js in const, var, and let

const  variable definition can not be amended and must be initialized

. 1  const B = 2 ; // correct 
2  // const B; // error must be initialized 
. 3 the console.log ( ' outside the defined function const B: ' + B); // the output value 
. 4  // B =. 5; 
. 5  // the console.log ( 'const definitions outside the function modification B:' + B); // can not be output

 

var  variable definitions can be modified, if not initialize prints undefined, not being given

. 1  var A = . 1 ;
 2  // var A; // not given 
. 3 the console.log ( ' outer var function definition A: ' + A); // can be output. 1 = A 
. 4  function Change () {
 . 5 A = . 4 ;
 . 6 the console.log ( ' the function defined var a: ' + a); // can be output. 4 = a 
. 7  } 
 . 8  Change ();
 . 9 the console.log ( ' after a function call is defined as a function of the internal var modification value : ' + A); // may output a = 4

 

let  a block-level scope, let the internal function definitions used, no effect on the external function

. 1 let C = . 3 ;
 2 the console.log ( ' external function definition let C: ' + C); // output. 3 = C 
. 3  function Change () {
 . 4 let C = . 6 ;
 . 5 the console.log ( ' the function let definition of C: ' + C); // output. 6 c = 
. 6  } 
 . 7  Change ();
 . 8 the console.log ( ' defines the internal function is not affected by the definition of the function call let C: ' + C); // output c = 3