Const and Let

The problem with just using var is that this is valid:

1var firstTime = "something original";
2var firstTime = "overwriting the original";

This example is easy to understand, but imagine a huge codebase with several thousand line files. If firstTime is declared on line 1, and then redeclared on line 1800, it can be easy to miss.

let and const are block-scoped. A block is simply a chunk of code wrapped by curly braces { and }. This means that any variables declared using either let or const are only available for use within the { and } symbols.

1if (true) {
2  let onlyHere = "this is only available here";
3}
4console.log(onlyHere); // Uncaught ReferenceError: onlyHere is not defined