Challenge - Modular structure

Build a counter module with private state using ES modules and closures.

Counter module

In this challenge you will build a counter module with private state. Only increment and read access are exposed; the count itself cannot be reached directly.

You use a closure for this. The createCounter function defines let count = 0 inside and returns increment and getCount methods.

Finally you export the function from the module with export default createCounter. Other files can import and use it.

Private state with closures

A closure is when an inner function reaches the outer function's variables. When createCounter is called a fresh count is created and only the returned methods can see it.

You cannot write counter.count from outside, because count is not a property of the object but a variable of the closure.

This technique is the basis of the module pattern: the public API is open, the inner state stays hidden.

Exercise Write the createCounter function

Write the createCounter() function. Inside it declare let count = 0. Return an object with increment and getCount methods: increment increases the counter by one, getCount returns the current value. End with the export default createCounter line.

Show example solution

function() { count = count + 1; }, getCount: function() { return count; } }. End the last line with export default createCounter;.

Example solution:
function createCounter() {
  let count = 0;
  return {
    increment: function() { count = count + 1; },
    getCount: function() { return count; }
  };
}
export default createCounter;

Test yourself

How do you write a default export in an ES module?
- export default createCounter
- module.exports = createCounter
- exports.createCounter
- public createCounter
Show answer

export default createCounter — Correct. export default ships one default value from the module.

Write the default export

export ____;

Show answer

export default createCounter;

Next lesson: Intermediate section exam

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