Final answer:
The key difference between 'let' and 'const' in JavaScript is that variables declared with 'let' are mutable and can be reassigned, while those declared with 'const' cannot be reassigned once set. Both are block-scoped, but 'const' enforces immutability on the variable's binding itself.
Step-by-step explanation:
The difference between let and const lies in the mutability of the variable they declare. The let keyword is used to declare a variable that can later be reassigned to a different value. In other words, it is mutable. For example:
let myVariable = 5;
myVariable = 10; // This is allowed with let
On the other hand, const is used to declare a variable that cannot be reassigned after its initial assignment. This means it is immutable, and an attempt to reassign it will result in an error. For example:
const myConstant = 5;
myConstant = 10; // This will cause an error
Both let and const are block-scoped, which means they are only accessible within the block they are defined in, unlike the var keyword, which defines a variable globally or locally to an entire function regardless of block scope. However, while variables declared with let can be updated within its scope, variables declared with const cannot be updated or re-declared.
It's important to note that while a const variable itself can't be reassigned, if it holds an object or array, the properties or elements of that object or array can still be modified; only the binding to the object or array itself is immutable.