How to map a map javascript

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How to map a map javascript

Pexels A map legend is a side table or box on a map that shows the meaning of the symbols, shapes, and colors used on the map. The map legend is sometimes called the map key. The map legend often also has a scale to help the map reader gauge distances. Map legends historically have been fixed elements on a printed map, but interactive digital maps often include dynamic map legends.What's on a Map Legend? A map legend shows colors, shapes, and symbols to define a certain characteristic of the map. On a physical location map, you might find areas of lakes, rivers, and mountain ranges highlighted in the map legend for the type of map that's being used. On a political map, you will find areas of influence pertaining to a political party or individual politician. A map legend will show colors and shapes for the political influences, such as parties, that are dominant in a particular state or region. Using a Map Legend Map legends are often found in a top or bottom of a map, with a color or symbol and a description for what these colors and symbols mean. Check your map area, and then consult the map key for a clearer definition of the part of the map you're seeing. The map legend's purpose is also to show relationships between certain things. You might be in a major metro area and consult the subway map, as an alternative to taking a bus or car. The map and its legend can highlight not only the distance but also the complexity or ease of your trip using a public subway. This value in highlighting spatial relationships is a key asset to a printed map legend. Types of Maps Legends There are many different types of maps, so the map legend varies according to the purpose of the map. On physical geographic area maps, the shapes and symbols likely show the location for towns and cities, rivers and lakes, government buildings, county borders, and highways. On more specialized maps, the map legend will differ. For example, on a map of a large building or complex, doors, windows, exits, stairwells, fences, property boundaries, and more will be highlighted in the map legend. Types of Printed Maps Since the dawn of man, mapmakers and cartographers have created maps for guidance by travelers. Early maps were first started getting made on tablets made from clay, and later onto parchment paper and finally to printed maps and book atlases. Today, there are many types of maps and their corresponding legends. You can find common foldable printed road maps and large road maps in bound atlas form. If you're a meteorologist, you will consult weather maps and climate maps. There are reference maps, political maps, population maps, gender maps, and more. Benefits of Printed Maps Printed maps have lost general usage during the past 20 years, due to the rise of satellite-aided GPS on mobile devices. Having a voice direct you in your car via your mobile device is an innovation few of us could live without today. However, printed maps and their legends can often guide us in ways on our travels that small screens cannot. For instance, looking at a larger map can give travelers an idea of what's in the surrounding area, and not just on the direct route to the location. If you are holding a hiking map, you can gauge the distance between your start and your turnaround loop using a printed map. MORE FROM The Map object holds key-value pairs and remembers the original insertion order of the keys. Any value (both objects and primitive values) may be used as either a key or a value. A Map object iterates its elements in insertion order -- a for...of loop returns an array of [key, value] for each iteration. Key equality is based on the sameValueZero algorithm. NaN is considered the same as NaN (even though NaN !== NaN) and all other values are considered equal according to the semantics of the === operator. In the current ECMAScript specification, -0 and +0 are considered equal, although this was not so in earlier drafts. See "Value equality for -0 and 0" in the Browser compatibility table for details. Object is similar to Map--both let you set keys to values, retrieve those values, delete keys, and detect whether something is stored at a key. For this reason (and because there were no built-in alternatives), Object has been used as Map historically. However, there are important differences that make Map preferable in some cases: Setting Object properties works for Map objects as well, and can cause considerable confusion. Therefore, this appears to work in a way: const wrongMap = new Map() wrongMap['bla'] = 'blaa' wrongMap['bla2'] = 'blaaa2' console.log(wrongMap) But that way of setting a property does not interact with the Map data structure. It uses the feature of the generic object. The value of 'bla' is not stored in the Map for queries. Other operations on the data fail: wrongMap.has('bla') wrongMap.delete('bla') console.log(wrongMap) The correct usage for storing data in the Map is through the set(key, value) method. const contacts = new Map() contacts.set('Jessie', {phone: "213-555-1234", address: "123 N 1st Ave"}) contacts.has('Jessie') contacts.get('Hilary') contacts.set('Hilary', {phone: "617-555-4321", address: "321 S 2nd St"}) contacts.get('Jessie') contacts.delete('Raymond') contacts.delete('Jessie') console.log(contacts.size) Map() Creates a new Map object. Map.prototype.size Returns the number of key/value pairs in the Map