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
| Method | Description |
|---|---|
| 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 |
| Property | Description |
|---|---|
| size | Returns 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:
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
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:
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.
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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