Node.js fs.filehandle.write() 方法

fs.filehandle.write()方法是文件系统模块中fs.filehandle类的内置应用程序编程接口，用于将数据从缓冲区写入特定文件。
句法：

const filehandle.write(buffer[, offset[, length[, position]]])

参数：此方法确实采用以下参数：

• 缓冲区：缓冲区，它将提供需要存储的数据。
• offset：缓冲区中的起点。
• 长度：要写入的字节数。
• 位置：文件的起点。

返回值：此方法返回一个未决的承诺，其中包含缓冲区和 bytesWritten 属性，该属性描述了写入的字节数。
下面的程序说明了 Node.js 中fs.filehandle.write()方法的使用：
示例 1：文件名：index.js

javascript

// Node.js program to demonstrate the
// filehandle.write() method
const fs = require('fs');
const fsPromises = fs.promises;
 
console.log("file before write operation :- "
    + fs.readFileSync('example.txt', 'utf8'));
 
// Initiating asyncrionise function
async function funct() {
 
    // Initializing following variables
    let filehandle = null;
    let prom = null;
    let buffer = Buffer.from('Geeks');
 
    try {
 
        // Creating and initiating filehandle
        filehandle = await
            fsPromises.open('example.txt', 'r+');
 
        // Writting the file by using
        // write() method
        prom = filehandle.write(
                buffer, 0, buffer.length, 0);
 
    } finally {
 
        if (filehandle) {
 
            // Display the result
            prom.then(function (result) {
                console.log(
                    "file after write operation :- "
                    + (result.buffer).toString());
            })
 
            // Close the file if it is opened.
            await filehandle.close();
        }
    }
}
 
funct().catch(console.error);

运行程序前的目录结构：

运行程序后的目录结构：

使用以下命令运行index.js文件：

node index.js

输出：

file before write operation :- Geeks for Geeks
file after write operation :- Geeks

示例 2：文件名：index.js

javascript

// Node.js program to demonstrate the
// filehandle.sync() method
const fs = require('fs');
const fsPromises = fs.promises;
 
// Data for the new file
let data = "This is a file containing"
        + " a collection of books.";
 
// Name of the file to be created
let file = "books.txt";
 
// Creating the new file 'books.txt'
fs.writeFile(file, data, (err) => {
 
    // Catching error
    if (err) {
        console.log(err);
    }
});
 
// Using fs.exists() method
fs.exists(file, (exists) => {
    if (exists) {
        console.log(
            "content of file before operation :- "
            + (fs.readFileSync(file)));
    }
});
 
// Initiating asyncrionise function
async function funct() {
 
    // Initializing filehandle
    let filehandle = null;
 
    // Initializing following variables
    let buffer = Buffer.from('ABCD');
 
    try {
 
        // Creating and initiating  filehandle
        filehandle = await
            fsPromises.open(file, 'r+');
 
        // Writting the file
        // by using write() method
        prom = filehandle.write(
            buffer, 0, buffer.length, 0);
 
    } finally {
 
        if (filehandle) {
 
            // Display the result
            prom.then(function (result) {
                console.log(
                    "file after write operation :- "
                    + (result.buffer).toString());
            })
 
            await filehandle.close();
        }
    }
}
 
funct().catch(console.error);

运行程序前的目录结构：

运行程序后的目录结构：

使用以下命令运行index.js文件：

node index.js

输出：

content of file before operation :- 
  This is a file containing a collection of books.
file after write operation :- ABCD

参考： https://nodejs.org/dist/latest-v12.x/docs/api/fs.html#fs_filehandle_write_buffer_offset_length_position