What is the difference between var and let in Typescript?

Main difference is scoping rules. Variables declared by var keyword are scoped to the immediate function body (hence the function scope) while let variables are scoped to the immediate enclosing block denoted by { } (hence the block scope

// demo: var
for(var i =0 ; i<5 ; i++){
   console.log(i) 
}//finally i =5
console.log(i) // i=5

// demo: let 
for(let i = 0; i<5;i++){
   console.log(i)
}
console.log(i)// i is undefined

function varTest() {
  var x = 1;
  if (true) {
    var x = 2;  // same variable!
    console.log(x);  // 2
  }
  console.log(x);  // 2
}

function letTest() {
  let x = 1;
  if (true) {
    let x = 2;  // different variable
    console.log(x);  // 2
  }
  console.log(x);  // 1
}