node . js fs . file handle . truncate()方法
原文:https://www . geesforgeks . org/node-js-fs-file handle-truncate-method/
fs.filehandle.truncate() 方法是文件系统模块中类 fs.filehandle 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()方法中作为整数传递的那部分字节。
语法:
const filehandle.truncate(len)
参数:该方法取该文件必须被截断的字节长度。 返回值:此方法返回一个不包含任何价值的待定承诺。
下面的程序说明了在 Node.js 中 fs.filehandle.truncate() 方法的使用:
示例 1: 文件名:index.js
java 描述语言
// 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
java 描述语言
// 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:以上程序不会在在线 JavaScript 和脚本编辑器上运行。 参考:https://nodejs . org/dist/latest-v 12 . x/docs/API/fs . html # fs _ file handle _ truncate _ len
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