READ THE FILE TEXT LINE BY LINE IN EXPRESS JS OR NODE USING PUG.

-If you are using this type of syntax- fs.appendFileSync(`./user/detail.txt`,`${data}`);

It is throwing error -SyntaxError: Unexpected token A in JSON at position 8

JSON.parse()(ref to read file at below) is used to convert a string containing JSON notation into a Javascript object.

Your code turns the object into a string (by calling .toString()) in order to try to parse it as JSON text.
The default .toString() returns "[object Object]", which is not valid JSON; hence the error.

Correct syntax

— -fs.appendFileSync(`./user/detail.txt`,data)

— -fs.writeFileSync(`./user/detail.txt`,data);

HERE IS THE READ FILE PROCESS

try {
    // read contents of the file
    const data = fs.readFileSync('./user/detail.txt', 'UTF-8')
  
    // split the contents by new line
    const lines = data.split(/\r?\n/)
  var arr=[]
    // print all lines
    
    lines.forEach(line => {
        const li=line.split(/,/)// it is split the 
        arr.push(`{"name":"${li[0]}","phone":"${li[1]}","email":"${li[2]}","age":"${li[3]}","city":"${li[4]}"}`);
    })  
   const result = arr.map(info => JSON.parse(info));
   console.log(arr);
    res.render("contactdetails",{result:result})
  } catch (err) {
    console.error(err)
  }
 })

If you download the file

app.get(`/single`,(req,res)=>{
    res.download(folderPath+'/detail.txt', function(err) {//locate the path of txt file
        if(err) {
            res.send("Error");
        }
        else{
            res.send("Downloaded")
        }
    })
})

Pug file

table.table
    thead
      tr
        th FULL NAME
        th PHONE NUMBER
        th EMAIL ADDRESS
        th Age 
        th City
    tbody
      - for (var i = 0; i < result.length-1; ++i) {
        tr
        td #{result[i].name}
        td #{result[i].phone}
        td #{result[i].email}
        td #{result[i].age}
        td #{result[i].city}  
      - }