What happens to the let variable in Javascript?

Question

I have executed the following code and I found that I was unable to access the variable all the time in global scope.

 console.log(b); let b = 1;
VM12265:1 Uncaught ReferenceError: b is not defined
    at <anonymous>:1:13
(anonymous) @ VM12265:1
let b = 1; console.log(b);
VM12318:1 Uncaught SyntaxError: Identifier 'b' has already been declared
    at <anonymous>:1:1
(anonymous) @ VM12318:1
console.log(b);
VM12368:1 Uncaught ReferenceError: b is not defined
    at <anonymous>:1:13

I would like to know what happened to the variable b in step 3 once the step 1 and step 2 is executed.

This is what I was looking for http://www.ecma-international.org/ecma-262/6.0/#sec-let-and-const-declarations-static-semantics-early-errors Thanks for sharing @gurvinder372 — kishorekumaru, Jan 12 '18 at 06:43
similar reference at https://stackoverflow.com/questions/31219420/are-variables-declared-with-let-or-const-not-hoisted-in-es6 — kishorekumaru, Jan 12 '18 at 10:36

score 2 · Accepted Answer · answered Jan 12 '18 at 06:39

For the first one

console.log(b); let b = 1; VM12265:1 Uncaught ReferenceError: b is not defined at :1:13 (anonymous) @ VM12265:1

Let's binding is not created till they are initialized and hence no reference is created. You accessed the value in temporal zone

let b = 1; console.log(b); VM12318:1 Uncaught SyntaxError: Identifier 'b' has already been declared at :1:1 (anonymous) @ VM12318:1

For the second one, as the message says, b has already been declared already. As per spec

It is a Syntax Error if the BoundNames of BindingList contains any duplicate entries.

score 1 · Answer 2 · answered Jan 12 '18 at 06:47

let does not define variables for the global scope. You can declare let variable once at the beginning of the block scope and set the variable to a new value without attempting to re-declare the variable using let more than once to avoid error

{
  let b;

  try {
    console.log(b);
    b = 1;
    console.log(b);
    b = 2;
  } catch (err) {
    console.error(err.message)
  }
  console.log(b);
}

score 0 · Answer 3 · answered Jan 12 '18 at 06:42

0

You haven't declared variable b

2 ways:

use var instead of let (That is because of JS Hoisting)
declare b first then use it after

answered Jan 12 '18 at 06:42

Xin

33,823
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85

score -1 · Answer 4 · edited Jul 02 '19 at 05:22

-1

I have checked in both the cases its working for me.

let b = 1;
var b1 = 2;
console.log(b);
console.log(b1);

edited Jul 02 '19 at 05:22

cs95

379,657
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746

answered Jan 12 '18 at 06:38

Amreesh Verma

1
2

I was looking for the riddle for step 3 not step 1 – kishorekumaru Jan 12 '18 at 06:46

What happens to the let variable in Javascript?

4 Answers4