How to Check if a Variable is Defined in JavaScript

let exampleVariable;
let isVariableDefined = "exampleVariable" in window;

Using the "typeof" operator

The “typeof” operator in JavaScript allows you to check if a variable is defined or not. It returns a string indicating the type of the unevaluated operand, or “undefined” if the variable has not been declared or defined.

let exampleVariable;
console.log(typeof exampleVariable);

exampleVariable = "This is a string";
console.log(typeof exampleVariable);

How it works

The ‘typeof’ operator checks the data type of its operand (the variable or value you’re checking). If the variable has been defined, it returns a string indicating the type of the operand, such as ‘string’, ‘number’, etc. If the variable is not defined, it returns ‘undefined’.

• First, we declare a variable ‘exampleVariable’ without assigning any value to it.
• Then, we use the ‘typeof’ operator to check the type of ‘exampleVariable’. As no value was assigned to the variable, it’s ‘undefined’. The console output at this stage will be ‘undefined’.
• Next, we assign a string value to ‘exampleVariable’.
• Finally, we again use ‘typeof’ operator to check the type of ‘exampleVariable’. Now it has a value, which is a string, so ‘typeof’ returns ‘string’. The console output at this stage will be ‘string’.

Using the "in" operator

In JavaScript, you can use the “in” operator to check if a property exists within an object. If the property is found, it will return true; otherwise, it will return false. This can be used to conveniently check if a variable is defined in the global scope (i.e., as a property of the window object).

let exampleVariable;
let isVariableDefined = "exampleVariable" in window;

How it works

The “in” operator in JavaScript checks if a certain property exists within a given object. In the context of checking if a variable is defined, we use the “window” object, which represents the global scope in a browser environment. If the variable is defined in the global scope, it will be a property of the window object.

• The variable “exampleVariable” is declared. Without assigning a value, JavaScript treats it as “undefined”, but the variable itself is defined in the global scope.
• Then, the “in” operator is used to check if “exampleVariable” is a property of the window object. The name of the variable is passed as a string to the “in” operator.
• If “exampleVariable” has been defined, “isVariableDefined” will be set to true. Otherwise, it will be false.

By trying to catch a ReferenceError exception using a "try...catch" statement

In JavaScript, we can check if a variable is defined or not by utilizing the “try…catch” statement. This method works by attempting to access a variable in the “try” block. If the variable is not defined, it will throw a ReferenceError, which will be caught in the “catch” block, allowing us to handle the error in a controlled manner.

let isDefined;
try {
 isDefined = typeof myVar !== 'undefined';
} catch (error) {
 if (error instanceof ReferenceError) {
 isDefined = false;
 }
}
console.log(isDefined);

How it works

This code sample is trying to access the variable ‘myVar’. If ‘myVar’ is not defined in the scope, it throws a ReferenceError. This error is then caught in the catch block where we check if the error is an instance of ReferenceError. If it is, we assume that the variable is not defined, so we set ‘isDefined’ to false.

• The code initializes an ‘isDefined’ variable.
• It then enters a ‘try’ block where it tries to access the ‘myVar’ variable. The ‘typeof’ operator is used to check the type of ‘myVar’. If ‘myVar’ is undefined, ‘typeof myVar’ will return ‘undefined’.
• If ‘myVar’ is not defined, JavaScript will throw a ReferenceError.
• This error is then caught in the ‘catch’ block. Inside this block, there is a conditional statement which checks if the error is an instance of ReferenceError.
• If the error is a ReferenceError, it means that ‘myVar’ is not defined, so the code sets ‘isDefined’ to false.
• Finally, the value of ‘isDefined’ is logged to the console. If ‘myVar’ was not defined, ‘isDefined’ would be false, otherwise it would be true.