Let and Const
let and const |
Hoisting problem |
let -- allows block scoping and hoisting problem in ES5 is solved in ES6.Variables declared with the var keyword can not have Block Scope. Variables declared inside a block {} can be accessed from outside the block. |
When we use a undeclared variable with var keyword in ES5 we get undefined variable name error. This is the example for hositing problem. |
const -- It does NOT define a constant value. It defines a constant reference to a value. Because of this, we cannot change constant primitive values, but we can change the properties of constant objects. |
Whereas when we use let keyword , hoisting problem is solved in ES6. We get the Error as Reference Error <variable name> not defined. |
This keyword
The JavaScript this keyword refers to the object it belongs to. It has different values depending on where it is used: In a method, this refers to the owner object. Alone, this refers to the global object |
The 4 rules of finding out the value of this keyword |
Rule 1 : When the keyword this is not inside the declared object then it refers to the global object |
Rule 2 : When the keyword this is inside the declared object , then it refers to the closest parent object |
Rule 3 : whenever the context of the object changes , we use call , apply and bind to set the value of this explicitly. |
Rule 4 : Whenever we create a object using new keyword inside the function definition, the this keyword refers to the new object that is being created |
Arrow Functions
Arrow function or fat arrow function -- shorter version of syntax when compared to the normal function |
We cannot manipulate the value of this keyword inside the arrow function when we use call,apply or bind |
We do not have access to the prototype field when we declare the function using fat arrow symbol |
Default function parameters
when we set up default function parameters we get access to the functions and the variables in the context |
data=(price,cost=0.07)=>{ console.log(price*cost) } data(5.00)
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Rest and spread operator
Rest |
Spread |
It allows to convert the no of parameters into an array |
It allows to convert the array into an parameters |
It is denoted by "..." in the function definition or function expression |
It is also denoted by the "..." , but used to destructure the array |
a = (...data)=>{ console.log(data) } a(2,3,3,3,3)
(5) [2, 3, 3, 3, 3]
a(2,3,3,3,3,)(5) [2, 3, 3, 3, 3]
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Spread operator can split the string into char |
The rest parameters must be at the end |
a = [...'acd'] (3) ["a", "c", "d"]
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Object Literal
It is shorthand for initializing the object properties and also method |
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Prototype
All JavaScript objects inherit properties and methods from a prototype. |
When we create the constructor function , prototype property is created for that constructor function |
The only inconvenience of using prototypes is that there is no easy way to create private methods or variables. |
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for of loop
//for of loop is used in iterable
var a = [1,2,2,2,2];
for ( let i of a) {
console.log(i);
}
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Octal and binary Literals
var a = 0O12; //octal literals either O or o is allowed
console.log(a)//12
var f = 0b111;
console.log(f);
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Template literals
It can create the multiline strings |
new.target
The new.target property lets you detect whether a function or constructor was called using the new operator. In constructors and functions instantiated with the new operator, new.target returns a reference to the constructor or function. In normal function calls, new.target is undefined. |
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