Category: JavaScript Answers

Sometimes, we want to write JSON object to a JSON file with fs.writeFileSync.

In this article, we’ll look at how to write JSON object to a JSON file with fs.writeFileSync.

How to write JSON object to a JSON file with fs.writeFileSync?

To write JSON object to a JSON file with fs.writeFileSync, we can convert the JSON object to a string before we write it.

For instance, we write

fs.writeFileSync('../data/freqs.json', JSON.stringify(output));

to call fs.writeFileSync with the JSON file path and the stringified JSON object.

We call JSON.stringify to stringify output.

Conclusion

To write JSON object to a JSON file with fs.writeFileSync, we can convert the JSON object to a string before we write it.

Sometimes, we want to make a collection store unique entries if the entry not null with Mongoose.

In this article, we’ll look at how to make a collection store unique entries if the entry not null with Mongoose.

How to make a collection store unique entries if the entry not null with Mongoose?

To make a collection store unique entries if the entry not null with Mongoose, we can specify a unique constraint when we define the schema.

For instance, we write

const UsersSchema = new Schema({
  name: {
    type: String,
    trim: true,
    index: true,
    required: true
  },
  email: {
    type: String,
    trim: true,
    index: {
      unique: true,
      partialFilterExpression: {
        email: {
          $type: "string"
        }
      }
    }
  }
});

to define UserSchema with the email field required to be unique for each entry.

The constraint is set by setting index.unique to true.

Conclusion

To make a collection store unique entries if the entry not null with Mongoose, we can specify a unique constraint when we define the schema.

Sometimes, we want to use Mongoose’s find method with $or.

In this article, we’ll look at how to use Mongoose’s find method with $or.

How to use Mongoose’s find method with $or?

To use Mongoose’s find method with $or, we can set $or to an array of properties and values we’re looking for.

For instance, we write

User.find({
    $or: [{
      '_id': objId
    }, {
      'name': param
    }, {
      'nickname': param
    }]
  },
  (err, docs) => {
    if (!err) {
      //...
    }
  });

to call User.find with an object with the $or property set to an array we’re looking for.

We’re searching for a User entry with _id set to objId or name set to param or nickname set to param.

Then callback is run when the query is done and the docs parameter has the query results.

Conclusion

To use Mongoose’s find method with $or, we can set $or to an array of properties and values we’re looking for.

Sometimes, we want to use Mongoose without defining a schema.

In this article, we’ll look at how to use Mongoose without defining a schema.

How to use Mongoose without defining a schema?

To use Mongoose without defining a schema, we can define a field with the Mixed data type.

For instance, we write

const Any = new Schema({ any: {} });

or

const Any = new Schema({ any: Schema.Types.Mixed });

to define the Any schema that has the any property set to the mixed type by setting the any property to {} or Schema.Types.Mixed.

As a result, we can set any value as the value of any.

Conclusion

To use Mongoose without defining a schema, we can define a field with the Mixed data type.

Sometimes, we want to parse huge log files and read it in line-by-line Node.js.

In this article, we’ll look at how to parse huge log files and read it in line-by-line Node.js.

How to parse huge log files and read it in line-by-line Node.js?

To parse huge log files and read it in line-by-line Node.js, we can use the event-stream package.

To install it, we run

npm install event-stream

Then we use it by writing

const fs = require('fs')
const es = require('event-stream');

let lineNr = 0;

const s = fs.createReadStream('very-large-file.csv')
  .pipe(es.split())
  .pipe(es.mapSync((line) => {
      s.pause();
      lineNr += 1;
      logMemoryUsage(lineNr);
      s.resume();
    })
    .on('error', (err) => {
      console.log('Error while reading file.', err);
    })
    .on('end', () => {
      console.log('Read entire file.')
    })
  );

to call fs.createReadStream with the file we want to read.

And we call pipe with es.split to split the lines from the files.

And then we call es.mapSync with a callback that calls s.pause to pause read the file.

And then we run the code we want to with line read from the file.

Then we call s.resume to resume reading the file.

Also, we call on with 'error' and 'end' to listen for those events.

'error' event is emitted when there’s an error reading the file.

'end' event is emitted when the file reading operation is done.

Conclusion

To parse huge log files and read it in line-by-line Node.js, we can use the event-stream package.