Basics: let directive

In JavaScript, let is a variable, first appeared in ES2015 (JavaScript Standard). It became one of two variable options to replace var variables, which were functionally deprecated by that time. The main advantage of let is that it's visible only within a block of code. This significantly increases code security and helps optimize memory use.

Key characteristics of let variables

1. Declaring a variable begins with a keyword let plus a unique identifier (name) for the variable.

let SomeIdentifier;

2. For a let variable identifier, it is forbidden to use JavaScript keywords, e.g. break, case, catch, class, const, delete, do, else, function, if, return, var, etc.

Wrong identifiers:

let if;
let return;

3. In the let variable you can store strings, numbers, bool, Arrays and Objects.

let text;
text = "words";

4. The name of a let variable can't start with a number and can include only English alphabet letters, numbers, symbols "$" and "_".

Correct identifiers:

let text;
let TEXT1;
let $text_;

Wrong identifiers:

let 2text;
let #text;

5. It can be initialized within the same line where it was declared.

let text = "story";

6. The values of let variables can be changed after the initial assignment.

let text = "characters";
text = "words";

7. Scope of let variables is only within the code block (denoted by curly braces).

If(SomeVariable){
    let words = "text";
}

8. let variables can be of global and functional scope.

let stories = "legends"; 
If(SomeVariable){
    let words = "text";
}

The stories variable has global scope, while the words variable has the functional (local) scope.

9. Declaration of the let variable should be located higher in the code than a value assignment.

Wrong declaration:

texts = "books";
let texts;

10. If let variables with the same identifier were initialized in global and local scopes, the variable from its local scope takes precedence.

let test;
function Func(){
    let test;
}