This tutorial will teach you the main difference between var,let,const step by step.
var let const difference
1.var is global scope.
2. let is block scope.
3.const is block scope.
variable declaration
1 2 3 | var mytest1 = 12; let mytest2 = "javascript"; const mytest3 = 2000; |
let we discuss about each
var
var is global scope.
1 2 | var mytest = 12; console.log(mytest); |
Output
12
i created the variable name mytest assign 12. you can see the output.
1 2 3 4 5 6 | console.log(mytest); var mytest = 12; var mytest = 120; var mytest = 12000; var mytest = 0; console.log(mytest); |
Output
undefined
0
above example i printed console.log(mytest); at the first time. no value assign thats way output display as undefined. same mytest name you can assign more than one value but when you print it out last declared value 0 as been printed. so that output displayed as 0.
1 2 3 4 5 6 7 8 | console.log(mytests); if(1000 > 500) { var mytests = 509; } console.log(mytests); |
Output
undefined
509
above example console.log(mytests); at the first time. no value assign thats way output display as undefined.
after that if the particular condition correct only print the value 509. 1000 is grater than 500 condition is corrent then print it out 509.
let
let is block scope.
1 2 | let mytestvar = 120; console.log(mytestvar); |
Output
120
1 2 3 | console.log(mytestvar); let mytestvar = 120; console.log(mytestvar); |
Error
ReferenceError: Cannot access ‘mytestvar’ before initialization
you cannot call console.log(mytestvar); at the firstline you get the error.
1 2 3 4 5 | let mytest = 12; let mytest = 120; let mytest = 12000; let mytest = 0; console.log(mytest); |
Error
SyntaxError: Identifier ‘mytest’ has already been declared
Same name you cannot use again by again.
1 2 3 4 5 6 7 | let myTestlett = 0; // you must create the variable first at the firstline then only works. if(1000 > 10) { mytestlet = 7000; } console.log(mytestlet); |
you must create the variable first at the firstline then only works because let is a block scope
Output as
7000
const
1 2 3 | const mytest = 12; console.log(mytest); |
Output
12
1 2 3 4 5 | const mytest = 12; const mytest = 120; const mytest = 12000; const mytest = 0; console.log(mytest); |
Error
SyntaxError: Identifier ‘mytest’ has already been declared.
Same name you cannot use again by again.
1 2 3 4 5 | const mytest = 200; mytest = 1200; console.log(mytest); |
Error
TypeError: Assignment to constant variable.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 | const myestarray = {name : "Tutusfunny", age: 300}; console.log(myestarray.name); const myestarray1 = {name : "Tutusfunny", age: 300}; myestarray.name = "Tutusfunny123"; console.log(myestarray1.name); const myestarray2 = [12,20,50]; myestarray2[0] = 60; console.log(myestarray2); |
Output
Tutusfunny
Tutusfunny
[ 60, 20, 50 ]
