How can I check if a `let` variable has been declared on ES6?

Question

Unlike traditional var-declared variables, which are attached to the entire enclosing, function scope regardless of where they appear — let declarations attach to the block scope but are not initialized until they appear in the block

So :

console.log( a ); // undefined
console.log( b ); // ReferenceError!

var a;
let b;

So it seems that hoisting is not applied here.

Question

If so , how can I safely check if the variable has been declared ?

NB - The option I see is try/catch and of course always put the let variables first at scope. but still my question remains

Answer 1

it seems that hoisting is not applied here.

Not exactly. The variable still covers the complete scope, the binding is created when the scope is entered just like with vars.

But you're right, in contrast to vars it is not initialised with undefined immediately, only when the let statement is evaluated. The area from the top of the scope to there is called temporal dead zone - the identifier is bound, but will always throw a ReferenceError when used.

How can I safely check if the variable has been declared?

You cannot, just as you cannot for vars¹. You don't need this anyway.

_{1: Let's ignore global variables² that become properties of the global object here.
2: var/function/function*-declared variables, I mean. Lexical bindings (let, const) indeed don't become global properties.}