Question

What’s the best way to overwrite a line in a large (2MB+) text file using node.js?

My current method involves

• copying the entire file into a buffer.
• Spliting the buffer into an array by the new line character (\n).
• Overwriting the line by using the buffer index.
• Then overwriting the file with the buffer after join with \n.

Answer 1

First, you need to search where the line starts and where it ends. Next you need to use a function for replacing the line. I have the solution for the first part using one of my libraries: Node-BufferedReader.

var lineToReplace = "your_line_to_replace";
var startLineOffset = 0;
var endLineOffset = 0;

new BufferedReader ("your_file", { encoding: "utf8" })
    .on ("error", function (error){
        console.log (error);
    })
    .on ("line", function (line, byteOffset){
        startLineOffset = endLineOffset;
        endLineOffset = byteOffset - 1; //byteOffset is the offset of the NEXT byte. -1 if it's the end of the file, if that's the case, endLineOffset = <the file size>

        if (line === lineToReplace ){
            console.log ("start: " + startLineOffset + ", end: " + endLineOffset +
                    ", length: " + (endLineOffset - startLineOffset));
            this.interrupt (); //interrupts the reading and finishes
        }
    })
    .read ();