Why check this instanceof boundFn in a bind polyfill?
To support the new operator. When a bound function is called with new, this should be the new instance, not the bound context. this instanceof boundFn is true only when called with new.
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More FAQs in bind Polyfill: Step-by-Step Explanation
this is the function on which myBind is called. For greet.myBind(obj), this is greet. Store it: const fn = this, and use it in the returned function.
presetArgs are passed to myBind: greet.myBind(obj, 'Hello'). laterArgs are passed when the bound function is called: bound('!'). Both are spread into fn.call: fn.call(context, ...presetArgs, ...laterArgs).
new boundFn() would set this to the bound context instead of a new instance. This is incorrect behavior. The real bind supports new, so the polyfill should too.
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