JS: const Declaration
New in JS2015.
const
is for declaring constants.
It's similar to let Declaration, except you can't change its value once set.
const name = val;
- declare and assign.
const name1 = val1, name2 = val2, name3 = val3 etc;
- declare and assign multiple constants
// variable declared with const cannot be changed const x = 3; x = 4; // TypeError: Assignment to constant variable.
const Must Have a Value When Declared
const
must have a value when declared.
// const must have a value const x; // SyntaxError: Missing initializer in const declaration x = 3;
Constant of Array
if a Array value is declared constant, the array itself can still change.
const ar = [3,4]; ar . push (5); console.log( ar );
Constant of Object
If a Object
value is declared const
, you can still add/delete/change properties of the object.
// if object is declared constant, the object properties can still be modified const ob = {}; ob.p = 3; console.log (ob); // { p: 3 }
Note, you can prevent add/delete/change properties by using Object.seal
No Name Hoisting
The const
doesn't have name hoisting as var
does.
That means, you can't use the name before its const
declaration.
[see var Declaration Order]