Learn JS #29

Posted on by Apostube

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]
]);

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);

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

Example

fruits.set("apples", 200);

The get() Method

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

Example

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

The size Property

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

Example

fruits.size;

The delete() Method

The delete() method removes a Map element:

Example

fruits.delete("apples");

The has() Method

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

Example

fruits.has("apples");

Try This:

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

// 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

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

Apostube

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37 thoughts on “Learn JS #29”

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