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JavaScript JavaScript Array Iteration Methods Combining Array Methods Working with Objects in Arrays

What if... 🤔 these two methods are the same. Then is it true .reduce() is just performing a variable assignment?

const acc = {}; // this assignment is the same as.. (see next comment) 👇🏽
for (let item of users) {
    acc[item.name] = item.age;
}

const assignedAges = users.reduce((acc, item) => {
    acc[item.name] = item.age;
    return acc;
}, {}) // making the assignment here?

Therefore acc is just a constant that needs to be returned every time the multi-line callback (wrapped in braces) does a loop through each element?

1 Answer

Steven Parker
Steven Parker
229,645 Points

The optional 2nd argument to a callback is indeed assigned to the accumulator before the callback is used.

The main difference between these two examples is that "acc" in the first case is a constant but in the second case it is a function parameter.

So effectively, acc in the second example is first declared as a variable in the parameter of the callback, and then assigned a value in the second parameter of the .reduce() method?

Steven Parker
Steven Parker
229,645 Points

Essentially, but normally the word "variable" is not used to refer to a parameter. It would be more common to just say that it is "declared as a parameter of the callback".