I was looking into Javascript `.every` documentation and it have a second parameter which is passed the description says 

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> Optional. Value to use as this when executing the callback. If a thisArg parameter is provided to every, it will be used as callback's this value. Otherwise, the value undefined will be used as its this value. The this value ultimately observable by the callback is determined according to the usual rules for determining the this seen by a function.
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which means that I can access that values as `this` in the callback function, what is a practical example of this?