Set (Hash Set)

• A Set is a collection of values, where each value may occur only once.

Methods

• new Set(iterable) – creates the set, optionally from an array of values (any iterable will do).
• set.add(value) – adds a value, returns the set itself.
• set.delete(value) – removes the value, returns true if value existed at the moment of the call, otherwise false.
• set.has(value) – returns true if the value exists in the set, otherwise false.
• set.clear() – removes everything from the set.
• set.size – is the elements count.

let set = new Set();

let john = { name: "John" };
let pete = { name: "Pete" };
let mary = { name: "Mary" };

// visits, some users come multiple times
set.add(john);
set.add(pete);
set.add(mary);
set.add(john);
set.add(mary);

// set keeps only unique values
alert( set.size ); // 3

for (let user of set) {
  alert(user.name); // John (then Pete and Mary)
}

• The alternative to Set could be an array of users, and the code to check for duplicates on every insertion using arr.find.
• The performance would be much worse, because this method walks through the whole array checking every element.
• Set is much better optimized internally for uniqueness checks.
• We can loop over a set either with for..of or using forEach:

let set = new Set(["oranges", "apples", "bananas"]);

for (let value of set) alert(value);

// the same with forEach:
set.forEach((value, valueAgain, set) => {
  alert(value);
});

• The forEach function in the Set has 3 arguments: a value, then again a value, and then the target object.
• Indeed, the same value appears in the arguments twice.
• That’s for compatibility with Map where forEach has three arguments.
  • It may help to replace Map with Set in certain cases with ease, and vice versa.
• The same methods Map has for iterators are also supported:
  • set.keys() – returns an iterable object for values,
  • set.values() – same as set.keys, for compatibility with Map,
  • set.entries() – returns an iterable object for entries [value, value], exists for compatibility with Map.

Map (Dictionary)