Uncaught ReferenceError: variable is not defined

Your browser console may also sometimes show an error saying, Uncaught ReferenceError: variable is not defined.

See the below image, especially the error message inside the red box.

check if variable is undefined in javascript

let checkVar = () => {
  alert(myName);        // variable myName is not defined anywhere. Therefore it will throw an error.
}

Try it

Don't confuse this error with the undefined error. Both are different and have different meanings.

In the above example, I have not declared (or defined) the variable myName anywhere in my JS code. It does not know if it’s a string, number or a Boolean. As a result, it throws the error when I am trying to "alert" the variable, expecting to see the value strored in the variable.

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