What is the difference between "let" and "var"?

tl;dr

The main difference between let and var is the scope in which they are defined. var is a keyword used to declare a variable in the global scope, meaning it can be accessed from anywhere in the code. let is a keyword used to declare a variable in the local scope, meaning it can only be accessed within the block of code in which it is declared.

For example:

var x = 10;

function myFunction() {
  let y = 20;
  console.log(x); // prints 10
  console.log(y); // prints 20
}

myFunction();
console.log(x); // prints 10
console.log(y); // throws an error

In the example above, x is declared using var and is accessible both inside and outside of the myFunction function. y is declared using let and is only accessible within the myFunction function.