When should I use let and var? [closed]

I would say that you should, as a principle, use let whenever it is not inconvenient to do so. Such as:

for (let i = 0; i < 100; i++) {    // Do something}if (condition) {    let msg = a + b + c;    console.log(msg);    alert(msg);}

The advantages to this approach is:

1. Less risk of overriding some global variable use for something else
2. Less risk of memory leaks due to variables staying in memory long after they have become irrelevant

Use let as a general rule, and var on occasion.

Block scoping is the standard and most readable choice, and will make debugging easier. Block scoping makes it easy to see exactly where a variable is in scope. Function scoping makes things a lot less apparent, and much easier to accidentally introduce bugs with scoping mistakes.

In general, the smaller the scope you can use, the better. Thus let over var.

In particular, it helps deal with the endless problem of closing over variables and not realising their value will change before the closure is executed:

for (var i = 1; i <= 5; i++) {  var item = document.createElement("LI");  item.appendChild(document.createTextNode("Item " + i));  let j = i;  item.onclick = function (ev) {    alert("Item " + j + " is clicked.");  };  list.appendChild(item);}

Well the answer is quite simple use var if you need function scoping and let if you need block scoping.