datatypes

Author: syedamir5560

object-datatypes.js

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+/*
+Set | Map | WeakSet | WeakMap
+*************MAP********** 
+
+ Map is a collection of keyed data items, just like an Object. But the main difference is that Map allows keys of any type.
+
+Methods and properties are:
+
+    new Map() – creates the map.
+    map.set(key, value) – stores the value by the key.
+    map.get(key) – returns the value by the key, undefined if key doesn’t exist in map.
+    map.has(key) – returns true if the key exists, false otherwise.
+    map.delete(key) – removes the element (the key/value pair) by the key.
+    map.clear() – removes everything from the map.
+    map.size – returns the current element count.
+
+let map = new Map();
+map.set("name", "John");//add key value
+map.set(1, "ali");
+map.delete(1);//delete elemenet by key
+console.log(map);   //
+console.log(map.size);  //return size of map
+
+
+********************iteration over map************************
+
+ For looping over a map, there are 3 methods:
+
+ map.keys() – returns an iterable for keys,
+ map.values() – returns an iterable for values,
+ map.entries() – returns an iterable for entries [key, value], it’s used by default in for..of. 
+
+ For instance:
+
+let recipeMap = new Map([
+    ['cucumber', 500],
+    ['tomatoes', 350],
+    ['onion', 50]
+]);
+
+ iterate over keys (vegetables)
+for (let vegetable of recipeMap.keys()) {
+    alert(vegetable); // cucumber, tomatoes, onion
+}
+
+ iterate over values (amounts)
+for (let amount of recipeMap.values()) {
+    alert(amount); // 500, 350, 50
+}
+
+ iterate    over [key, value] entries
+for (let entry of recipeMap) { // the same as of recipeMap.entries()
+    alert(entry); // cucumber,500 (and so on)
+}
+
+
+ *********************************SET****************************
+
+  A Set is a special type collection – “set of values” (without keys), where each value may occur only once.
+
+Its main methods are:
+
+new Set([iterable]) – creates the set, and if an iterable object is provided (usually an array), copies values from it into the set.
+set.add(value) – adds a value, returns the set itself.
+set.delete(value) – removes the value, returns true if value existed at the moment of the call, otherwise false.
+set.has(value) – returns true if the value exists in the set, otherwise false.
+set.clear() – removes everything from the set.
+set.size – is the elements count.
+
+let set = new Set();
+
+let john = { name: "John" };
+let pete = { name: "Pete" };
+let mary = { name: "Mary" };
+
+ visits, some users come multiple times
+set.add(john);
+set.add(pete);
+set.add(mary);
+set.add(john);
+set.add(mary);
+
+ set keeps only unique values
+alert(set.size); // 3
+
+for (let user of set) {
+    alert(user.name); // John (then Pete and Mary)
+}
+
+
+********************iteration over set******************
+let set1 = new Set(["oranges", "apples", "bananas"]);
+
+for (let value of set) alert(value);
+
+ the same with forEach:
+set.forEach((value, valueAgain, set) => {
+    alert(value);
+});
+
+ ****************************** WEAKMAP *******************************
+
+ The first difference between Map and WeakMap is that keys must be objects, not primitive values:
+
+let weakMap = new WeakMap();
+
+let obj = {};
+
+weakMap.set(obj, "ok"); // works fine (object key)
+
+can't use a string as the key
+weakMap.set("test", "Whoops");
+
+WeakMap does not support iteration and methods keys(), values(), entries(), so there’s no way to get all keys or values from it.
+
+WeakMap has only the following methods:
+
+weakMap.set(key, value)
+weakMap.get(key)
+weakMap.delete(key)
+weakMap.has(key)  
+
+*************************** WEAKSET *******************************************
+    WeakSet behaves similarly:
+
+It is analogous to Set, but we may only add objects to WeakSet(not primitives).
+
