What’s the difference between a global variable and a ‘window.variable’ in JavaScript?

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No difference. They both have the same effect (In the browser, where window is the global context1).

  • window.foo = "bar" sets the property foo on window.
  • foo = "bar" indicates either a typo or intentionally global.

Since I have to double check whether it’s a typo or not, I personally find it more readable to set window.foo directly.

Also, in ES5 strict mode, foo = "bar" is an illegal assignment because foo is not declared and will throw a Error.

Edit:

As noted in the comments, foo = "bar" will look all the way up the scope chain for the variable foo and re-assign it with "bar" if it’s found. If it’s not found, it will create a new global variable.

Also with window.foo = "bar" you’re just assigning a property to an object, which can be deleted using delete window.foo.

In ES5 strict mode it is invalid to delete a variable.

1 In other environments, such as node.js and Web Workers, there may be another name for the global object and window may not exist at all. Node.js uses global and Web Workers use self.

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