Understanding the Difference between let Keyword and Not Defined Type Variables in TypeScript

A comprehensive guide to understanding the differences between the `let` keyword and variables declared without it in TypeScript. Learn proper usage and best practices for writing TypeScript code.
---

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Typescript let keyword and not defined type variable difference

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding the Difference between let Keyword and Not Defined Type Variables in TypeScript

As a beginner in TypeScript, you might be familiar with traditional JavaScript variable declarations using var, let, and const. However, the nuances distinguishing the let keyword from not explicitly defined variables can be confusing, especially when you're starting with classes. In this guide, we’ll break down common pitfalls and clarify how to use the let keyword effectively within your TypeScript classes.

The Problem: Confusion with Variable Declarations

In your TypeScript code, you have encountered issues when trying to declare variables within a class using let. Here’s an excerpt of the code causing confusion:

[[See Video to Reveal this Text or Code Snippet]]

You may wonder why declaring num with let results in an error while assigning a value to num without it works perfectly fine. Similarly, you are confused about the errors thrown while using variables like varName in different contexts.

Understanding the Solution

Let’s dissect the code to understand what modifications are necessary and why certain declarations cause errors.

Class Member Declaration

Inside a class in TypeScript, you can only declare properties that belong to that class. This means:

You cannot use var, let, or const to declare new variables.

Class properties must start with visibility keywords: public, private, protected, or static. If no modifier is specified, the default is public.

Example: Correct Declaration of Class Members

[[See Video to Reveal this Text or Code Snippet]]

In this example, public num: number = 10; works because num is defined as a public property of the ClassName class, adhering to TypeScript's class syntax.

Assignment vs Declaration

While using a variable declaration like let num: number = 10; inside a class will throw an error, directly assigning a value using num = 10; can work because it is interpreted as setting the property of the class instance that is already defined.

Understanding the Syntax Error

Let's clarify the part concerning variable assignment outside class declarations:

varName: ClassName = new ClassName(); is incorrect because varName is not declared with a keyword (let, var, or const), making it not a valid syntax for variable assignment in TypeScript.

Correct Usage Example:

[[See Video to Reveal this Text or Code Snippet]]

In this statement, you use let to declare a variable named varName that holds an instance of ClassName, thus conforming to TypeScript’s design principles and syntax rules.

Summary

Use of let inside classes: You cannot declare variables using let, var, or const inside class bodies. Use visibility keywords instead to set class members.

Variable Assignment: Direct assignment (num = 10;) works if the variable is already defined as a class member.

Correct Syntax for Instances: Always declare your variables with let, const, or var when creating instances outside of class bodies.

By understanding these concepts and practicing the correct syntax, you will be able to confidently navigate variable declarations in TypeScript. Happy coding!