In Javascript we can declare variables using var, const and let keywords

var

var is the old way (pre es6/es2015) of declaring variables. It's a "weak" variable declaration because it doesn't help us understanding if binding could change durign execution.

let

We use let to declare bindings that could chage during execution (eg. a loop counter, or a variable used in math).

const

const stands for constant. We use const for bindings that will not change during execution.

Pay attention: this means that identifiers declared with const cannot be re-assigned, but in case of objects we can change its attributes after binding:

const str = "foobar"
str = "barfoo"
Uncaught TypeError: Assignment to constant variable.

const obj = { foo: 'foo', bar: 'bar' }
obj = { foo: 'foo2', bar: 'bar2' }
Uncaught TypeError: Assignment to constant variable.
obj.foo = 'foo3'
obj
{ foo: 'foo3', bar: 'bar' }

Scoping

Variables declared with var are only function-scoped or globally-scoped.

On the other end, the scope of variables declared with const and let is the enclosing block (code between curly braces { } ) and not the whole function.

Global object (window)

Variables declared outside functions (aka in the top level of program) with var become properties of the global object (window), while variables declared with let or const don't.

var foo = 'foo';
let bar = 'bar';
window.foo;
('foo');
window.bar;
undefined;

Conclusion

I don't use var anymore
I use let only when it's required that binding can change during execution (eg. loop variables)
I use const almost for everything