Node.js fs.filehandle.truncate() 方法

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，用于截断特定的文件对象，并且只保留在 truncate() 中作为整数传递的那个字节方法。

句法：

const filehandle.truncate(len)

参数：此方法采用必须截断此文件的字节长度。
返回值：此方法返回一个未包含任何值的待处理承诺。

下面的程序说明了 Node.js 中fs.filehandle.truncate()方法的使用：

示例 1：文件名：index.js

Javascript

// Node.js program to demonstrate the
// filehandle.truncate() method
const fs = require('fs');
const fsPromises = fs.promises;
 
// Initiating asyncrionise function
async function funct() {
 
    // Initializing 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

示例 2：文件名：index.js

Javascript

// 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) => {
 
    // 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;
 
    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：上述程序不会在在线 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