ReferenceError: can't access lexical declaration`X' before initialization

The JavaScript exception "can't access lexical declaration `variable' before initialization" occurs when a lexical variable was accessed before it was initialized. This happens within any block statement, when let or const declarations are accessed before they are defined.

ReferenceError: Use before delaration (Edge)
ReferenceError: can't access lexical declaration `X' before initialization (Firefox)
ReferenceError: 'x' is not defined (Chrome)

ReferenceError

A lexical variable was accessed before it was initialized. This happens within any block statement, when let or const declarations are accessed before they are defined.

In this case, the variable "foo" is redeclared in the block statement using let.

function test() {
  let foo = 33;
  if (true) {
    let foo = (foo + 55);
    // ReferenceError: can't access lexical
    // declaration `foo' before initialization
  }
}
test();

To change "foo" inside the if statement, you need to remove the let that causes the redeclaration.

function test(){
   let foo = 33;
   if (true) {
      foo = (foo + 55);
   }
}
test();

• Temporal Dead Zone and errors with let