VAR
Var is functioned-scoped and variables defined in the function CAN NOT be accessed from its outer function. But variables can be accessed from its inner function.
Children-scoped variables can USE its parents variables.
Parents can not use its children variables – because … well, yeah, children like to keep secrets 😀
Var is hoisted
Hoisting is when a variable declaration set upfront at the beginning of function execution while its assignment or value initialization will be done later, when the execution reaches the statement.
Example for Hoisting :
console.log(myDog) // undefined -- but no error raised var myDog = 2; // although the assignment happens after the variable gets called
Basically that is the same like :
var myDog; console.log(myDog); // undefined myDog = 2; // because the assignment happens here
Var is Function-Scoped
function myFunction() { var myVar = "Noodles"; console.log(myVar) // works! Same scope } console.log(myVar) // ReferenceError, the child is too shy!
VAR-declared variables can not be accessed outside the function
More subtle example :
function myFunction() { var myVar = "Noodles"; if (true) { var myVar = "Woof!"; console.log(myVar); // "Woof!" // actually, with myVar being function-scoped, we just erased the previous myVar value "Noodles" for "Woof!" } console.log(myVar); // "Woof!" - notice the assignment in the if block has reassigned this value for good } console.log(myVar); // Throws a ReferenceError, myVar is not accessible outside the function.
Var can be redeclared
var myVar = 2; var myVar = 3; console.log(myVar) // 3
LET
Let declared variable are block-scoped, can not be redeclared in the same scope.
function myFunction() { let myVar = "Noodles"; if (true) { let myVar = "Woof!"; console.log(myVar); // "Woof!" // with let myVar being block-scoped, we just created a new variable called myVar } console.log(myVar); // "Noodles", it then accesses the same-scoped variable where console log is invoked } console.log(myVar); // Throws a ReferenceError, myVar is not accessible outside the function.
Let is not hoisted
console.log(myDog); // a ReferenceError! let myDog = "Noodles"; // because the assignment happens after the variable gets called
Let can not be redeclared
let myVar = 2; let myVar = 3; console.log(myVar) // SyntaxError: Duplicate declaration
CONST
Const variables can not be reassigned and redeclared.
const dog = "Noodles"; // declaration with value assignment dog = "Meow"; // error - reassignment is not allowed const dog = "Meow"; // error - redeclaration is not allowed
BUT! const Object and Array are mutable!
For objects:
const dog = { name: 'Noodles' }; dog.name = 'Meow'; // this will work ! dog variable is not completely reassigned, but mutated console.log(dog.name) // "Woof" dog = "Noodles" // raises an error, because reassignment is not allowed with const declared variables
For arrays :
const dogs = []; dogs.push('Pillow'); // this will work ! dogs array is not reassigned, but mutated console.log(dog[0]) // "Pillow" dogs = ["Noodles"] // raises an error, because reassignment is not allowed with const declared variables![]()
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