How to check if a variable is loaded in JavaScript?

In this article, we will explore five different methods to check if a variable is loaded in JavaScript.

By "loaded," we mean determining whether a variable has been declared and assigned a value. This is essential to avoid errors and handle situations where variables might be undefined or null.

Using the typeof Operator #

The typeof operator in JavaScript allows us to determine the type of a variable. By using it on a variable, we can check if the variable has been defined or loaded.

1. The typeof operator returns a string representing the type of the variable.
2. typeof variable !== 'undefined': This condition checks if the variable is not equal to undefined.

Utilizing the in Operator #

The in operator is used to check if a property exists in an object. We can utilize it to verify if a variable is loaded.

1. The isPropertyExist function takes two parameters: obj (the object) and property (the property to check).
2. It uses the in operator to check if property exists as a property of the obj object.
3. The function returns true if the property exists in the object, and false otherwise.

Checking with a Default Value #

By using a default value, we can determine if a variable is loaded or not.

variable !== undefined : This condition checks if the variable is not equal to undefined.

Using try and catch #

With try and catch blocks, we can handle potential errors and infer if a variable is loaded or not.

1. try : The code inside the `try` block is attempted to be executed.
2. catch (e) : If any error occurs, it is caught, and we can infer that the variable is not loaded.
3. The isVariableLoaded function takes a parameter variable, which is the variable we want to check if it is loaded.
4. Inside the function, we use a try block to attempt to execute the code.
5. return variable !== undefined;: This line checks if the variable is not equal to undefined. If it is defined, the function will return true, indicating that the variable is loaded.
6. If an error occurs during the execution (for example, if the variable is not declared), the catch block will catch the error, and the function will return false, indicating that the variable is not loaded. However, in most cases, this catch block won't be triggered.

Checking with Double Exclamation #

The double exclamation (!!) can be used to convert a value to its boolean equivalent, helping us check if a variable is loaded.

!!variable : The double exclamation converts variable into its boolean equivalent. It will be false for undefined, null, 0, or an empty string.

Conclusions: #

In this article, we explored five different methods to check if a variable is loaded in JavaScript.

We learned about using the `typeof` operator, the `in` operator, default value comparison, `try` and `catch` blocks, and the double exclamation technique.