generate a js file dynamically using fs node

Question

i am trying to generate a .js file by running a node script. here's my code.

const dynamicCode = [
    {
        "ClassCode" : "9620",
        "Rate" : 0.99,
        "IsActive" : "TRUE",
        "StartDate" : "2021-10-19T17:00:00.000-07:00",
        "EndDate" : "9999-12-30T16:00:00.000-08:00"
    },
    {
        "ClassCode" : "0038",
        "Rate" : 12.19,
        "IsActive" : true,
        "StartDate" : "2021-05-31T17:00:00.000-07:00",
        "EndDate" : "9999-12-30T16:00:00.000-08:00"
    }
]

const bdCode = module.exports = {
    async up(db, client) {
      await db.collection('Configuration_Lookup').deleteMany();
      await db.collection('Configuration_Lookup').insertMany(dynamicCode)
    },
  
    async down(db, client) {
      // TODO write the statements to rollback your migration (if possible)
      // Example:
      // await db.collection('albums').updateOne({artist: 'The Beatles'}, {$set: {blacklisted: false}});
    }
  };

fs.writeFileSync("bds.js", JSON.stringify(bdCode, null, 2), 'utf-8');

while this worked for .txt files containing simple texts, or non-executable codes like json objects. it is executing before running the writeFileSync function. so the data inserting into the js file is

{}

how can we generate a js file with working code inside it

Does this question help? stackoverflow.com/q/53501756/14032355 — ikhvjs
– ikhvjs, Commented Jan 19, 2022 at 10:43
Why do you want to do that don't you think it will not be secure as anyone can write the code and try to execute it on the server? What do you want to achieve? — Apoorva Chikara
– Apoorva Chikara, Commented Jan 19, 2022 at 11:08
Hi Apoorva, this snippet is just to automate a daily process, this is not going to be hosted on server, from xlsx file i need to create a mongo migration script to update data in db. — Shubham Shaw
– Shubham Shaw, Commented Jan 19, 2022 at 16:50

Shubham Shaw · Accepted Answer · 2022-01-19 16:56:59Z

i solved the issue by following code.

const dynamicCode = [
    {
        "ClassCode" : "9620",
        "Rate" : 0.99,
        "IsActive" : "TRUE",
        "StartDate" : {"$date": "2021-10-19T00:00:00Z"},
        "EndDate" : {"$date": "9999-12-30T23:59:59Z"}
    },
    {
        "ClassCode" : "0038",
        "Rate" : 12.19,
        "IsActive" : true,
        "StartDate" : {"$date": "2021-10-19T00:00:00Z"},
        "EndDate" : {"$date": "9999-12-30T23:59:59Z"}
    }
]

const bdCode = `module.exports = {
    async up(db, client) {
      await db.collection('Configuration_Lookup').deleteMany();
      await db.collection('Configuration_Lookup').insertMany(${JSON.stringify(dynamicCode, null, 2)})
    },
  
    async down(db, client) {
      // TODO write the statements to rollback your migration (if possible)
      // Example:
      // await db.collection('albums').updateOne({artist: 'The Beatles'}, {$set: {blacklisted: false}});
    }
  };`

fs.writeFileSync("bds.js", bdCode);

Apoorva Chikara · Accepted Answer · 2022-01-19 17:00:07Z

1

You can run the code as a string by creating a function from it using the Function constructor.

const code = "let x = 100; console.log(x + 1)";  
const run = new Function(code);  
run();

Write this string to the JS file and run the js file using child process.

answered Jan 19, 2022 at 17:00

Apoorva Chikara

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3 Comments

Daniel Over a year ago

Woah, that's interesting. I had no idea you could do that with the Function constructor lol

Apoorva Chikara Over a year ago

Yes, this way you can actually run the code. You can use eval(string), but it has security flaws.

Daniel Over a year ago

Yeah I would always use eval when I needed to do something like this; However, I always tried to avoid it for the aforementioned security flaws. How does the function constructor avoid those flaws?

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