JavaScript ReferenceError – Can’t access lexical declaration`variable’ before initialization

This JavaScript exception can’t access the lexical declaration `variable’ before initialization occurs if a lexical variable has been accessed before initialization. This could happen inside any block statement when let or const declarations are accessed when they are undefined.

Message:

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

Error Type:

ReferenceError

Cause of the error: Somewhere in the code, there is a lexical variable that was accessed before initialization.

Example 1: In this example, the const keyword is used with the variable inside the if statement, So the error has occurred.

Javascript

function GFG() {
    const var_1 = "This is";
    if (true) {
        const var_1 = var_1 + "GeeksforGeeks";
    }
}
function Geeks() {
    try {
        GFG();
        console.log(
            "'Can't access lexical declaration" +
            "`variable'before initialization' " +
            "error has not occurred");
    } catch (e) {
        console.log(
            "'Can't access lexical declaration" +
            "`variable' before initialization'" +
            " error has occurred");
    }
}
Geeks()

Output

'Can't access lexical declaration`variable' before initialization' error has occurred

Example 2: In this example, the keyword is used with the variable, So the error has occurred.

Javascript

function GFG() {
    let var_1 = 3;
    if (true) {
        var_1 = var_1 + 5;
    }
}
function Geeks() {
    try {
        GFG();
        console.log(
            "'Can't access lexical declaration" +
            "`variable' before initialization'" +
            " error has not occurred");
    } catch (e) {
        console.log(
            "'Can't access lexical declaration" +
            "`variable'before initialization'" +
            " error has occurred");
    }
}
Geeks()

Output

'Can't access lexical declaration`variable' before initialization' error has not occurred