PRDV402: Introduction to JavaScript II

Using the Set object

const mySet1 = new Set();

mySet1.add(1); // Set(1) { 1 }
mySet1.add(5); // Set(2) { 1, 5 }
mySet1.add(5); // Set(2) { 1, 5 }
mySet1.add("some text"); // Set(3) { 1, 5, 'some text' }
const o = { a: 1, b: 2 };
mySet1.add(o);

mySet1.add({ a: 1, b: 2 }); // o is referencing a different object, so this is okay

mySet1.has(1); // true
mySet1.has(3); // false, since 3 has not been added to the set
mySet1.has(5); // true
mySet1.has(Math.sqrt(25)); // true
mySet1.has("Some Text".toLowerCase()); // true
mySet1.has(o); // true

mySet1.size; // 5

mySet1.delete(5); // removes 5 from the set
mySet1.has(5); // false, 5 has been removed

mySet1.size; // 4, since we just removed one value

mySet1.add(5); // Set(5) { 1, 'some text', {...}, {...}, 5 } - a previously deleted item will be added as a new item, it will not retain its original position before deletion

console.log(mySet1); // Set(5) { 1, "some text", {…}, {…}, 5 }

Iterating sets

The iteration over a set visits elements in insertion order.

for (const item of mySet1) {
  console.log(item);
}
// 1, "some text", { "a": 1, "b": 2 }, { "a": 1, "b": 2 }, 5

for (const item of mySet1.keys()) {
  console.log(item);
}
// 1, "some text", { "a": 1, "b": 2 }, { "a": 1, "b": 2 }, 5

for (const item of mySet1.values()) {
  console.log(item);
}
// 1, "some text", { "a": 1, "b": 2 }, { "a": 1, "b": 2 }, 5

// key and value are the same here
for (const [key, value] of mySet1.entries()) {
  console.log(key);
}
// 1, "some text", { "a": 1, "b": 2 }, { "a": 1, "b": 2 }, 5

// Convert Set object to an Array object, with Array.from
const myArr = Array.from(mySet1); // [1, "some text", {"a": 1, "b": 2}, {"a": 1, "b": 2}, 5]

// the following will also work if run in an HTML document
mySet1.add(document.body);
mySet1.has(document.querySelector("body")); // true

// converting between Set and Array
const mySet2 = new Set([1, 2, 3, 4]);
console.log(mySet2.size); // 4
console.log([...mySet2]); // [1, 2, 3, 4]

// intersect can be simulated via
const intersection = new Set([...mySet1].filter((x) => mySet2.has(x)));

// difference can be simulated via
const difference = new Set([...mySet1].filter((x) => !mySet2.has(x)));

// Iterate set entries with forEach()
mySet2.forEach((value) => {
  console.log(value);
});
// 1
// 2
// 3
// 4

Implementing basic set operations

function isSuperset(set, subset) {
  for (const elem of subset) {
    if (!set.has(elem)) {
      return false;
    }
  }
  return true;
}

function union(setA, setB) {
  const _union = new Set(setA);
  for (const elem of setB) {
    _union.add(elem);
  }
  return _union;
}

function intersection(setA, setB) {
  const _intersection = new Set();
  for (const elem of setB) {
    if (setA.has(elem)) {
      _intersection.add(elem);
    }
  }
  return _intersection;
}

function symmetricDifference(setA, setB) {
  const _difference = new Set(setA);
  for (const elem of setB) {
    if (_difference.has(elem)) {
      _difference.delete(elem);
    } else {
      _difference.add(elem);
    }
  }
  return _difference;
}

function difference(setA, setB) {
  const _difference = new Set(setA);
  for (const elem of setB) {
    _difference.delete(elem);
  }
  return _difference;
}

// Examples
const setA = new Set([1, 2, 3, 4]);
const setB = new Set([2, 3]);
const setC = new Set([3, 4, 5, 6]);

isSuperset(setA, setB); // returns true
union(setA, setC); // returns Set {1, 2, 3, 4, 5, 6}
intersection(setA, setC); // returns Set {3, 4}
symmetricDifference(setA, setC); // returns Set {1, 2, 5, 6}
difference(setA, setC); // returns Set {1, 2}

Relation to arrays

const myArray = ["value1", "value2", "value3"];

// Use the regular Set constructor to transform an Array into a Set
const mySet = new Set(myArray);

mySet.has("value1"); // returns true

// Use the spread syntax to transform a set into an Array.
console.log([...mySet]); // Will show you exactly the same Array as myArray

Remove duplicate elements from an array

// Use to remove duplicate elements from an array

const numbers = [2, 3, 4, 4, 2, 3, 3, 4, 4, 5, 5, 6, 6, 7, 5, 32, 3, 4, 5];

console.log([...new Set(numbers)]);

// [2, 3, 4, 5, 6, 7, 32]

Relation to strings

const text = "India";

const mySet = new Set(text); // Set(5) {'I', 'n', 'd', 'i', 'a'}
mySet.size; // 5

// case sensitive & duplicate omission
new Set("Firefox"); // Set(7) { "F", "i", "r", "e", "f", "o", "x" }
new Set("firefox"); // Set(6) { "f", "i", "r", "e", "o", "x" }

Use a set to ensure the uniqueness of a list of values

const array = Array.from(document.querySelectorAll("[id]")).map((e) => e.id);

const set = new Set(array);
console.assert(set.size === array.length);

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Source: Mozilla, https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set#examples
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