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

Samuel Kleos
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Samuel Kleos
Front End Web Development Techdegree Student 13,307 Points
Posted by Samuel Kleos
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Samuel Kleos
Front End Web Development Techdegree Student 13,307 Points

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
231,236 Points
Steven Parker
Steven Parker
231,236 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.

Samuel Kleos
seal-mask
.a{fill-rule:evenodd;}techdegree
Samuel Kleos
Front End Web Development Techdegree Student 13,307 Points
Samuel Kleos
.a{fill-rule:evenodd;}techdegree
Samuel Kleos
Front End Web Development Techdegree Student 13,307 Points

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
231,236 Points
Steven Parker
Steven Parker
231,236 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".