnValue, args });
        return returnValue;
      },
    });
  }

  return {
    calls() {
      return calls;
    },
    lastCall() {
      return calls.length > 0 ? calls[calls.length - 1] : undefined;
    },
    wasCalled() {
      return calls.length > 0;
    },
    reset() {
      calls = [];
    },
    restore() {
      if (isGetterSetter) {
        Object.defineProperty(obj, method, {
          configurable,
          enumerable,
          get,
          set,
        });
      } else {
        Object.defineProperty(obj, method, {
          configurable,
          enumerable,
          writable,
          value,
        });
      }
    },
  };
}

module.exports = spyOn;