object. Map.prototype[@@iterator]() Returns a new Iterator object that contains an array of [key, value] for each element in the Map object in insertion order. Map.prototype.keys() Returns a new Iterator object that contains the keys for each element in the Map object in insertion order. Map.prototype.values() Returns a new Iterator object that contains the values for each element in the Map object in insertion order. Map.prototype.entries() Returns a new Iterator object that contains an array of [key, value] for each element in the Map object in insertion order. Map.prototype.forEach(callbackFn[, thisArg]) Calls callbackFn once for each key-value pair present in the Map object, in insertion order. If a thisArg parameter is provided to forEach, it will be used as the this value for each callback. const myMap = new Map() const keyString = 'a string' const keyObj = {} const keyFunc = function() {} myMap.set(keyString, "value associated with 'a string'") myMap.set(keyObj, 'value associated with keyObj') myMap.set(keyFunc, 'value associated with keyFunc') myMap.size myMap.get(keyString) myMap.get(keyObj) myMap.get(keyFunc) myMap.get('a string') myMap.get({}) myMap.get(function() {}) NaN can also be used as a key. Even though every NaN is not equal to itself (NaN !== NaN is true), the following example works because NaNs are indistinguishable from each other: const myMap = new Map() myMap.set(NaN, 'not a number') myMap.get(NaN) const otherNaN = Number('foo') myMap.get(otherNaN) Maps can be iterated using a for..of loop: const myMap = new Map() myMap.set(0, 'zero') myMap.set(1, 'one') for (const [key, value] of myMap) { console.log(key + ' = ' + value) } for (const key of myMap.keys()) { console.log(key) } for (const value of myMap.values()) { console.log(value) } for (const [key, value] of myMap.entries()) { console.log(key + ' = ' + value) } Maps can be iterated using the forEach() method: myMap.forEach(function(value, key) { console.log(key + ' = ' + value) }) const kvArray = [['key1', 'value1'], ['key2', 'value2']] const myMap = new Map(kvArray) myMap.get('key1') console.log(Array.from(myMap)) console.log([...myMap]) console.log(Array.from(myMap.keys())) Just like Arrays, Maps can be cloned: const original = new Map([ [1, 'one'] ]) const clone = new Map(original) console.log(clone.get(1)) console.log(original === clone) Note: Keep in mind that the data itself is not cloned. Maps can be merged, maintaining key uniqueness: const first = new Map([ [1, 'one'], [2, 'two'], [3, 'three'], ]) const second = new Map([ [1, 'uno'], [2, 'dos'] ]) const merged = new Map([...first, ...second]) console.log(merged.get(1)) console.log(merged.get(2)) console.log(merged.get(3)) Maps can be merged with Arrays, too: const first = new Map([ [1, 'one'], [2, 'two'], [3, 'three'], ]) const second = new Map([ [1, 'uno'], [2, 'dos'] ]) const merged = new Map([...first, ...second, [1, 'eins']]) console.log(merged.get(1)) console.log(merged.get(2)) console.log(merged.get(3)) SpecificationECMAScript Language Specification # sec-map-objectsBCD tables only load in the browserSee also A polyfill of Map is available in core-js Set WeakMap WeakSet Summary: in this tutorial, you will learn about the JavaScript Map object that maps a key to a value.Introduction to JavaScript Map objectBefore ES6, we often used an object to emulate a map by mapping a key to a value of any type. But using an object as a map has some side effects:An object always has a default key like the prototype.A key of an object must be a string or a symbol, you cannot use an object as a key.An object does not have a property that represents the size of the map.ES6 provides a new collection type called Map that addresses these deficiencies.By definition, a Map object holds key-value pairs where values of any type can be used as either keys or values. In addition, a Map object remembers the original insertion order of the keys.To create a new Map, you use the following syntax:let map = new Map([iterable]);Code language: JavaScript (javascript)The Map() accepts an optional iterable object whose elements are key-value pairs.Useful JavaScript Map methodsclear() ? removes all elements from the map object. delete(key) ? removes an element specified by the key. It returns if the element is in the map, or false if it does not. entries() ? returns a new Iterator object that contains an array of [key, value] for each element in the map object. The order of objects in the map is the same as the insertion order. forEach(callback[, thisArg]) ? invokes a callback for each key-value pair in the map in the insertion order. The optional thisArg parameter sets the this value for each callback. get(key) ? returns the value associated with the key. If the key does not exist, it returns undefined. has(key) ? returns true if a value associated with the key exists, otherwise, return false. keys() ? returns a new Iterator that contains the keys for elements in insertion order. set(key, value) ? sets the value for the key in the map object. It returns the map object itself therefore you can chain this method with other methods. values() returns a new iterator object that contains values for each element in insertion order.JavaScript Map examplesCreate a new Map objectSuppose you have a list of user objects as follows:let john = {name: 'John Doe'}, lily = {name: 'Lily Bush'}, peter = {name: 'Peter Drucker'};Code language: JavaScript (javascript)Assuming that you have to create a map of users and roles. In this case, you