assert.js

// Copyright (c) 2013 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

/**
 * @fileoverview Assertion support.
 */

/**
 * Verify |condition| is truthy and return |condition| if so.
 * @template T
 * @param {T} condition A condition to check for truthiness.  Note that this
 *     may be used to test whether a value is defined or not, and we don't want
 *     to force a cast to Boolean.
 * @param {string=} opt_message A message to show on failure.
 * @return {T} A non-null |condition|.
 */
function assert(condition, opt_message) {
  if (!condition) {
    var message = 'Assertion failed';
    if (opt_message)
      message = message + ': ' + opt_message;
    var error = new Error(message);
    var global = function() { return this; }();
    if (global.traceAssertionsForTesting)
      console.warn(error.stack);
    throw error;
  }
  return condition;
}

/**
 * Call this from places in the code that should never be reached.
 *
 * For example, handling all the values of enum with a switch() like this:
 *
 *   function getValueFromEnum(enum) {
 *     switch (enum) {
 *       case ENUM_FIRST_OF_TWO:
 *         return first
 *       case ENUM_LAST_OF_TWO:
 *         return last;
 *     }
 *     assertNotReached();
 *     return document;
 *   }
 *
 * This code should only be hit in the case of serious programmer error or
 * unexpected input.
 *
 * @param {string=} opt_message A message to show when this is hit.
 */
function assertNotReached(opt_message) {
  assert(false, opt_message || 'Unreachable code hit');
}

/**
 * @param {*} value The value to check.
 * @param {function(new: T, ...)} type A user-defined constructor.
 * @param {string=} opt_message A message to show when this is hit.
 * @return {T}
 * @template T
 */
function assertInstanceof(value, type, opt_message) {
  // We don't use assert immediately here so that we avoid constructing an error
  // message if we don't have to.
  if (!(value instanceof type)) {
    assertNotReached(opt_message || 'Value ' + value +
                     ' is not a[n] ' + (type.name || typeof type));
  }
  return value;
}