Node.js fs.filehandle.truncate()用法及代码示例

fs.filehandle.truncate()方法是文件系统模块中fs.filehandle类的内置应用程序编程接口，用于截断特定文件对象，并且仅保留那么多的字节，该字节作为整数在truncate()方法中传递。

用法:

const filehandle.truncate(len)

参数：此方法占用此文件必须被截断的字节长度。

返回值：此方法返回未包含的承诺。

以下示例程序旨在说明Node.js中fs.filehandle.truncate()方法的使用：

范例1： 文件名：index.js

// Node.js program to demonstrate the  
// filehandle.truncate() method 
const fs = require('fs'); 
const fsPromises = fs.promises; 
  
// Initiating asyncrionise function 
async function funct() { 
  
    // Initializng following variables 
    let filehandle = null; 
    let prom = null; 
  
    try { 
  
        // Data before operation 
        console.log("data before operation:" + 
            fs.readFileSync('example.txt', 'utf8')); 
  
        // Creating and initiating  filehandle 
        filehandle = await  
            fsPromises.open('example.txt', 'r+'); 
  
        // Truncating the file 
        // by using truncate() method 
        prom = filehandle.truncate(5); 
  
    } finally { 
  
        if (filehandle) { 
  
            // Data after operation 
            console.log("data after operation:" + 
                fs.readFileSync('example.txt', 'utf8')); 
  
            // Close the file if it is opened. 
            await filehandle.close(); 
        } 
    } 
} 
  
funct().catch(console.error);

使用以下命令运行index.js文件：

node index.js

输出：

data before operation:ABCDEFGHIJK
data after operation:ABCDE

范例2： 文件名：index.js

// Node.js program to demonstrate the  
// filehandle.truncate() 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) => { 
  
    // Cathing 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() { 
  
    // Initializng filehandle 
    let filehandle = null; 
  
    try { 
      
        // Creating and initiating filehandle 
        filehandle = await fsPromises.open(file, 'r+'); 
  
        // Truncating the file 
        // by using truncate() method 
        prom = filehandle.truncate(4); 
  
    } finally { 
  
        if (filehandle) { 
  
            // Close the file if it is opened. 
            console.log( 
                "content of file after operation:"
                + (fs.readFileSync(file))); 
  
            await filehandle.close(); 
        } 
    } 
} 
  
funct().catch(console.error);

运行程序之前的目录结构：

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.
content of file after operation:This

注意：以上程序无法在在线JavaScript和脚本编辑器上运行。

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

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注：本文由纯净天空筛选整理自RohitPrasad3大神的英文原创作品 Node.js fs.filehandle.truncate() Method。非经特殊声明，原始代码版权归原作者所有，本译文未经允许或授权，请勿转载或复制。