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TypeScript Variables

A variable is the storage location, which is used to store value/information to be referenced and used by programs. It acts as a container for value in a program. It can be declared using the var keyword. It must be declared before the use. TypeScript variable follows the same naming rule as of JavaScript variable declaration. These rules are-

  • The variable name must be an alphabet or numeric digits.
  • The variable name cannot start with digits.
  • The variable name cannot contain spaces and special character, except the underscore(_) and the dollar($) sign.

In ES6, we can define variables using let and const keyword. These variables have similar syntax for variable declaration and initialization but differ in scope and usage. In TypeScript, there is always recommended to define a variable using let keyword because it provides the type safety.

let keyword is similar to var keyword in some respects, and const is an augmentation of let that prevents re-assignment to a variable.

Variable Declaration

We can declare a variable in one of the four ways:

1. Declare type and value in a single statement

2. Declare type without value. Then variable will be set to undefined.

3. Declare its value without type. Then variable will be set to any.

4. Declare without value and typ. Then variable will be set to any and initialized with undefined.

Let's understand all the three variable keywords one by one.

var keyword

Generally, var keyword is used to declare a variable in JavaScript.

We can also declare a variable inside the function:

We can also access a variable of one function with the other function:

Scoping rules

For other language programmers, they are getting some odd scoping rules for var declaration in JavaScript. Variables declared in TypeScript with the var keyword have function scope. This variable has global scope in the function where they are declared. It can also be accessed by any function which shares the same scope.

Example:

NOTE: Since var declarations are accessible anywhere within their containing module, function, global scope or namespace, some people call this var-scoping or function-scoping. Parameters are also called function scoped.


let declarations

The let keyword is similar to the var keyword. The var declaration has some problems in solving a programs, so ES6 introduced let keyword to declare a variable in TypeSript and JavaScript. The let keyword has some restriction in scoping in comparison of var keyword. The let keyword can greatly enhance our code readability and decreases the chance of programming error.

The let statement are written as same syntax as the var statement:

The key difference between var and let is not in the syntax, but it differs in the semantics. The Variable declared with the let keyword are scoped to the nearest enclosing block which can be smaller than a function block.

Block Scoping

When the variable declared using the let keyword, it uses block scoping or lexical scoping. Unlike variable declared using var keyword whose scopes leak out to their containing function, block scoped variable not visible outside of its containing block.

Here, we have two local variables x and y. x's scope is limited to the body of f() while y's scope is limited to the containing if statement's block.

NOTE- The variables declared in a try catch clause also have similar scoping rules. For example:

Re-declaration and Shadowing

With var declaration, it did not matter how many time's we declared variables; we just got only one. In the below example, all declarations of x refer to the same x. This is perfectly valid. But there is some bug, which can be found by the let declaration.

Example without let keyword:

Example with let keyword:

Shadowing is the act of introducing a new name in a more nested scope. It declares an identifier that has already been declared in an outer scope. This is not incorrect, but there might be confusion, as well as it will make the outer identifier unavailable inside the loop where the loop variable is shadowing it. It can introduce certain bugs on its own in the event of accidental shadowing, while it is also preventing certain bugs.

Example:

The above example has a global variable name that shares the same name as the inner method. The inner variable will be used only in that function. Other functions will use the global variable declaration.

Shadowing usually avoided in writing of clearer code. While in some scenarios, if there may be fitting to take advantage of it, we should use it with the best judgment.

Hoisting

Hoisting of var

Hoisting is a mechanism of JavaScript. In hoisting, variables and function declarations are moved to the top of their enclosing scope before code execution. This can result in confusing bugs. We can understand it with the given example.

Note: Hoisting does not happen if we initialize the variable.

Example:

Output:

undefined
4

Hoisting of let

A variable declared with let keyword is not hoisted. If we try to use a let variable before it is declared, then it will result in a ReferenceError.

Example:

const declarations

  • const declaration is used to declare permanent value which cannot be changed later.
  • It has fixed value.
  • const declarations are block scoped.
  • const cannot be updated or re-declared.

Note: According to the naming standards, the const variable must be declared in capital letters. Naming standards should be followed to maintain the code for the long run.

Example:

Output:

Value is: 10

What will happen when we try to re-assign the const variable?

If we try to re-assign the existing const variable in a code, the code will throw an error. So, we cannot re-assign any new value to an existing const variable.

Example:

Output:

SyntaxError: Identifier 'VAR' has already been declared.




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