option_a.js

// Library:
Email = {}
Email.Message = function(raw_source) {
  var parts = raw_source.split(/\r\n\r\n/);
  this.headers = parts[0];
  this.body = parts[1];
}

Email.Message.prototype.header(header_name, value) {
    if(!value) {
      // Look up the header +header_name+
    } else {
      // Set the header +header_name+ to +value+
    }
  }  
}

// Client code:
var email = new Email.Message(source),
    date  = email.header("date");

option_b.js

// Library:
Email = {}
Email.Message = function(raw_source) {
  var parts = raw_source.split(/\r\n\r\n/);
  this.header = {};
  for(var i = 0; i <= parts[0].size; i++) {
    var header_parts = parts[0][i].split(/:/, 2);
    this.header[header_parts[0]] = header_parts[1];
  }
  this.body = parts[1];
}

// Client code:
var email = new Email.Message(source),
    date  = email.header.date;

option_c.js

// Library:
Email = {}
Email.Message = function(raw_source) {
  var parts = raw_source.split(/\r\n\r\n/);
  this.header = {};
  for(var i = 0; i <= parts[0].size; i++) {
    var header_parts = parts[0][i].split(/:/, 2);
    this.header._values[header_parts[0]] = header_parts[1];
    this.header[header_parts[0]] = function(value) {
      if(typeof(value) == "undefined") {
        return this.header._values[header_parts[0]];
      } else {
        this.header._values[header_parts[0]] = value;
      }
    }
  }
  this.body = parts[1];
}

// Client code:
var email = new Email.Message(source);
if(email.header.date) {
  var date = email.header.date();
}