Friday 16 February 2024

What is the difference between let and var keyword Typescript ?057

 What is the difference between let and var keyword Typescript ?

Both let and var keywords are used to declare variables in TypeScript, but they have significant differences in how they behave:


  • let: Variables declared with let have block-level scope, meaning they are only accessible within the block they are declared in (e.g., if statement, loop, function). Redeclaring let variables within the same block will cause a redeclaration error.

  • var: Variables declared with var have function-level scope by default. They are accessible anywhere within the function they are declared in, even within nested blocks. Redeclaring var variables within the same function is allowed, but it can lead to unexpected behavior due to hoisting.


  • let: Does not undergo hoisting. You cannot access a let variable before it is declared in the block.

  • var: Undergoes hoisting. You can access a var variable anywhere within its function scope, even before its declaration. However, the value will be undefined until the declaration is reached.

Temporal Dead Zone (TDZ):

  • let: Introduced a concept called TDZ, which represents the time between the start of a block and the declaration of a let variable. Accessing a let variable within its TDZ results in a reference error.

  • var: Does not have a TDZ due to hoisting. You can access a var variable as soon as you enter its function scope.


  • let: Cannot be redeclared within the same block.

  • var: Can be redeclared within the same function scope, but it can lead to unexpected behavior.

Best Practices:

  • Use let for most variable declarations to avoid ambiguity and prevent accidental hoisting-related issues.

  • Use const when the value of a variable cannot change.

  • Avoid using var due to its potential for confusing behavior, especially in modern JavaScript and TypeScript contexts.

In summary, use let for modern variable declarations with clear block-level scope and to avoid hoisting-related issues. Use const for variables with constant values. Only use var if you specifically need its function-level scope and hoisting behavior, understanding the potential side effects.

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