Declarations let, const and class Can’t be Redeclared
Version: ES2015
Level: Beginner
When declaring variables with let, const, and class, you’ll need to be sure not to duplicate declarations of the same name in the same scope. Doing so will create an error.
let num = 1;
let num = 2;
// SyntaxError: Identifier 'num' has already been declared
This is not the case with var and function declarations. They can be repeated without producing an error.
var num = 1;
var num = 2; // Ok, num = 2
If you mix these declarations, for example using both let and var with the same identifier name, the let behavior will be used; that is, the redeclaration will not be allowed.
var num = 1;
let num = 2;
// SyntaxError: Identifier 'num' has already been declared
You can redeclare variables of the same name in different scopes, even if just an arbitrary block scope (when using let, const, and class which are block scoped).
let num = 1;
{
let num = 2; // Ok, no conflict
}
Note: As of Chrome version 80, you can also redeclare let and class in multiple entries within the JavaScript console without there being an error. Attempting to redeclare a const, however, will still produce a SyntaxError.
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