comparing two reduce() methods

Question

const fruitBasket = ['banana', 'cherry', 'orange', 'apple', 'cherry', 'orange', 'apple', 'banana', 'cherry', 'orange', 'fig' ];

why does this -

const count = fruitBasket.reduce((tally, fruit) => {
  if (!tally[fruit]) {
    tally[fruit] = 1;
  } else {
    tally[fruit] = tally[fruit] + 1; //??//
  }
  return tally;
}, {});

is equal to this -

//const count = fruitBasket.reduce( (tally, fruit) => {
//  tally[fruit] = (tally[fruit] || 0) + 1 ;
//  return tally;
//} , {})

i dont seem to understand - (tally[fruit] || 0) + 1 ;

Answer 1 · 2019-02-08T04:16:28Z

February 8, 2019 4:16am

// At the start of the reduce, tally is an empty object: {}
{} //tally
// if the first entry is 'banana', then we begin to run the function on 'banana'.
// We're going to set tally['banana'] to the evaluated value of (tally['banana'] || 0 ) + 1
tally['banana'] || 0 //Here either tally[fruit] exists in which case we grab the value 
// associated with the 'banana' key, OR not, in which case we get 0. 'banana' 
// doesn't exist on tally because tally is empty, so we get 0.
// If there isn't a key on the object with any said value, the right side of the 
// OR statement will be the result of that evaluation.
// you can try this yourself:
//**********
const x = {}
x['hello'] || 1 // 1

const x = {}
x['hello'] || NaN//  NaN

const x = {}
x['hello'] || false //  false

// On the other hand, if the key exists, the left side will always be the result
const x = {'hello': 2}
x['hello'] || false //  2
***********/
+ 1 // we add 1 to the determined value, so 0 + 1 = 1.
tally[fruit] = 1 // since 'banana' isn't a key on tally, we create one, and assign the value 1.
{'banana': 1} // tally
// Now we move on to the next value in the array, lets say it's also 'banana'.
tally['banana'] = (tally['banana'] || 0 ) + 1 // evaluates to tally['banana'] = 1 + 1
{'banana': 2} // tally

Hope that makes sense!

Answer 2 · 2019-02-08T19:53:51Z

February 8, 2019 7:53pm

thanks :)

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comparing two reduce() methods

2 Answers

Cooper Runstein

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yoav green

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