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.

Message

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

Error type

ReferenceError

What went wrong?

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.

Examples

Invalid cases

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();

Valid cases

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();