import Keyword

In this example, we create a re-usable module that exports a function to get all primes within a given range.

// getPrimes.js
/**
 * Returns a list of prime numbers that are smaller than `max`.
 */
function getPrimes(max) {
  const isPrime = Array.from({ length: max }, () => true);
  isPrime[0] = isPrime[1] = false;
  isPrime[2] = true;
  for (let i = 2; i * i < max; i++) {
    if (isPrime[i]) {
      for (let j = i ** 2; j < max; j += i) {
        isPrime[j] = false;
      }
    }
  }
  return [...isPrime.entries()]
    .filter(([, isPrime]) => isPrime)
    .map(([number]) => number);
}

import { getPrimes } from '/modules/getPrimes.js';

console.log(getPrimes(10)); // [2, 3, 5, 7]

The identifier being imported is a live binding, because the module exporting it may mutate it and the imported value would change. However, the module importing it cannot re-assign it.

// my-module.js
export let myValue = 1;
setTimeout(() => {
  myValue = 2;
}, 500);

// main.js
import { myValue } from '/modules/my-module.js';
console.log(myValue); // 1
setTimeout(() => {
  console.log(myValue); // 2; my-module has updated its value
  myValue = 3; // TypeError: Assignment to constant variable.
  // The importing module can only read the value but can't re-assign it.
}, 1000);