JavaScript Maps
A Map holds key-value pairs where the keys can be any datatype.
A Map remembers the original insertion order of the keys.
Essential Map Methods
| Method | Description |
|---|---|
| new Map() | Creates a new Map |
| set() | Sets the value for a key in a Map |
| get() | Gets the value for a key in a Map |
| delete() | Removes a Map element specified by the key |
| has() | Returns true if a key exists in a Map |
| forEach() | Calls a function for each key/value pair in a Map |
| entries() | Returns an iterator with the [key, value] pairs in a Map |
| Property | Description |
| size | Returns the number of elements in a Map |
How to Create a Map
You can create a JavaScript Map by:
- Passing an Array to
new Map() - Create a Map and use
Map.set()
The new Map() Method
You can create a Map by passing an Array to the new Map() constructor:
Example
// Create a Map
const fruits = new Map([
["apples", 500],
["bananas", 300],
["oranges", 200]
]);
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The set() Method
You can add elements to a Map with the set() method:
Example
// Create a Map
const fruits = new Map();
// Set Map Values
fruits.set("apples", 500);
fruits.set("bananas", 300);
fruits.set("oranges", 200);
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The set() method can also be used to change existing Map values:
The get() Method
The get() method gets the value of a key in a Map:
The size Property
The size property returns the number of elements in a Map:
The delete() Method
The delete() method removes a Map element:
The has() Method
The has() method returns true if a key exists in a Map:
Example
fruits.has("apples");
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Try This:
fruits.delete("apples");
fruits.has("apples");
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JavaScript Objects vs Maps
Differences between JavaScript Objects and Maps:
| Object | Map | |
|---|---|---|
| Iterable | Not directly iterable | Directly iterable |
| Size | Do not have a size property | Have a size property |
| Key Types | Keys must be Strings (or Symbols) | Keys can be any datatype |
| Key Order | Keys are not well ordered | Keys are ordered by insertion |
| Defaults | Have default keys | Do not have default keys |
The forEach() Method
The forEach() method calls a function for each key/value pair in a Map:
Example
// List all entries
let text = "";
fruits.forEach (function(value, key) {
text += key + ' = ' + value;
})
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The entries() Method
The entries() method returns an iterator object with the [key, values] in a Map:
Example
// List all entries
let text = "";
for (const x of fruits.entries()) {
text += x;
}
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Browser Support
JavaScript Maps are supported in all browsers, except Internet Explorer:
| Chrome | Edge | Firefox | Safari | Opera |