Basics of JavaScript for technical interviews

Do you remember how to read from files?

Even if you use JavaScript on a daily basis, you might have forgotten some of the very basics of JavaScript that are required for some technical interviews. Let’s refresh our minds on these:

• How to read from the console
• How to process command line arguments
• How to read and write from and to files
• How to parse CSV or text files
• How to read the length of string
• How to declare and use 2D arrays

How to read from the console

To read from the console using JavaScript, you can use readline module from Node.js.

import readline from "readline";
const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
});
rl.question("Input data: ", (data) => {
  console.log(`Data: ${data}`);
  rl.close();
});

And the result is like this:

Input data: apple,watermelon,lemon
Data: apple,watermelon,lemon

You can read more about it in the Node.js doc.

How to process command line arguments

In the Node.js environment, command line arguments are stored in process.argv. So you can simply output the arguments like:

console.log(`Arguments: ${process.argv}`);

process.argv is an array of strings and the first two elements contain the interpreter and the file path. So just to extract arguments you can use:

const argvs = process.argv.slice(2);
for (const argv of argvs) {
  console.log(`Argument: ${argv}`);
}

In recent versions of the Node.js, you can also import process using import or require:

import process from 'node:process';
const process = require('node:process');

And the output is:

yarn run-script src/02-process-command-line-argv.ts option1=apple option2=orange
Argument: option1=apple
Argument: option2=orange

You can read more about it in the doc.

How to read and write from and to files

To process files, you can use the fs module. Reading from a file and making a copy can be like this:

import fs from "node:fs/promises";
const main = async () => {
  const sampleFile = await fs.readFile("sample.csv", "utf-8");
  console.log(sampleFile);
  await fs.writeFile("sample-copy.csv", sampleFile);
};
main();

In the output, you can see the content of the sample.csv. You can also see sample-copy.csv is created.

These APIs have three versions: a promise version, a callback version, and a synchronous version. You can choose whatever is best for your situation.

To read more about fs, visit the doc here.

How to parse CSV or text files

To parse CSV or text files, first, you can split each line with the new line character (\n). After that, you can split each line with a comma.

const CSV = `apple,orange,banana
pear,grape,pineapple
milk,cheese,yogurt
`;
const main = async () => {
  const lines = CSV.trim().split("\n");
  for (const line of lines) {
    const elements = line.split(",");
    console.log(elements);
  }
};
main();

And the output is:

[ 'apple', 'orange', 'banana' ]
[ 'pear', 'grape', 'pineapple' ]
[ 'milk', 'cheese', 'yogurt' ]

It is straight forward and you just need to keep in mind two things:

• Remove unwanted spaces between elements or new line characters at the end of the file.
• If the element is not a string type, you need to convert them into the type you want. For example, you can convert the element into a number using Number(element).

How to split strings

How to read the length of string

How to declare and use 2D arrays

Happy coding 😎