How can I write an array to a file in nodejs and keep the square brackets?

Question

I want to write a matrix to a .js file. When I use console.log(matrix) everything is fine but when I write it to the file it comes out differently.

var fs = require("fs");
var matrix = new Array(10);
for(var i=0;i<matrix.length;i++) matrix[i]=[];

for (var i = 0; i < 100 ; i++)
{ 
    var n = i%10;
    matrix[n].push(i);      
}

console.log(matrix);

//write it as a js array and export it (can't get brackets to stay)
fs.writeFile("./matrixtest.js", matrix, function(err) {
if(err) {
        console.log(err);
  } 
  else {
    console.log("Output saved to /matrixtest.js.");
    }
});     

So the console.log gives me [[0,10,20,30,...100],...,[1,11,21,31,...91]] and so on. But opening up matrixtest.js it's only this:

0,10,20,30,40,50...

All the numbers separated by commas with no brackets. How do I prevent it from converting to that format? Thank you.

Answer 1

stringify it (JSON.stringify) before saving it then parse it (JSON.parse) when reading it back in.

fs.writeFile("./matrixtest.js", JSON.stringify(matrix), function(err) {
  if(err) {
        console.log(err);
  } 
  else {
    console.log("Output saved to /matrixtest.js.");
  }
}); 

then when reading back in

var matrix = JSON.parse(contents);

Answer 2

When you are writing an Array to a file, it is getting converted to a string as JavaScript cannot figure out how to write an array as it is. That is why it loses the format. You can convert an array to a string like this and check

var array = [1, 2, 3, 4];
console.log(array.toString());
// 1,2,3,4

So, to solve this problem, you might want to convert it to a JSON string like this

fs.writeFile("./matrixtest.js", JSON.stringify(matrix), function(err) {
    ...
}

Answer 3

The system doesn't know that you wanna store the array into the file with []. It just puts the contents of the array to the file.

• So, first you need to convert the data you wanna write to file into JSON format. The JSON.stringify() method converts a JavaScript value to a JSON string
• Then, write the JSON string to the file.

• Then, while reading, use JSON.parse(). JSON.parse() method parses a JSON string, constructing the JavaScript value or object described by the string

   fs.writeFile('./matrix.js', JSON.stringify(matrix), function (err) { if(err) { console.log(err); } }) fs.readFile('./matrix.js', function(err, data) { console.log(JSON.parse(data)); //Do whatever you want with JSON.parse(data) });