joinpursuit / pursuit-core-npm-and-modules-lab

Playing with modules in Node.js Lab

In this lab you will be learning how to use npm to install 3rd party modules as well as how to create your own modules, how to export it, import it and use it.

1. Chalk

Setup

• In a new folder trychalk, create an app.js file.
• While being in the trychalk directory, in the command line, you will want to execute npm init¹, which will begin setting up a package.json file. You can do npm init --yes to accept all the defaults, or just npm init to have more control of what gets put on your initial package.json. Take a look at the newly created package.json in your folder.
• Enter npm install chalk. This will install the chalk module as a dependency for your project.
  • In your package.json, under dependencies, you should see chalk followed by a version number.
  • A node_modules folder should've been created, with a number of sub-folders. These should include the folder for the chalk module and a folder for each module that chalk depends on.
• In your app.js file, require the chalk module. With npm modules(3rd party modules), like with the core node modules, you can just enter the name of the module as the string for require. You do not need to enter the entire path.
• Read the chalk module documentation

Exercises

1. Write a function called helloBlue that will log the string 'Hello world' in blue.
2. Write a function called helloRed that will log the string 'Hello world' in red.
3. Write a function called stringToColor. It should take two arguments: a string and a color. The function should log the string to the console in the given color. For example, stringToColor('this is a test', 'red') will log this is a test in red.
4. Write a function called evensBlueOddsYellow that takes a string as an argument. It should log all words at an even position/order in blue and all words in an odd position in yellow. For example, evensBlueOddsYellow('this is a test') will log 'this' in yellow, 'is' in blue, 'a' in yellow, and 'test' in blue.
5. Write a function called angryText that takes a string as an argument. It should log that string in red, underlined, and bold capital letters.
6. Write a function called backgroundCyan that takes a string as an argument, and logs that string in white font color and cyan background-color (check the chalk documentation to see how to change background colors).
7. Write a function called boldFirstUnderlineLast that takes a string as an argument and logs the string, with the first word in bold and the last word underlined.
8. Write a function called commandLineChalk that takes the arguments entered in the command line. The function should log out whatever was entered into the terminal, in a color of your choice. Try adding more command line argument options so you can specify the color, background color, etc. Hint: Take a look at process.argv.

2. Build your own module

Setup

• Create a new folder.
• Add a main.js file to the folder.
• Add a math-module.js file to your folder

Exercises

• In the math-module.js file add a function called sum. The sum function should take two arguments and return their sum.

  Example:

  sum(3, 6); //returns 9

• Add a module.exports statement at the bottom of the math-module.js file.

• Add the sum function to module.exports.

• In the main.js file, use the require keyword to import the math-module.js file.

• Call the sum function from the main.js file and save the result to a new variable.

• Add a console.log statement that logs the saved variable.

• Open the terminal and run the main.js file. You should see the output from the sum function

• Add three more functions to the math-module.js file:

  1. A multiply function (takes two arguments and returns their product).
  2. A divide function (takes two arguments and returns the first argument divided by the second).
  3. A square function (takes one argument and returns its square).

  Examples:

  multiply(2, 5); //returns 10
  divide(20, 10); //returns 2
  square(5); //returns 25

• Export the multiply, divide, and square functions by adding them to module.exports in the math-module.js file.

• In the main.js file, use the imported math-module.js file to call the three new functions, and save the results as new variables

• Log the saved variable.

• Open the terminal and run the main.js file. You should see the output from all the functions.

3. String Module

• Create a new file called strings-module.js that contains at least the string functions
  • firstChar - returns the first character.
  • compare - compares string A to string B if they are equal return true.
  • reverse - reverse the string passed as an argument
  • Come up with one of your own functions.
• Import the string-module into the main.js file and try calling and logging the functions from string-module.
• Can you also import the string-module into the math-module and use it in there? Or vice versa?

4. Bonus/Challenge - Files By Extension

The following is a code for an program that takes a user's input of a folder and an extension, and lists all the files in that folder that have the given extension.

const fs = require('fs')
const path = require('path')

const folder = process.argv[2]
const ext = '.' + process.argv[3]

fs.readdir(folder, (err, files) => {
  if (err) {
    console.error(err);
    return;
  }

  files.forEach((file) => {
    if (path.extname(file) === ext) {
      console.log(file)
    }
  })
})

In a new folder, create the files filterFiles.js and main.js.

1. Paste the above code into filterFiles.js and rewrite it as follows:

• Do not take any input from the user (i.e. remove lines that make use of the process object).
• Wrap the code in a function that takes as arguments a folder name, an extension, and a callback function.
• Change the forEach method call to filter: instead of logging the files, return a new array that contains only the files with the desired extension. Save the filtered array to a new variable.
• If there is an error while reading the folder, invoke the callback with the error.
• Otherwise, invoke the callback at the bottom of the readDir function with two arguments: null and the filtered file array.
• Export the function using module.exports.

2. In main.js do the following:

• Import the function from filterFiles.js as filterFilesFn.
• Read the input for folder and extension from the user (using the process object).
• invoke the function with the following arguments:
  • the folder
  • the extension
  • a callback function that takes as arguments an error object and a list. If the error object is not null, it logs the string: 'there was an error' followed by the error. Otherwise, it logs the list, with each element in a separate line.

References

1. npm init docs page.