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：

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：

buf = new Buffer(256);

 

len = buf.write('\u00bd + \u00bc = \u00be', 0);

 

console.log(len + " bytes: " + buf.toString('utf8', 0, len));

源码：

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;

};