JavaScript Sets
A JavaScript Set is a collection of unique values.
Each value can only occur once in a Set.
A Set can hold any value of any data type.
Set Methods
MethodDescription
new Set()Creates a new Set
add()Adds a new element to the Set
delete()Removes an element from a Set
has()Returns true if a value exists
clear()Removes all elements from a Set
forEach()Invokes a callback for each element
values()Returns an Iterator with all the values in a Set
keys()Same as values()
entries()Returns an Iterator with the [value,value] pairs from a Set
PropertyDescription
sizeReturns the number elements in a Set
How to Create a Set
You can create a JavaScript Set by:
Passing an Array to new Set()
Create a new Set and use add() to add values
Create a new Set and use add() to add variables
The new Set() Method
Pass an Array to the new Set() constructor:
Example
// Create a Set
const letters = new Set(["a","b","c"]);
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Create a Set and add literal values:
Example
// Create a Set
const letters = new Set();
// Add Values to the Set
letters.add("a");
letters.add("b");
letters.add("c");
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Create a Set and add variables:
Example
// Create Variables
const a = "a";
const b = "b";
const c = "c";
// Create a Set
const letters = new Set();
// Add Variables to the Set
letters.add(a);
letters.add(b);
letters.add(c);
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The add() Method
Example
letters.add("d");
letters.add("e");
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If you add equal elements, only the first will be saved:
Example
letters.add("a");
letters.add("b");
letters.add("c");
letters.add("c");
letters.add("c");
letters.add("c");
letters.add("c");
letters.add("c");
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The forEach() Method
The forEach() method invokes a function for each Set element:
Example
// Create a Set
const letters = new Set(["a","b","c"]);
// List all entries
let text = "";
letters.forEach (function(value) {
text += value;
})
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The values() Method
The values() method returns an Iterator object containing all the values in a Set:
Example
letters.values() // Returns [object Set Iterator]
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Now you can use the Iterator object to access the elements:
Example
// Create an Iterator
const myIterator = letters.values();
// List all Values
let text = "";
for (const entry of myIterator) {
text += entry;
}
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The keys() Method
A Set has no keys.
keys() returns the same as
values().
This makes Sets compatible with Maps.
Example
letters.keys() // Returns [object Set Iterator]
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The entries() Method
A Set has no keys.
entries() returns [value,value] pairs
instead of [key,value] pairs.
This makes Sets compatible with Maps:
Example
// Create an Iterator
const myIterator = letters.entries();
// List all Entries
let text = "";
for (const entry of myIterator) {
text += entry;
}
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Sets are Objects
For a Set, typeof returns object:
typeof letters; // Returns object
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For a Set, instanceof Set returns true:
letters instanceof Set; // Returns true
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Reference: https://www.w3schools.com/js/js_object_sets.asp