Yes, in some execution contexts, this is undefined, which is falsy in JavaScript:

In strict mode, however, if the value of this is not set when entering an execution context, it remains as undefined, as shown in the following example:

Here is at least one convoluted scenario in which the following conditions must be met:

• global or function execution in strict mode
• the function was defined at the top-level (effectively being a function of window)
• that function was called directly without invoking its parent (i.e. foo() and not window.foo())
• the function returned this

function foo() {
  'use strict'; // see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode
  return this;
}

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/this