Read and write on a File using JS and Node

Question

I'm trying to write a function that reads a file and when it finds a specific line it deletes everything below it and then it appends another set of lines. I have managed to read the file and find the string I need:

function read() {
    lineReader.on('line', function (line) {
        console.log('Line from file: ' + line)
        if(line.trim() === 'Examples:'){
            console.log('Found what I needed, delete everything below this line');
        }
    });
}

What I'm unable to see is how to delete everything below this line and then append the text I need. I am new to JS and Node.js.

boxmein · Accepted Answer · 2017-10-25 15:29:49Z

1

You could do this by opening a file writing stream at the same time.

In the lineReader event handler, put all the lines before "Example" into a separate file stream. When Example comes along, just append your desired set of lines into the second file stream and close the lineReader.

So add something like this:

// before the code
var writer = fs.createWriteStream('new.txt');
// load custom lines from customLines.txt
var customLines = fs.readFileSync('customLines.txt');

// in the on('line') callback:
  writer.write(line + "\n");

// if 'Examples:' was found:
  writer.write(customLines);
  writer.end();
  lineReader.close();

answered Oct 25, 2017 at 15:29

boxmein

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3 Comments

Phix Over a year ago

@jfriend00 it doesn't have to be, hence the sync variation.

jfriend00 Over a year ago

@Phix - Well, if it's server code (anywhere other than startup), then it should be async because blocking, synchronous I/O can ruin server scalability.

boxmein Over a year ago

It doesn't have to be async. fs.readFileSync() will block the execution until the file contents have been slurped into a string. It is shorter to write for a StackOverflow example, and often times works well enough. There are cases where you should prefer the async version, and this includes larger files and production/serverside/scalable usage.

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