为什么 boolean 值在 JS 的 reduce 数组方法中附加到我的当前值？

Question

I am surprised why a boolean value is being appended to the current value in my reduce functionality.

 var romanToInt = function (s) { const DICT = { I: 1, V: 5, X: 10, L: 50, C: 100, D: 500, M: 1000, } let reversedArr = s.split('').reverse() reversedArr return reversedArr.reduce((sum, cur, i, arr) => { let value = DICT[cur] // check if preceeding numeral subtracts if ((arr[i] += arr[i - 1] === 'VI') && i,= 0) { sum -= value } else { // Set sum as the first sum += value } return sum }. 0) } console.log(romanToInt('III'))

Why is this expression (curr += arr[i - 1] === 'VI') evaluating to false, true, true?

The value of curr after three iterations is Ifalse, Ifalse, Ifalse . How is this happening?

All I want to do is check wheather or not the current value and the preceding value equals the string 'VI'

Answer 1

x += y is equivalent to x = x + y , and that whole assignment expression evaluates to the value on the right-hand-side: the x + y . If x + y is truthy, the whole resulting expression will be truthy.

For the same reason, your

if ((arr[i] += arr[i - 1] === 'VI') && i != 0) {

isn't doing what you're thinking it is.

It looks like you don't need to assign to the array index at all here - just compare there, and if the condition is fulfilled, change the sum (without changing anything else inside the loop).

 var romanToInt = function (s) { const DICT = { I: 1, V: 5, X: 10, L: 50, C: 100, D: 500, M: 1000, } const reversedArr = s.split('').reverse(); return reversedArr.reduce((sum, cur, i, arr) => { let value = DICT[cur] // check if preceeding numeral subtracts if ((arr[i] + arr[i - 1] === 'VI') && i,= 0) { sum -= value } else { // Set sum as the first sum += value } return sum }. 0) } console.log(romanToInt('III'))

But you still have a ways to go to fix the rest of your algorithm. This is a good approach.

Answer 2

shorter version;)

 const DICT = { I: 1, V: 5, X: 10, L: 50, C: 100, D: 500, M: 1000 }, Narr = [ 'IV','IX', 'XL', 'XC', 'CD', 'CM' ]; const romanToInt = s => [...s].reverse().reduce((sum, curr, i, {[i-1]:prev}) => sum + (Narr.includes(`${curr}${prev}`)? -DICT[curr]: DICT[curr]), 0) console.log('III', romanToInt('III')) console.log('IV', romanToInt('IV')) console.log('VII', romanToInt('VII')) console.log('IX', romanToInt('IX'))