We Keep Coding

How do I go about using variable from multiple files in node js?

I want to get a variable from one .js file to another .js file. Right now I have
main.js
const balances = require('./balance');
console.log(balances.balanceBTC)
and I have
balance.js
const balanceBTC = () => {
return arrayCleaned[0];
};
exports.balanceBTC = balanceBTC;
And I am getting the error
const balances = require('./balance');
ReferenceError: require is not defined
I am running this code via windows PowerShell and the node version is: v14.10.1

NodeJS might be treating your code as an ES Module. And CommonJS variables like "require" are not available in ES modules. Try one of the below:
As mentioned
here,
declare require before using it.
import { createRequire } from 'module';
const require = createRequire(import.meta.url);
const balances = require('./balance');
[...]
If you have "type" : "module" in your package.json, remove it

It looks like the problem is coming from the environment where you are running your code.
Check the following links and you'lle find the answser:
Node | Require is not defined
https://www.thecrazyprogrammer.com/2020/05/require-is-not-defined.html
Require is not defined nodejs
https://github.com/nodejs/node/issues/33741

Write to file without overwriting with fs

I want a logger for my project. Everytime I call myConsole.log it overwrites the existing log. what can i do that the it just write it in the next line and not delete everything
const fs = require('fs');
const myConsole = new console.Console(fs.createWriteStream('./output.log'));
myConsole.log(var)
myConsole.log(var2)
myConsole.log(var3)

I'd reccomend looking at Winston, which is a package that does great logging.
But with your code, you need to add the a (append) flag to your fs writer so it writes properly rather than overwriting (default flag is w), like so:
const myConsole = new console.Console(fs.createWriteStream('./output.log', { flags: 'a' }));
See docs on createwritestream and system flags.

You can pass { flags: 'a'} as options argument to the createWriteStream call.
This flag will open the file for appending instead of overwriting

Update (write to) an object in a separate JS file using Node

I'm fairly new to Node, and am wracking my brains on how to achieve the following:
I have a config file that looks something like this:
// various es imports
export default {
input: {
index: 'src/index.ts',
Button: 'src/Button/index.ts',
Spinner: 'src/Spinner/index.ts',
'icons/Notification': 'src/_shared/components/icons/Notification.tsx',
'icons/Heart': 'src/_shared/components/icons/Heart.tsx',
},
//.. other properties
}
From my node script, i need to somehow read this file and do the following:
Delete any entries in the input object that have a key starting
with icons/
Append new entries to the input object.
Write these changes back to the original config file.
Is there a recommended way to do this in Node, i've been looking at a couple of libs, like replace-in-file but none seem to be suited to this particular case.

Just faced the same concern, here is how I solved it :
1. Gets your file content
If it is not a .js file, then use fs.readFileSync (or fs.readFile) like so :
const fs = require('fs');
const path = require('path');
const myObjectAsString = fs.readFileSync(
path.join( process.cwd(), 'my-file.txt' ), // use path.join for cross-platform
'utf-8' // Otherwise you'll get buffer instead of a plain string
);
// process myObjectAsString to get it as something you can manipulate
Here I am using process.cwd(), in case of a CLI app, it will gives you the current working directory path.
If it is a .js file (eg. a JavaScript config file like webpack.config.js for instance), then simply use require native function, as you would do with regular internal file or NPM module, and you will get all module.export content :
const path = require('path');
const myObject = require( path.join( process.cwd(), 'myFile.js') );
2. Modify your object as you want
// ...
myObject.options.foo = 'An awesome option value';
3. Then rewrite it back to the file
You can simply use fs.writeFileSync to achieve that :
// ...
fs.writeFileSync( path.join(process.cwd(), 'my-file.txt', myObject );
If you want to write a plain JavaScript Object, then you can use util.inspect() native method and you may also use fs.appendFileSync :
// ...
// If we wants to adds the 'module.exports = ' part before
fs.writeFileSync( process.cwd() + '/file.js', 'module.exports = ');
// Writes the plain object to the file
fs.appendFileSync( process.cwd() + '/file.js', util.inspect(options));

Simple node module command line example

I want to make a simple node module that can be run from the command line that I can input files into, then it might change every instance of 'red' to 'blue' for example and then save that as a new file. Is there a simple example out there somewhere that I can edit to fit my purposes? I've tried looking but couldn't find one that was sufficiently simple to understand how to modify it. Can anyone help?

A simple example of replace.js (both old and new files are supposed to be in UTF-8 encoding):
'use strict';
const fs = require('fs');
const oldFilePath = process.argv[2];
const newFilePath = process.argv[3];
const oldFileContent = fs.readFileSync(oldFilePath, 'utf8');
const newFileContent = oldFileContent.replace(/red/g, 'blue');
fs.writeFileSync(newFilePath, newFileContent);
How to call:
node replace.js test.txt new_test.txt
Documentation on used API:
process.argv
fs.readFileSync()
fs.writeFileSync()

Load text as JavaScript object in Node.js

I have a javascript file for node.js:
module.exports = {
someString: 'blblalb'
}
I want to able to read the file as a javascript object, using fs.readFileSync. I can't use require because I am using a variable that may be modified in runtime to load the file.
Is that possible?

You can use eval('JavaScript string') but is highly recommended not to. It is a serious security risk if you cannot 100% trust the source of the text. If a malicious user figures out a way to modify the text they have complete control of your system. It is not a path I would take or recommend.
const text = 'console.log("Hello")';
eval(text);
If I saw that code when I was doing a code review we would definitely be having some words.

it's possible to evaluate a file or string variable as child module in hacky yet valid way.
The problem is that Node.js module environment should be unaware of these operations. Child module may load other modules and contain require(...), it will fail if there is no require function (there is none if it is evaluated with vm) or it uses wrong module relative path (this will happen with eval), also there will be no dedicated module.exports for a module. This can be fixed by wrapping module source with Node.js module wrapper that was rewired to match child module location.
const fs = require('fs');
const Module = require('module');
const path = require('path');
const childModuleAbsPath = path.resolve('./foo/bar.js');
const childModuleBody = fs.readFileSync(childModuleAbsPath);
const childModuleObj = { exports: {} };
const { dir: childModuleDirname, base: childModuleFilename } = path.parse(childModuleAbsPath);
const childRequire = modulePath => module.require(childModuleAbsPath);
require('vm').runInThisContext(Module.wrap(childModuleBody))(
childModuleObj.exports,
childRequire,
childModuleObj,
childModuleDirname,
childModuleFilename
);
In this case childModuleObj.exports.someString === 'blblalb' after bar child module was evaluated.
This is XY problem that should be addressed in another way.
If the intention is to reevaluate a module with new variables, this can be done by invalidating Node module cache by modifying require.cache, e.g. with decache:
decache('./foo/bar');
const reloadedBar = require('./foo/bar');