Because reload() needs window.location as this. In other words – it is a method of window.location. When you say:

var fun = window.location.reload;

fun();

You are calling reload() function without any this reference (or with implicit window reference).

This should work:

setTimeout(window.location.reload.bind(window.location), 250);

The window.location.reload.bind(window.location) part means: take window.location.reload function and return a function that, when called, will use window.location as this reference inside reload().

See also

• How can I pass an argument to a function called using setTimeout?
• Why doesn’t console.log work when passed as a parameter to forEach?
• Preserve ‘this’ reference in javascript prototype event handler