chai/lib/chai/utils/overwriteProperty.js

BDD/TDD assertion library for node.js and the browser. Test framework agnostic.

/*!
 * Chai - overwriteProperty utility
 * Copyright(c) 2012-2014 Jake Luer <jake@alogicalparadox.com>
 * MIT Licensed
 */

import {Assertion} from '../assertion.js';
import {flag} from './flag.js';
import {isProxyEnabled} from './isProxyEnabled.js';
import {transferFlags} from './transferFlags.js';

/**
 * ### .overwriteProperty(ctx, name, fn)
 *
 * Overwrites an already existing property getter and provides
 * access to previous value. Must return function to use as getter.
 *
 *     utils.overwriteProperty(chai.Assertion.prototype, 'ok', function (_super) {
 *         return function () {
 *             var obj = utils.flag(this, 'object');
 *             if (obj instanceof Foo) {
 *                 new chai.Assertion(obj.name).to.equal('bar');
 *             } else {
 *                 _super.call(this);
 *             }
 *         }
 *     });
 *
 * Can also be accessed directly from `chai.Assertion`.
 *
 *     chai.Assertion.overwriteProperty('foo', fn);
 *
 * Then can be used as any other assertion.
 *
 *     expect(myFoo).to.be.ok;
 *
 * @param {object} ctx object whose property is to be overwritten
 * @param {string} name of property to overwrite
 * @param {Function} getter function that returns a getter function to be used for name
 * @namespace Utils
 * @name overwriteProperty
 * @public
 */
export function overwriteProperty(ctx, name, getter) {
  let _get = Object.getOwnPropertyDescriptor(ctx, name),
    _super = function () {};

  if (_get && 'function' === typeof _get.get) _super = _get.get;

  Object.defineProperty(ctx, name, {
    get: function overwritingPropertyGetter() {
      // Setting the `ssfi` flag to `overwritingPropertyGetter` causes this
      // function to be the starting point for removing implementation frames
      // from the stack trace of a failed assertion.
      //
      // However, we only want to use this function as the starting point if
      // the `lockSsfi` flag isn't set and proxy protection is disabled.
      //
      // If the `lockSsfi` flag is set, then either this assertion has been
      // overwritten by another assertion, or this assertion is being invoked
      // from inside of another assertion. In the first case, the `ssfi` flag
      // has already been set by the overwriting assertion. In the second
      // case, the `ssfi` flag has already been set by the outer assertion.
      //
      // If proxy protection is enabled, then the `ssfi` flag has already been
      // set by the proxy getter.
      if (!isProxyEnabled() && !flag(this, 'lockSsfi')) {
        flag(this, 'ssfi', overwritingPropertyGetter);
      }

      // Setting the `lockSsfi` flag to `true` prevents the overwritten
      // assertion from changing the `ssfi` flag. By this point, the `ssfi`
      // flag is already set to the correct starting point for this assertion.
      let origLockSsfi = flag(this, 'lockSsfi');
      flag(this, 'lockSsfi', true);
      let result = getter(_super).call(this);
      flag(this, 'lockSsfi', origLockSsfi);

      if (result !== undefined) {
        return result;
      }

      let newAssertion = new Assertion();
      transferFlags(this, newAssertion);
      return newAssertion;
    },
    configurable: true
  });
}