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 The reduce method contains two parameters, the first parameter is a callback function, which contains four parameters, the second parameter is an optional parameter, as the first parameter of the callback function. It executes the callback function for each element in the array in turn, excluding elements that are deleted or never assigned in the array, and accepts four parameters: the initial value (or the return value of the last callback function), the current element value, the current Index, the array on which reduce is called.

 Deep copy is to re-open a new reference address for the variable, which is completely separated from the original, which is equivalent to dividing a new space for itself. Although the content in it is the same, even if one of them is changed, the other is not. will be affected. Shallow copy means that the reference address of the reference type in the stack cannot be changed. The two variables share the same reference address and change the things in the same place. Therefore, the shallow copy causes the change of one variable to cause the same change of the other variable. .

 Functions and methods are an important knowledge point in programming. The key point of a function is how to call and execute. The way it is called has a huge impact on how the code inside the function is executed, especially in the creation of the this parameter. In addition to the most basic calling method such as function name (), what other methods are there? The following briefly introduces the three most basic (excluding, bind, apply, call).