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Questions and Answers
Explain the difference between variables declared with var, let, and const in JavaScript.
Explain the difference between variables declared with var, let, and const in JavaScript.
Variables declared with var are function-scoped, meaning they are only visible within the function where they are declared. Variables declared with let have block scope, which means they are only visible within the block (enclosed by curly braces) where they are declared. Variables declared with const also have block scope and are used for constants, meaning once a value is assigned to a const variable, it cannot be reassigned.
What happens if you try to reassign a value to a const variable in JavaScript?
What happens if you try to reassign a value to a const variable in JavaScript?
It would result in an error, as you cannot reassign a const variable once a value is assigned to it.
How are variables declared with var, let, and const scope in JavaScript?
How are variables declared with var, let, and const scope in JavaScript?
Variables declared with var are function-scoped, while variables declared with let and const have block scope.
Explain the concept of block scope in JavaScript.
Explain the concept of block scope in JavaScript.
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Provide an example of using a const variable in a JavaScript function.
Provide an example of using a const variable in a JavaScript function.
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Study Notes
Variable declaration in JavaScript
-
var,let, andconstare keywords used to declare variables in JavaScript. -
varwas the original way to declare variables in JavaScript, but it has some quirks that make it less desirable in modern JavaScript. -
letwas introduced in ES6, and it addresses some of the limitations ofvar, providing more control over variable scope. -
constwas also introduced in ES6, and it is used to declare variables that cannot be reassigned after they are initialized.
Variable Scope
-
Global scope: Variables declared with
varoutside of any function have global scope. This means they are accessible from anywhere in the code. -
Function scope: Variables declared with
varinside a function have function scope, meaning they are only accessible within that specific function. -
Block scope: Variables declared with
letandconsthave block scope, meaning they are only accessible within the block of code they are declared in. This includes if/else blocks, loops, and functions.
const Variables
-
constvariables are immutable, meaning their value cannot be changed after they are initialized. - Attempting to reassign a value to a
constvariable results in an error. -
constvariables do not prevent the modification of properties of objects or elements of arrays.
Block Scope
- Block scope is a feature of
letandconstvariables. - It means the variable is only accessible within the block of code where it is declared. This is different from
var, where variables can be accessed outside of the block they are declared in.
Example of Using a const Variable in a Function
function calculateArea(width, height) {
const area = width * height;
return area;
}
- This function takes two arguments,
widthandheight, and uses aconstvariableareato store the calculated value. - Once
areais initialized, its value cannot be changed within the function.
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Description
Test your knowledge of JavaScript variable declaration and scope with this quiz. Learn the differences between var, let, and const and understand their impact on variable visibility within functions and blocks.
