Today I encountered a variable defined by const for the first time. I checked the relevant information and sorted out this article. The main content is: the difference between const, var, and let in js.
The defined variables cannot be modified and must be initialized.
1 const b = 2;//correct 2 //const b;//Error, must be initialized 3 ('External const definition b:' + b);//There is an output value 4 // b = 5; 5 //('Modify const definition b outside the function:' + b);// Cannot output
The defined variables can be modified. If they are not initialized, undefined will be output and no error will be reported.
1 var a = 1; 2 //var a;//No errors will be reported 3 ('External function var definition a:' + a);//Can output a=1 4 function change(){ 5 a = 4; 6 ('Var within the function defines a:' + a);//Can output a=4 7 } 8 change(); 9 ('After the function call var is defined as the internal modification value of the function:' + a);//Can output a=4
It is a block-level scope. After using let's internal definition, it has no effect on the external function.
1 let c = 3; 2 ('Function outside let definition c:' + c);//Output c=3 3 function change(){ 4 let c = 6; 5 ('The function let define c:' + c);//Output c=6 6 } 7 change(); 8 ('The let definition c after the function is called is not affected by the internal definition of the function:' + c);//Output c=3