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const countOccurrences = (arr, value) => arr.reduce((a, v) => v === value ? a + 1 : a + 0, 0);
countOccurrences([1,1,2,1,2,3], 1) -> 3
//使用Array.reduce()在每次遇到数组中的特定值时递增计数器

var arr = 'abcdaabc';
var info = arr.split('').reduce((p, k) => (p[k]++ || (p[k] = 1), p), {});
console.log(info);
//{ a: 3, b: 2, c: 2, d: 1 }