use the following code:let userRoles = new Map();Code language: JavaScript (javascript)The userRoles is an instance of the Map object and its type is an object as illustrated in the following example:console.log(typeof(userRoles)); console.log(userRoles instanceof Map); Code language: JavaScript (javascript)Add elements to a MapTo assign a role to a user, you use the set() method:userRoles.set(john, 'admin');Code language: JavaScript (javascript)The set() method maps user john with the admin role. Since the set() method is chainable, you can save some typings as shown in this example:userRoles.set(lily, 'editor') .set(peter, 'subscriber'); Code language: JavaScript (javascript)Initialize a map with an iterable objectAs mentioned earlier, you can pass an iterable object to the Map() constructor:let userRoles = new Map([ [john, 'admin'], [lily, 'editor'], [peter, 'subscriber'] ]);Code language: JavaScript (javascript)Get an element from a map by keyIf you want to see the roles of John , you use the get() method:Code language: JavaScript (javascript)If you pass a key that does not exist, the get() method will return undefined.let foo = {name: 'Foo'}; userRoles.get(foo); Code language: JavaScript (javascript)Check the existence of an element by keyTo check if a key exists in the map, you use the has() method.userRoles.has(foo); userRoles.has(lily); Code language: JavaScript (javascript)Get the number of elements in the mapThe size property returns the number of entries of the map.console.log(userRoles.size); Code language: JavaScript (javascript)Iterate over map keysTo get the keys of a Map object, you use the keys() method. The keys() returns a new iterator object that contains the keys of elements in the map.The following example displays the username of the users in the userRoles map object.let john = { name: 'John Doe' }, lily = { name: 'Lily Bush' }, peter = { name: 'Peter Drucker' }; let userRoles = new Map([ [john, 'admin'], [lily, 'editor'], [peter, 'subscriber'], ]); for (const user of userRoles.keys()) { console.log(user.name); }Code language: JavaScript (javascript)Output:John Doe Lily Bush Peter DruckerIterate over map valuesSimilarly, you can use the values() method to get an iterator object that contains values for all the elements in the map:let john = { name: 'John Doe' }, lily = { name: 'Lily Bush' }, peter = { name: 'Peter Drucker' }; let userRoles = new Map([ [john, 'admin'], [lily, 'editor'], [peter, 'subscriber'], ]); for (let role of userRoles.values()) { console.log(role); }Code language: JavaScript (javascript)Output:Iterate over map elementsAlso, the entries() method returns an iterator object that contains an array of [key,value] of each element in the Map object:let john = { name: 'John Doe' }, lily = { name: 'Lily Bush' }, peter = { name: 'Peter Drucker' }; let userRoles = new Map([ [john, 'admin'], [lily, 'editor'], [peter, 'subscriber'], ]); for (const role of userRoles.entries()) { console.log(`${role[0].name}: ${role[1]}`); }Code language: JavaScript (javascript)To make the iteration more natural, you can use destructuring as follows:let john = { name: 'John Doe' }, lily = { name: 'Lily Bush' }, peter = { name: 'Peter Drucker' }; let userRoles = new Map([ [john, 'admin'], [lily, 'editor'], [peter, 'subscriber'], ]); for (let [user, role] of userRoles.entries()) { console.log(`${user.name}: ${role}`); } Code language: JavaScript (javascript)In addition to for...of loop, you can use the forEach() method of the map object:let john = { name: 'John Doe' }, lily = { name: 'Lily Bush' }, peter = { name: 'Peter Drucker' }; let userRoles = new Map([ [john, 'admin'], [lily, 'editor'], [peter, 'subscriber'], ]); userRoles.forEach((role, user) => console.log(`${user.name}: ${role}`));Code language: JavaScript (javascript)Convert map keys or values to a arraySometimes, you want to work with an array instead of an iterable object, in this case, you can use the spread operator.The following example converts keys for each element into an array of keys:var keys = [...userRoles.keys()]; console.log(keys);Code language: JavaScript (javascript)Output:[ { name: 'John Doe' }, { name: 'Lily Bush' }, { name: 'Peter Drucker' } ]Code language: JavaScript (javascript)And the following converts the values of elements to an array:let roles = [...userRoles.values()]; console.log(roles);Code language: JavaScript (javascript)Output[ 'admin', 'editor', 'subscriber' ]Code language: JSON / JSON with Comments (json)Delete an element by keyTo delete an entry in the map, you use the delete() method.Code language: CSS (css)Delete all elements in the mapTo delete all entries in the Map object, you use the clear() method:Code language: CSS (css)Hence, the size of the map now is zero.console.log(userRoles.size); Code language: JavaScript (javascript)WeakMapA WeakMap is similar to a Map except the keys of a WeakMap must be objects. It means that when a reference to a key (an object) is out of scope, the corresponding value is automatically released from the memory.A WeakMap only has subset methods of a Map object: get(key) set(key, value) has(key) delete(key)Here are the main difference between a Map and a WeekMap:Elements of a WeakMap cannot be iterated.Cannot clear all elements at once.Cannot check the size of a WeakMap.In this tutorial, you have learned how to work with the JavaScript Map object and its useful methods to manipulate entries in the map.Was this tutorial helpful ?

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