方法说明：
写入文件（根据文件描述符），功能与 fs.writeFile() 类似，但该方法提供更底层的操作，实际应用中建议使用多 fs.writeFile() 。
该方法有两种形式：
1、fs.write(fd, buffer, offset, length[, position], [callback(err, bytesWritten, buffer)])
这种写法将buffer写入文件（根据文件描述符fd来查找文件）。
2、fs.write(fd, data[, position[, encoding]], [callback(err, written, string)])
这种写法将数据data写入文件（根据文件描述符fd来查找文件）。如果数据不是一个缓冲区的实例值将被强制转换为一个字符串。
语法：
fs.write(fd, buffer, offset, length[, position], [callback(err, bytesWritten, buffer)]) fs.write(fd, data[, position[, encoding]], [callback(err, written, string)]) //fs.write(fd, buffer, offset, length[, position], [callback(err, bytesWritten, buffer)]) //执行结果：bytesWritten = 8 , buffer =var fs = require('fs'); fs.open('content.txt', 'a', function(err,fd){ if(err){ throw err; } var data = '123123123 hello world'; var buf = new Buffer(8); fs.write(fd, buf, 0, 8, 0, function(err, bytesWritten, buffer){ if(err){ throw err; } console.log(bytesWritten); console.log(buffer); fs.close(fd,function(err){ if(err){ throw err; } console.log('file closed'); }) }) }) //fs.write(fd, data[, position[, encoding]], [callback(err, written, string)]) //执行结果：written = 21 , string = var fs = require('fs'); fs.open('content.txt', 'a', function(err,fd){ if(err){ throw err; } var data = '123123123 hello world'; fs.write(fd, data, 0 , 'utf-8', function(err, written, string){ if(err){ throw err; } console.log(written); console.log(string); fs.close(fd,function(err){ if(err){ throw err; } console.log('file closed'); }) }) }) // usage: // fs.write(fd, buffer, offset, length[, position], callback); // OR // fs.write(fd, string[, position[, encoding]], callback); fs.write = function(fd, buffer, offset, length, position, callback) { if (util.isBuffer(buffer)) { // if no position is passed then assume null if (util.isFunction(position)) { callback = position; position = null; } callback = maybeCallback(callback); var wrapper = function(err, written) { // Retain a reference to buffer so that it can't be GC'ed too soon. callback(err, written || 0, buffer); }; return binding.writeBuffer(fd, buffer, offset, length, position, wrapper); } if (util.isString(buffer)) buffer += ''; if (!util.isFunction(position)) { if (util.isFunction(offset)) { position = offset; offset = null; } else { position = length; } length = 'utf8'; } callback = maybeCallback(position); position = function(err, written) { // retain reference to string in case it's external callback(err, written || 0, buffer); }; return binding.writeString(fd, buffer, offset, length, position); };
由于该方法属于fs模块，使用前需要引入fs模块（var fs= require(“fs”) ）
接收参数：
fd 文件描述符。
buffer 缓冲区，数据将被写入。buffer尺寸的大小设置最好是8的倍数，效率较高。
offset buffer写入的偏移量
length （integer） 指定文件读取字节数长度
position （integer） 指定文件读取的起始位置，如果该项为null，将从当前文件指针的位置开始读取数据。
callback 回调传递了三个参数，err， bytesRead， buffer
· err 异常
· bytesRead:读取的字节数
· buffer:缓冲区对象
第二种形式：
encoding 字符编码
callback
· err 异常
· written 指定多少字符数将被写入到文件。
· string 返回的Buffer
例子：
fs.write(fd, buffer, offset, length[, position], [callback(err, bytesWritten, buffer)]) fs.write(fd, data[, position[, encoding]], [callback(err, written, string)]) //fs.write(fd, buffer, offset, length[, position], [callback(err, bytesWritten, buffer)]) //执行结果：bytesWritten = 8 , buffer =var fs = require('fs'); fs.open('content.txt', 'a', function(err,fd){ if(err){ throw err; } var data = '123123123 hello world'; var buf = new Buffer(8); fs.write(fd, buf, 0, 8, 0, function(err, bytesWritten, buffer){ if(err){ throw err; } console.log(bytesWritten); console.log(buffer); fs.close(fd,function(err){ if(err){ throw err; } console.log('file closed'); }) }) }) //fs.write(fd, data[, position[, encoding]], [callback(err, written, string)]) //执行结果：written = 21 , string = var fs = require('fs'); fs.open('content.txt', 'a', function(err,fd){ if(err){ throw err; } var data = '123123123 hello world'; fs.write(fd, data, 0 , 'utf-8', function(err, written, string){ if(err){ throw err; } console.log(written); console.log(string); fs.close(fd,function(err){ if(err){ throw err; } console.log('file closed'); }) }) }) // usage: // fs.write(fd, buffer, offset, length[, position], callback); // OR // fs.write(fd, string[, position[, encoding]], callback); fs.write = function(fd, buffer, offset, length, position, callback) { if (util.isBuffer(buffer)) { // if no position is passed then assume null if (util.isFunction(position)) { callback = position; position = null; } callback = maybeCallback(callback); var wrapper = function(err, written) { // Retain a reference to buffer so that it can't be GC'ed too soon. callback(err, written || 0, buffer); }; return binding.writeBuffer(fd, buffer, offset, length, position, wrapper); } if (util.isString(buffer)) buffer += ''; if (!util.isFunction(position)) { if (util.isFunction(offset)) { position = offset; offset = null; } else { position = length; } length = 'utf8'; } callback = maybeCallback(position); position = function(err, written) { // retain reference to string in case it's external callback(err, written || 0, buffer); }; return binding.writeString(fd, buffer, offset, length, position); };
源码：
fs.write(fd, buffer, offset, length[, position], [callback(err, bytesWritten, buffer)]) fs.write(fd, data[, position[, encoding]], [callback(err, written, string)]) //fs.write(fd, buffer, offset, length[, position], [callback(err, bytesWritten, buffer)]) //执行结果：bytesWritten = 8 , buffer =var fs = require('fs'); fs.open('content.txt', 'a', function(err,fd){ if(err){ throw err; } var data = '123123123 hello world'; var buf = new Buffer(8); fs.write(fd, buf, 0, 8, 0, function(err, bytesWritten, buffer){ if(err){ throw err; } console.log(bytesWritten); console.log(buffer); fs.close(fd,function(err){ if(err){ throw err; } console.log('file closed'); }) }) }) //fs.write(fd, data[, position[, encoding]], [callback(err, written, string)]) //执行结果：written = 21 , string = var fs = require('fs'); fs.open('content.txt', 'a', function(err,fd){ if(err){ throw err; } var data = '123123123 hello world'; fs.write(fd, data, 0 , 'utf-8', function(err, written, string){ if(err){ throw err; } console.log(written); console.log(string); fs.close(fd,function(err){ if(err){ throw err; } console.log('file closed'); }) }) }) // usage: // fs.write(fd, buffer, offset, length[, position], callback); // OR // fs.write(fd, string[, position[, encoding]], callback); fs.write = function(fd, buffer, offset, length, position, callback) { if (util.isBuffer(buffer)) { // if no position is passed then assume null if (util.isFunction(position)) { callback = position; position = null; } callback = maybeCallback(callback); var wrapper = function(err, written) { // Retain a reference to buffer so that it can't be GC'ed too soon. callback(err, written || 0, buffer); }; return binding.writeBuffer(fd, buffer, offset, length, position, wrapper); } if (util.isString(buffer)) buffer += ''; if (!util.isFunction(position)) { if (util.isFunction(offset)) { position = offset; offset = null; } else { position = length; } length = 'utf8'; } callback = maybeCallback(position); position = function(err, written) { // retain reference to string in case it's external callback(err, written || 0, buffer); }; return binding.writeString(fd, buffer, offset, length, position); };