Variables in typescript

Variable is the name of reserved memory location. It means when we declare a variable some part of memory is reserved.

Rules for declaring a TypeScript variable:

TypeScript variable name must begin with a letter, underscore, or dollar sign.
TypeScript variable names are case sensitive.
TypeScript reserved keywords like abstract, boolean etc can’t be used as TypeScript variable name.

Variable Declaration in TypeScript

TypeScript provides the following 4 ways to declare a variable. Common syntax for a variable declaration is to include a colon (:) after the variable name which followed by its type. We can use var or let keyword to declare a variable.

Declare a variable with type and value.
By using var keyword:
```
var variableName:data_type = value;
```
By using let keyword:
```
let variableName:data_type = value;
```
Declare a variable with type but no value. Variable’s value will be set to undefined by default.
By using var keyword:
```
var variableName:data_type;
```
By using let keyword:
```
let variableName:data_type;
```
Declare a variable with value but no type. Variable’s type will be set to any by default.
By using var keyword:
```
var variableName = value;
```
By using let keyword:
```
let variableName = value;
```
Declare a variable without type and value. Variable’s type will be set to any and value will be set to undefined by default.
By using var keyword:
```
var variableName;
```
By using let keyword:
```
let variableName;
```