Which statement is illegal in TypeScript but legal in JavaScript?

Which statement is illegal in TypeScript but legal in JavaScript?

• A. let x="1"; x="2";
• B. let x=1; x="2"
• C. let x=1; x=2
• D. let x; x=1; x="2"

Answer: (B)

In TypeScript, every variable has a type that is determined either explicitly or through inference, which adds a layer of type safety. When you declare a variable, TypeScript keeps track of the assigned type throughout its usage. In the case of declaring a variable with `let x=1;`, this initializes `x` with a number type. TypeScript expects that `x` will continue to hold values of that type. Therefore, when you subsequently try to assign a string value with `x="2"`, it conflicts with the original number type, resulting in a type error. This behavior is one of TypeScript's key features aimed at preventing potential runtime errors by enforcing stricter type checks at compile-time. In contrast, JavaScript is a dynamically typed language, which means variables can hold values of any type without such restrictions. As a result, reassigning `x` to a string after it was initialized as a number is completely legal in JavaScript. This distinction between TypeScript and JavaScript showcases TypeScript’s strengths in providing more robust compile-time checks to help developers catch potential errors ahead of time.

In TypeScript, every variable has a type that is determined either explicitly or through inference, which adds a layer of type safety. When you declare a variable, TypeScript keeps track of the assigned type throughout its usage.

In the case of declaring a variable with let x=1;, this initializes x with a number type. TypeScript expects that x will continue to hold values of that type. Therefore, when you subsequently try to assign a string value with x="2", it conflicts with the original number type, resulting in a type error. This behavior is one of TypeScript's key features aimed at preventing potential runtime errors by enforcing stricter type checks at compile-time.

In contrast, JavaScript is a dynamically typed language, which means variables can hold values of any type without such restrictions. As a result, reassigning x to a string after it was initialized as a number is completely legal in JavaScript.

This distinction between TypeScript and JavaScript showcases TypeScript’s strengths in providing more robust compile-time checks to help developers catch potential errors ahead of time.