JS exercise ''find the mistake'' not understood well

[Chris]

Hello Coding Forums Community,

I have to identify the errors in the piece of code below and I have the correction but I don't fully understand it:

"use strict";

const prefix = "username_";

let userName = "Jack";
const userName = "Adam";

let prefixedUserName;
const prefixedUserName;

userName = "John";
prefixedUserName = prefix + userName;

console.log(prefixedUserName + prefixedUserName2);
console.log(prefixedUserName2);

Uncaught SyntaxError: Identifier 'userName' has already been declared

****************************************************************************************************

The answer:

"use strict";

const prefix = "username_";

let userName = "Jack";
// const userName = "Adam";

let prefixedUserName;
// const prefixedUserName;

userName = "John";
prefixedUserName = prefix + userName;

console.log(prefixedUserName /+ prefixedUserName2/);
// console.log(prefixedUserName2);

****************************************************************************************************

I got prefixedUserName2 was never declared, thus error. For const it seems to be a redundant declaration, right?
But why cannot keep const instead of let?

"use strict";

const prefix = "username_";

// let userName = "Jack";
const userName = "Adam";

// let prefixedUserName;
const prefixedUserName;

userName = "John";
prefixedUserName = prefix + userName;

console.log(prefixedUserName /+ prefixedUserName2/);
// console.log(prefixedUserName2);

Uncaught SyntaxError: Missing initializer in const declaration

 

[WhiteCube]

The difference between a constant and a variable is that a constant may be assigned only once.

// let userName = "Jack";
const userName = "Adam";

// let prefixedUserName;
const prefixedUserName;
*This line needs a value.*

userName = "John";
*This line is trying to reassign.*

 

[VBService]

Check: The Difference Between let and const in JavaScript and Best Practices

 

[Chris]

@WhiteCube

Could you please be more precise? Do you have a clear example in mind?

@VBService

Article is unrelated to the question.

 

[WhiteCube]

1 let userName = "Jack";

This line assigns "Jack" to userName, userName is a variable so it can be reassigned later.

2 const userName = "Adam";

This line assigns "Adam" to userName, userName is a constant so it can not be reassigned later.

3 let prefixedUserName;

This line declares a variable. Declaring a variable without an initial value, is valid.

4 const prefixedUserName;

This line is invalid, it is trying to declare a constant without a value.

5 userName = "John";

This line is valid, if userName is a variable. Invalid if userName is a constant.

But why cannot keep const instead of let?

answer: because of line 5