chai/lib/chai/utils/addProperty.js

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

/*!
 * Chai - addProperty 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';

/**
 * ### .addProperty(ctx, name, getter)
 *
 * Adds a property to the prototype of an object.
 *
 *     utils.addProperty(chai.Assertion.prototype, 'foo', function () {
 *         var obj = utils.flag(this, 'object');
 *         new chai.Assertion(obj).to.be.instanceof(Foo);
 *     });
 *
 * Can also be accessed directly from `chai.Assertion`.
 *
 *     chai.Assertion.addProperty('foo', fn);
 *
 * Then can be used as any other assertion.
 *
 *     expect(myFoo).to.be.foo;
 *
 * @param {object} ctx object to which the property is added
 * @param {string} name of property to add
 * @param {Function} getter function to be used for name
 * @namespace Utils
 * @name addProperty
 * @public
 */
export function addProperty(ctx, name, getter) {
  getter = getter === undefined ? function () {} : getter;

  Object.defineProperty(ctx, name, {
    get: function propertyGetter() {
      // Setting the `ssfi` flag to `propertyGetter` 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', propertyGetter);
      }

      let result = getter.call(this);
      if (result !== undefined) return result;

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