javascript - 编辑文件并在nodejs中再次读取编辑后的文件

Question

我有一个包含大约 1000 行数据的文件，因为我将“FFFF”附加到特定行。附加工作正常，文件已更改。但是，当我在追加后立即重新读取文件时，并没有发生变化——它显示的是旧数据而不是新的变化。

下面是代码：

const fs = require('fs');

const parseTxt = async (txtFile) => {
    const data = await fs.readFileAsync(txtFile);
    const str = data.toString('utf8');
    const lines = str.split('\r\n');
    
    var ff_string = 'FFFF';
    var append_FF = linedata.substring(0, line_len - 2) + ff_string + linedata.substring(line_len - 2);
    replace_line(linedata, append_FF, txtFile);

    /* Re-Read the File with Changed/Appended data FF */
    var re_data = re_read_file(txtFile);
    const re_str = re_data.toString('utf8');
    const re_lines = re_str.split('\r\n');
    console.log('Re Lines Data:=========',re_str);
}

parseTxt('file.txt').then(() => { 
    console.log('parseTxt===');
})

function replace_line(linedata, append_FF, txtFile){
    fs.readFile(txtFile, 'utf8', function(err,data) {
        var formatted = data.replace(linedata, append_FF);
        fs.writeFile(txtFile, formatted, 'utf8', function (err) {
            if (err) return console.log(err);
        });
    });
    return;
}

function re_read_file(txtFile){
  try {
    const data = fs.readFileSync(txtFile)
    console.log('Re-readed File data',data);
  } catch (err) {
    console.error(err)
  }
}

变量 'linedata' 和 'line_len' 我从不同的函数中得到它，因为它是一个巨大的函数，所以我没有包含在其中。

标签： javascriptnode.jsfile

Answer 1

The reason for your issues is that writing to the file is asynchronous. Here's what happens:

// you do some magic and create a string that you'd like to write to a file
replace_line(linedata, append_FF, txtFile); // you call "write this to a file (1)

/* Re-Read the File with Changed/Appended data FF */
// you've called `replace_line`, and you get here
// but since reading/writing to the file is asynchronous, it's working somewhere
// and your code continues to execute
// you read the file again, happens immediately! check (2)
var re_data = re_read_file(txtFile);
const re_str = re_data.toString('utf8');
const re_lines = re_str.split('\r\n');
// you print the result
console.log('Re Lines Data:=========',re_str);


// you execute this method from (1)
function replace_line(linedata, append_FF, txtFile) {
    // you start reading the file, but it doesn't happen immediately
    // you provide a callback to execute when read is done
    fs.readFile(txtFile, 'utf8', function(err,data) {
        var formatted = data.replace(linedata, append_FF);
        // same goes here - you start writing the file, but it's asynchronous
        // you pass a callback to execute when it's done
        fs.writeFile(txtFile, formatted, 'utf8', function (err) {
            if (err) return console.log(err);
        });
    });
    return;
}

function re_read_file(txtFile){ // (2)
  try {
    // here you use read file SYNC, so it happens immediately!
    const data = fs.readFileSync(txtFile)
    console.log('Re-readed File data',data);
  } catch (err) {
    console.error(err)
  }
}

So the overall flow is like this:

• You do some string manipulation
• You try to write that to a file
  • it makes asynchronous read of the file
    • it makes asynchronous write of the new data
• You read the file synchronously (read immediately)
• You write the result of that read
• -> at some point the asynchronous reading/writing to the file is done

You should either use sync read/write, or you should somehow wait for the updating of the document to be done and then re-read it again (Promises, await/async, callbacks - whatever you like).