What does the Array method `reduce` do?

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Taken from here, arr.reduce() will reduce the array to a value, specified by the callback. In your case, it will basically sum the elements of the array.
Steps:

  • Call function on 0,1 ( 0 is the initial value passed to .reduce() as the second argument. Return sum od 0 and 1, which is 1.
  • Call function on previous result ( which is 1 ) and next array element. This returns sum of 1 and 2, which is 3
  • Repeat until last element, which will sum up to 21

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