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7.2 JS 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 1: JS Map Objects

// Create a Map
const fruits = new Map([
  ["apples", 500],
  ["bananas", 300],
  ["oranges", 200]
]);

The set() Method

You can add elements to a Map with the set() method:

Example 2: JS Map Objects - Map.set()

// Create a Map
const fruits = new Map();

// Set Map Values
fruits.set("apples", 500);
fruits.set("bananas", 300);
fruits.set("oranges", 200);

The set() method can also be used to change existing Map values:

Example 3: JS Map Objects - Map.set() part 2

fruits.set("apples", 200);

The get() Method

The get() method gets the value of a key in a Map:

Example 4: JS Map Objects Map.get()

fruits.get("apples");    // Returns 500

The size Property

The size property returns the number of elements in a Map:

Example 5: JS Map Objects - Map.size

fruits.size;

The delete() Method

The delete() method removes a Map element:

Example 6: JS Map Objects - Map.delete()

fruits.delete("apples");

The has() Method

The has() method returns true if a key exists in a Map:

Example 7: JS Map Objects - Map.has()

fruits.has("apples");

Try This: JS Map Objects - Map.has() part 2

fruits.delete("apples");
fruits.has("apples");

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 8: JS Map Objects - Map.forEach()

// List all entries
let text = "";
fruits.forEach (function(value, key) {
  text += key + ' = ' + value;
})

The entries() Method

The entries() method returns an iterator object with the [key, values] in a Map:

Example 9: JS Map Objects - Map.entries()

// List all entries
let text = "";
for (const x of fruits.entries()) {
  text += x;
}

Browser Support

JavaScript Maps are supported in all browsers, except Internet Explorer:


Chrome	Edge	Firefox	Safari	Opera

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Example files created in this module:
JS Sets Map Objects part 1 JS Sets Map Objects part 2 JS Sets Map Objects part 3 JS Sets Map Objects part 4 JS Sets Map Objects part 5 JS Sets Delete Map Objects JS Sets Map.has() part 1 JS Sets Map.has() part 2 JS Sets Map Objects forEach() JS Sets Map Objects Map.entries() JS Map Objects JS Map Objects - Map.set() part 1 JS Map Objects - Map.set() part 2 JS Map Objects Map.get() JS Map Objects - Map.size JS Map Objects - Map.delete() JS Map Objects - Map.has() part 1 JS Map Objects - Map.has() part 2 JS Map Objects - Map.forEach() JS Map Objects - Map.entries() JS typeof Operator part 1 JS typeof Operator part 2 JS typeof Operator part 3 JS typeof Properties - constructor Property part 1 JS typeof Properties - constructor Property part 2 JS typeof Properties - Array Object JS typeof Operator - Date Object JS typeof Operator - Date Function JS typeof Operator - undefined part 1 JS typeof Operator - undefined part 2 JS typeof Operator - empty values JS typeof Operator - null JS typeof Operator - undefined Data Type JS instanceof Operator JS void Operator JS Operators - typeof Operator JS Properties - constructor Property JS Arrays - isArray() function part 1 JS Arrays - isArray() function part 2 JS Date Object - isDate() function part 1 JS Date Object - isDate() function part 2 JS Converting Numbers to Strings part 1 JS Converting Numbers to Strings part 2 JS typeof Operator part 1 JS typeof Operator part 2 JS Bitwise AND JS Bitwise OR JS Bitwise XOR JS Bitwise NOT JS Bitwise Left JS Bitwise Right JS Unsigned Bitwise Right JS Convert Decimal to Binary JS Convert Binary to Decimal JS String Methods JS Regular Expressions part 1 JS String Methods - String replace() JS Regular Expressions part 2 JS Regular Expressions part 3 JS Error Handling part 1 JS try catch JS Errors - RangeError JS Errors - ReferenceError JS Errors - SyntaxError JS Errors - TypeError JS Errors - URIError JS Scope - Local Variables JS Scope - GLOBAL Variable JS Global Variables JS Scope - Global HTML variables JS Hoisted Declarations part 1 JS Hoisted Declarations part 2 JS Hoisting part 1 JS Hoisting part 2 JS Initializations are Not Hoisted part 1 JS Initializations are Not Hoisted part 2 JS Initializations are Not Hoisted part 3 JS "use strict" Mode part 1 JS "use strict" Mode part 2 JS this Keyword JS this Keyword [object Window] part 1 JS this Keyword - window object JS this Keyword [object Window] part 2 JS this Keyword - Function JS this Keyword - Event Handler JS this Keyword - Event Handler JS this Keyword - person object part 1 JS this Keyword - person object part 2 JS this Keyword - Explicit Function Binding JS this Keyword - Function bind() JS this Keyword - Arrow Function part 1 JS this Keyword - Arrow Function part 2