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The ‘ReferenceError x is not defined’ – occurs when the variable or function isn’t declared, i.e. it doesn’t exist in the current scope.

This can happen when:

You’re Using A Variable That Hasn’t Been Declared Yet (Possibly Typo)

var foo = x; // ReferenceError: x is not defined

If you have not declared a variable using var, let, const and you try to access it, you will get this error. To fix this, declare the variable before using it.

var x = 1;

var foo = x; // no error

When You’re Trying To Access A Variable Outside Of Its Scope

function foo(){

  var x = 1;

}

console.log(x); // ReferenceError: x is not defined

This error occurs because x is declared inside the foo function, which means it can only be accessed inside that function. To fix this, move the declaration of x outside of the function:

var x = 1;

function foo(){

  x = 10;

}

foo();

console.log(x); // no error

Now, x is defined in global scope and can be used inside the foo function as well as outside the function. So, when you initialize x as 1 outside the function, and later change its value to 10 inside the function, you can see the updated value when you try to log it outside the function.

If You’re Using A Function That Hasn’t Been Defined Yet

function foo(){

  console.log('foo');

}

bar(); // ReferenceError: bar is not defined

This error occurs because the bar function hasn’t been declared yet, so it doesn’t exist. To fix this, you need to declare the function before calling it:

function bar(){

  console.log('bar');

}

function foo(){

  bar();

}

Conclusion

The “ReferenceError: X is not defined” error in JavaScript can occur for various reasons.

The most common reason is that the variable or function doesn’t exist in the current scope. To fix this, you need to make sure that the variable or function is declared before trying to access it.

Another common reason for this error is trying to access a variable outside of its scope. To fix this, you need to move the declaration of the variable outside of the function.

Finally, this error can also occur if you’re trying to use a function that hasn’t been declared yet. To fix this, you need to declare the function before calling it.

I hope this article has helped you understand the “ReferenceError: X is not defined” error in JavaScript.

If you have any questions, feel free to leave a comment below.