+An object exists in the set while it is reachable from somewhere else.
+
+Like Set, it supports add, has and delete, but not size, keys() and no iterations.
+
+    For instance, we can add users to WeakSet to keep track of those who visited our site:
+
+let visitedSet = new WeakSet();
+let john = { name: "John" };
+let pete = { name: "Pete" };
+let mary = { name: "Mary" };
+
+visitedSet.add(john); // John visited us
+visitedSet.add(pete); // Then Pete
+visitedSet.add(john); // John again
+
+ visitedSet has 2 users now
+
+ check if John visited ?
+    alert(visitedSet.has(john)); // true
+
+check if Mary visited ?
+    alert(visitedSet.has(mary)); // false
+
+john = null;   
+
+
+ ----------------------------------------------------------------------
+********************************************Date Object********************************
+
+To create a new Date object call new Date() with one of the following arguments:
+
+new Date()
+
+ex=>
+      let x= new Date();
+      console.log("Current date:",x);// Current date
+
+****there are methods to access day,month and year from the date
+
+getFullYear()
+    Get the year (4 digits)
+          let x=new Date();
+          console.log(x.getDay());
+
+getMonth()
+    Get the month, from 0 to 11.
+getDate()
+    Get the day of month, from 1 to 31, the name of the method does look a little bit strange.
+getHours(), getMinutes(), getSeconds(), getMilliseconds()
+    Get the corresponding time components.
+
+
+
+--------------------------------------------------------------------
+**************************************JSON OBJECT******************************
+
+
+JSON is java script object notation
+JSON is text formating for storing and transporing data
+JSON is language independant 
+
+=>This example is a JSON string:
+'{"name":"John", "age":30, "car":null}'
+
+JSON Values
+
+=>In JSON, values must be one of the following data types:
+
+a string
+a number
+an object
+an array
+a boolean
+null
+
+JavaScript provides methods:
+
+JSON.stringify to convert objects into JSON.
+JSON.parse to convert JSON back into an object.
+
+
+=> Example - Parsing JSON
+
+Imagine we received this text from a web server:
+'{"name":"John", "age":30, "city":"New York"}'
+
+Use the JavaScript function JSON.parse() to convert text into a JavaScript object:
+const obj = JSON.parse('{"name":"John", "age":30, "city":"New York"}');
+
+=>JSON.parse()=> mathod in js
+
+When receiving data from a web server, the data is always a string.
+Parse the data with JSON.parse(), and the data becomes a JavaScript object.
+
+use JSON.parse method to convert string into obj
+
+let x='{"name":"John", "age":30, "city":"New York"}';
+let y=JSON.parse(x);
+console.log(y);
+
+=> convert date 
+let x='{"name":"John", "date":"2012-5-14", "city":"New York"}';
+let y=JSON.parse(x );
+y.date=new Date(y.date)
+console.log(y);
+
+Or, you can use the second parameter, of the JSON.parse() function, called reviver.
+    let x='{"name":"John", "date":"2012-12-14", "city":"New York"}';
+
+let y=JSON.parse(x, function(key,value){
+        if(key=="date"){
+            return new Date(value);
+        }
+        else{
+            return value;
+        }
+});
+console.log(y);
+
+=>JSON.stringify method=>
+
+When sending data to a web server, the data has to be a string.
+Convert a JavaScript object into a string with JSON.stringify().
+
+const obj = {name: "John", age: 30, city: "New York"};
+const myJSON = JSON.stringify(obj);
+
+console.log(myJSON);
+
+=>Stringify Dates
+
+In JSON, date objects are not allowed. The JSON.stringify() function will convert any dates into strings.
+
+Example=>
+
+const obj = {name: "John", today: new Date(), city : "New York"};
+const myJSON = JSON.stringify(obj);
+
+=> we can conver function to string =>
+
+const obj = {name: "John", age: function () {return 30;}, city: "New York"};
+obj.age = obj.age.toString();
+const myJSON = JSON.stringify(obj);
+
+*/
+
+
+