im.doctor/test/server/node_modules/should/lib/assertion-error.js

var util = require('./util');

/**
 * should AssertionError
 * @param {Object} options
 * @constructor
 * @memberOf should
 * @static
 */
var AssertionError = function AssertionError(options) {
  util.merge(this, options);

  if(!options.message) {
    Object.defineProperty(this, 'message', {
        get: function() {
          if(!this._message) {
            this._message = this.generateMessage();
            this.generatedMessage = true;
          }
          return this._message;
        },
        configurable: true,
        enumerable: false
      }
    );
  }

  if(Error.captureStackTrace) {
    Error.captureStackTrace(this, this.stackStartFunction);
  } else {
    // non v8 browsers so we can have a stacktrace
    var err = new Error();
    if(err.stack) {
      var out = err.stack;

      if(this.stackStartFunction) {
        // try to strip useless frames
        var fn_name = util.functionName(this.stackStartFunction);
        var idx = out.indexOf('\n' + fn_name);
        if(idx >= 0) {
          // once we have located the function frame
          // we need to strip out everything before it (and its line)
          var next_line = out.indexOf('\n', idx + 1);
          out = out.substring(next_line + 1);
        }
      }

      this.stack = out;
    }
  }
};


var indent = '    ';
function prependIndent(line) {
  return indent + line;
}

function indentLines(text) {
  return text.split('\n').map(prependIndent).join('\n');
}


// assert.AssertionError instanceof Error
AssertionError.prototype = Object.create(Error.prototype, {
  name: {
    value: 'AssertionError'
  },

  generateMessage: {
    value: function() {
      if(!this.operator && this.previous) {
        return this.previous.message;
      }
      var actual = util.format(this.actual);
      var expected = 'expected' in this ? ' ' + util.format(this.expected) : '';
      var details = 'details' in this && this.details ? ' (' + this.details + ')' : '';

      var previous = this.previous ? '\n' + indentLines(this.previous.message) : '';

      return 'expected ' + actual + (this.negate ? ' not ' : ' ') + this.operator + expected + details + previous;
    }
  }
});

module.exports = AssertionError;