Learn ECMAScript - Second Edition

When to use var/let/const 

The difference between const and let is that const makes sure that rebinding will not happen. That means you cannot reinitialize a const variable, but a let variable can be reinitialized (but not redeclared).

Within a particular scope, a const variable always refers to the same object. Because let can change its value at runtime, there is no guarantee that a let variable always refers to the same value. Therefore, as a rule of thumb, you can (not strictly) follow these:

• Use const by default if you know that you'll not change the value (max performance boost)
• Only use let if you think reassignment is required/can happen somewhere in your code (modern syntax)
• Avoid using var (let does not create global variables ...