13.4. Exporting Modules

We learned how to pull in useful code in the form of modules, but what if we write clever code that we want to share? Fortunately, Node allows us to make our code available for use in other programs.

First, some basic points:

  1. Every Node.js file is treated as a module (also called a package).
  2. From a file, we can export a single function or a set of functions.

13.4.1. Starter Code

We will use the following code sample to practice how to export our work---making it available to import as a module.

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function isPalindrome(str){
   return str === str.split('').reverse().join('');
}

function evenOrOdd(num){
   if (num%2===0){
      return "Even";
   } else {
      return "Odd";
   }
}

function randomArrayElement(arr){
   let index = Math.floor(Math.random()*arr.length);
   return arr[index];
}

These functions are in the practiceExports.js file, and our goal is to import them into index.js.

13.4.2. Exporting a Single Function

Let's start by exporting the isPalindrome function. At the bottom of the practiceExports.js code, add the line module.exports = isPalindrome;. This makes the function available to other files.

In index.js, we import practiceExports.js with a require statement. isPalindrome gets pulled in and assigned to the new variable palindromeCheck, and we can now call the function from within index.js.

Try It

Add the following code to index.js, then click "Run".

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const palindromeCheck = require('./practiceExports.js');

console.log(typeof palindromeCheck);
console.log(palindromeCheck('that'));
console.log(palindromeCheck('radar'));

Console Output

function
false
true

There are several points to make about the code and output.

  1. By setting module.exports equal to isPalindrome, we exported that single function.
  2. Even though we require the file practiceExports.js, it only assigns isPalindrome to the variable palindromeCheck. Thus, typeof palindromeCheck returns function.
  3. palindromeCheck now behaves in the same way as isPalindrome, so calling palindromeCheck('that') evaluates to false, since 'that' is not a palindrome.

13.4.3. Exporting Multiple Functions

practiceExports.js contains three functions, and to export all of them we use a different syntax for module.exports. Instead of setting up a single function, we will create an object.

To export multiple functions, the syntax is:

module.exports = {
   isPalindrome: isPalindrome,
   evenOrOdd: evenOrOdd,
   randomArrayElement: randomArrayElement
}

Within the {}, we create a series of key:value pairs. The keys will be the names used in index.js to call the functions. The values are the functions themselves.

Note

We do not have to make the key match the name of the function, but doing so helps maintain consistency between files.

Warning

You might be tempted to use three statements to export the three functions:

module.exports = isPalindrome;
module.exports = evenOrOdd;
module.exports = randomArrayElement;

This will NOT work, because Node expects only ONE module.exports statement in a file. No error will be thrown if you use more than one, but require('./practiceExports.js') will only pull in the information from the LAST statement.

13.4.3.1. Try It

Use the object syntax as shown above to modify module.exports in practiceExports.js. We could include only one or two of the functions, but for this practice let's use all of them.

Next, modify index.js as follows and click "Run":

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const practice = require('./practiceExports.js');

console.log(typeof practice);
console.log(practice);

typeof indicates that practice is an object, and printing practice gives us a list of its key/value pairs (e.g. isPalindrome: [Function: isPalindrome]).

All of the functions from practiceExports are included in the practice object. To call them, we use dot notation--- practice.functionName(argument).

Modify index.js again and click "Run":

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const practice = require('./practiceExports.js');
let arr = ['Hello', 'World', 123, 987, 'LC101'];

console.log(practice.isPalindrome('mom'));
console.log(practice.evenOrOdd(9));

for (i=0; i < 3; i++){
   console.log(practice.randomArrayElement(arr));
}

Console Output

true
Odd
123
World
LC101

Success! You exported your first module.

13.4.4. What If

You might be wondering, If I have 20+ functions in a file, and I want to export them ALL, do I really need to type 20+ key/value pairs in module.exports?

The quick answer is, Yes. require only pulls in items identified in module.exports. The longer answer is, Hmmm, you missed the point.

Just like functions, we want to keep modules small and specific. Each module should focus on a single idea and contain only a few related functions. With this in mind, we see that practiceExports falls short of the goal. Even though it is small in size, isPalindrome, evenOrOdd, and randomArrayElement do not really compliment each other. They would be better placed in different modules.

If you find yourself writing lots of functions in a single file, consider splitting them up into smaller, more detailed modules. Doing this makes debugging easier, organizes your work, and helps you identify which modules to import into a new project. A module titled cleverLC101Work is not nearly as helpful as one called arraySortingMethods.

13.4.5. Check Your Understanding

Question

A module in Node.js is:

  1. A file containing JavaScript code intended for use in other Node programs.
  2. A separate block of code within a program.
  3. One line of code in a program.
  4. A function.
  5. A file that contains documentation about functions in JavaScript.

Question

Assume you have the following at the end of a circleStuff.js module:

module.exports = {
   areaOfCircle: areaOfCircle,
   circumference: circumference,
   findRadius: findRadius,
   arcLength: arcLength
}

Inside your project, you import circleStuff:

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const circleStuff = require('./circleStuff.js');

Which of the following is the correct way to find the circumference of a circle from within your project?

  1. circleStuff(argument)
  2. circleStuff.circumference(argument)
  3. circleStuff(circumference(argument))
  4. circumference(argument)