JavaScript Variable Declaration: var, let, and const

Overview

• Three ways to declare variables in JavaScript: var, let, const.
• var has been in JavaScript from the beginning.
• let and const were introduced in ES6 to improve upon the limitations of var.

Importance

• Understanding how they work is crucial, especially for technical interviews.
• Interviewers often evaluate JavaScript knowledge based on these concepts.

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Types of Questions

1. Scope

   • Scope refers to the visibility of variables in different parts of the code.

   • Types of scopes:

     • Global Scope
     • Function Scope
     • Block Scope (unique to let and const)

   • Example of var:

     var a = 5;  
     console.log(a); // 5  
     

   • Example of let:

     {  
         let b = 10;  
     }  
     console.log(b); // ReferenceError  
     

   • Example of const:

     {  
         const c = 15;  
     }  
     console.log(c); // ReferenceError  
     

2. Variable Shadowing

   • Refers to when a variable in a certain scope has the same name as another variable in an outer scope.

   • Example:

     var hello = 'Hello';  
     {  
         let hello = 'Hi';  
         console.log(hello); // High  
     }  
     console.log(hello); // Hello  
     

   • Note: Cannot shadow let or const with var (illegal shadowing).

3. Declaration

   • var: Can be redeclared without error.
   • let: Cannot be redeclared in the same scope; results in an error.
   • const: Same behavior as let, with an additional error for initializing without a value.
   • Example:

     var a = 1;  
     var a = 2; // No error  
     let b = 1;  
     let b = 2; // Error  
     const c = 1;  
     const c = 2; // Error  
     

4. Initialization

   • var: Can be declared without initial value.
   • let: Can be declared without initial value.
   • const: Must be initialized at declaration; otherwise, it results in an error.

5. Hoisting

   • Process where declarations are moved to the top of their scope during creation phase.
   • var: Hoisted, initialized to undefined.
   • let and const: Hoisted but cannot be accessed before initialization (causing error - Temporal Dead Zone).
   • Example of hoisting in var:

     console.log(a); // undefined  
     var a = 5;  
     

   • Example of hoisting in let/const:

     console.log(b); // ReferenceError  
     let b = 10;  
     

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Temporal Dead Zone

• The period between the declaration and initialization of let and const variables where they exist but cannot be accessed.

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Conclusion

• Questions about var, let, and const can cover various aspects, including scope, declaration, initialization, and hoisting.
• Understanding these key concepts is valuable for tackling JavaScript interviews effectively.

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Additional Notes

• The discussion included recommendations to follow for more insights and potential future topics to cover.