And the undefined type has exactly one value that is undefined. That is not a nice thought, anyhow, ask them nicely and then add the eslint-plugin-no-null plugin to your. The undefined is a primitive type in JavaScript. Okay okay, this might make sense, how can I force all my colleagues into submission? Keeping such properties just increases the payload size that needs to be sent over the network or stored in a file without actually giving you any extra benefits. If you use null in some cases, that it will require you to always check for both types each time we need to be able to tell undefined or null apart from other falsy values like 0, '', or false: if (value = undefined || value = null), but why would you do that? If the property someProperty has no value, it's better to just exclude that property completely. As the variable already has no value there is absolutely no point in declaring that again with another type that in the end has the same meaning. When a variable or a property on an object does not have a value it is undefined, and for a value to be null you need to assign it a null value. I find that where JavaScript offers alternatives, stick to one of them. snack has not been created and does not exist.To write simple code you need to keep complexity and variation down. We get a reference error because snack is undeclared. The typeof operator is used to determine the data type of a value.

In the following example, we print snack. Using the typeof Operator to check if variable is undefined. It means the variable does not exist because it has not been created. The syntax would look like this, let snack.Ī variable is undeclared when it has not been declared or initialized. Note that we can declare a variable without initializing it. The syntax looks like this, let snack = 'popcorn'.

Variables are declared using the keywords let and const, and initialized with the = operator.

When we declare and initialize a variable, we are creating a variable and storing something inside the variable. undeclaredīefore we dive into what undeclared means, let's first go over what it means to declare and initialize a variable. In this blog, we'll walk through what undeclared, undefined, and null mean. Surprise, they don't actually mean the same thing! null is one of the primitive value in JavaScript. At some point, I thought they meant the same thing. undefined and null keywords have different meanings in JavaScript. When I learned JavaScript, I often got undeclared, undefined, and null mixed up.

Javascript is undefined