Failure to specify the variable type can lead to the creation of automatic global variables

Recently, I stumbled upon this http://www.w3schools.com/js/js_scope.asp page which introduced me to the concept of "Automatic Global variables". Here is an example of how it works:

// You can use carName variable here

function myFunction() {
    carName = "Volvo";

        // You can also use carName variable here
}

However, I have concerns about its scope. If it's truly global, could other files or controllers (such as those in AngularJS) access it? How far does its reach extend?

Answer №1

When working with JavaScript, not specifying var when creating a variable is equivalent to defining it as a property of the global object (in the case of browsers, this would be window):

nameWithoutVar = 1;
// the above is essentially
window.nameWithoutVar = 1;

As a result, any other script loaded on the same page can access nameWithoutVar, just like they can access built-in properties such as location, document, etc.

The use of global variables is generally discouraged due to the risk of namespace collision. If the use of a global variable is absolutely necessary, it should be properly documented and ideally placed within its own namespace to minimize potential conflicts with other variables.

If in doubt, it's best to avoid using global variables altogether.

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