call, apply, bind: choosing this yourself

Setting this explicitly

Every function has three built-in methods that let you pick this yourself:

• fn.call(thisValue, a, b): call now, arguments listed one by one.
• fn.apply(thisValue, [a, b]): call now, arguments in an array (apply → array).
• fn.bind(thisValue): don't call. Returns a new function with this permanently locked in.

call and apply are for one-off calls. bind is for handing a function to someone else to call later.

You need these the moment a function travels. Handing a method to setTimeout, to forEach, or to an event system is everyday code, and that hand-off is exactly where this gets lost — bind is the standard repair, so this bug (and its fix) shows up in nearly every interview loop.

The classic bug: losing this in a callback

When you pass a method somewhere as a callback, like setTimeout(dog.speak, 100) or array.forEach(obj.method), the receiver later calls it as a plain function (Rule 2 from lesson 2-1). The dot is long gone, so this is lost.

Two standard fixes:

1. dog.speak.bind(dog): lock this before handing it over.
2. Wrap it as () => dog.speak(), so the arrow performs a real method call when invoked.

bind can also preset arguments

bind accepts arguments after the this value and locks those in too: multiply.bind(null, 2) returns a function whose first parameter is permanently 2 (the null just fills the this slot for a function that never uses this). Making a specific function out of a general one like this is called partial application.

apply has a classic use of its own: Math.max.apply(null, nums) hands an array to a function as separate arguments — for years this was the standard way to take the max of an array.