方法说明：

将string使用指定的encoding写入到buffer的offset处。

返回写入了多少个八进制字节。

如果Buffer没有足够的空间来适应整个string,那么将只有string的部分被写入。

语法：

复制代码 代码如下:
buffer.write(string, [offset], [length], [encoding])

接收参数：

string                  String,被写入buffer的数据.
offet                   number,可选,默认0.数据写入到buffer的位置.
length                Number,可选,默认:buffer.length – offset,要写入数据的长度
encoding           String,需要使用的编码格式,可选,默认为”utf8″.

例子：

复制代码 代码如下:
buf = new Buffer(256);
 
len = buf.write('\u00bd + \u00bc = \u00be', 0);
 
console.log(len + " bytes: " + buf.toString('utf8', 0, len));

源码：

复制代码 代码如下:
Buffer.prototype.write = function(string, offset, length, encoding) {
  // allow write(string, encoding)
  if (util.isString(offset) && util.isUndefined(length)) {
    encoding = offset;
    offset = 0;
  // allow write(string, offset[, length], encoding)
  } else if (isFinite(offset)) {
    offset = ~~offset;
    if (isFinite(length)) {
      length = ~~length;
    } else {
      encoding = length;
      length = undefined;
    }
  // XXX legacy write(string, encoding, offset, length) - remove in v0.13
  } else {
    if (!writeWarned) {
      if (process.throwDeprecation)
        throw new Error(writeMsg);
      else if (process.traceDeprecation)
        console.trace(writeMsg);
      else
        console.error(writeMsg);
      writeWarned = true;
    }
    var swap = encoding;
    encoding = offset;
    offset = ~~length;
    length = swap;
  }
  var remaining = this.length - offset;
  if (util.isUndefined(length) || length > remaining)
    length = remaining;
  encoding = !!encoding ? (encoding + '').toLowerCase() : 'utf8';
  if (string.length > 0 && (length < 0 || offset < 0))
    throw new RangeError('attempt to write beyond buffer bounds');
  var ret;
  switch (encoding) {
    case 'hex':
      ret = this.hexWrite(string, offset, length);
      break;
    case 'utf8':
    case 'utf-8':
      ret = this.utf8Write(string, offset, length);
      break;
    case 'ascii':
      ret = this.asciiWrite(string, offset, length);
      break;
    case 'binary':
      ret = this.binaryWrite(string, offset, length);
      break;
    case 'base64':
      // Warning: maxLength not taken into account in base64Write
      ret = this.base64Write(string, offset, length);
      break;
    case 'ucs2':
    case 'ucs-2':
    case 'utf16le':
    case 'utf-16le':
      ret = this.ucs2Write(string, offset, length);
      break;
    default:
      throw new TypeError('Unknown encoding: ' + encoding);
  }
  return ret;
};