使用map和reduce来获得对象数组

Question

I'm having an array of

var c = [{name: 'John'}, {name: 'John'}, {name: 'Tom'}];

I want to use the reduce method and map to get this following result:

result = [
    { name: "John", occurence: 2 },
    { name: "Tom", occurence: 1 }
  ]

Here's an attempt I have tried but still not fully getting it. https://jsfiddle.net/joeSaad/up3ddfzz/

 c = [{ name: 'John' }, { name: 'John' }, { name: 'Simon' }]; var d = c.reduce((countMap, word) => { countMap[word.name] = ++countMap[word.name] || 1; return countMap }, []); var e = c.reduce((countMap, word) => { q = []; countMap[word.name] = ++countMap[word.name] || 1; var p = { name: word.name, occurence: countMap[word.name] } q.push(p); return q }, []); console.log(e); 

Help much appreciated. Thanks in advance

Answer 1

You're definitely on the right track working with reduce, but I would separate the computations into two distinct steps

 let c = [{name: 'John'}, {name: 'John'}, {name: 'Tom'}]; const pairs = o=> Object.keys(o).map(k=> [k, o[k]]) let count = c.reduce((acc, {name}) => { if (acc[name] === undefined) return Object.assign(acc, { [name]: 1 }) else return Object.assign(acc, { [name]: acc[name] + 1 }) }, {}) var result = pairs(count).map(([name, occurences]) => ({name, occurences})) console.log(result) //=> [ { name: 'John', occurences: 2 }, { name: 'Tom', occurences: 1 } ] 

---

You could abstract these behaviours into distinct functions like this

 // convert an object to [[key,value]] pairs const pairs = o=> Object.keys(o).map(k=> [k, o[k]]) // count unique instances of a property value in an array of objects const countBy = (prop, xs)=> { return xs.reduce((acc, x)=> { let y = x[prop] if (acc[y] === undefined) return Object.assign(acc, { [y]: 1 }) else return Object.assign(acc, { [y]: acc[y] + 1 }) }, {}) } // your data let c = [{name: 'John'}, {name: 'John'}, {name: 'Tom'}] // then chain it all together let result = pairs(countBy('name', c)).map(([name, occurences]) => ({name, occurences})) console.log(result) //=> [ { name: 'John', occurences: 2 }, { name: 'Tom', occurences: 1 } ] 

---

ES6 also offers Map which is pretty good for doing this kind of computation.

This code reads a little nicer to me and is a little more semantically correct. Our previous code was doing the exact same thing, only it was using a plain object. Map is a data structure specifically designed for this key->value data, so that makes Map a better choice.

The only problem with this is, if your codebase isn't already using Maps in some way, introducing them just for this might be the wrong move.

 // convert a Map to an array of pairs const mpairs = m=> [...m.entries()] // countBy this time uses Map const countBy = (prop, xs)=> { return xs.reduce((m, x)=> { let y = x[prop] if (m.has(y)) return m.set(y, m.get(y) + 1) else return m.set(y, 1) }, new Map) } // your data let c = [{name: 'John'}, {name: 'John'}, {name: 'Tom'}] // then chain it all together let result = mpairs(countBy('name', c)).map(([name, occurences]) => ({name, occurences})) console.log(result) //=> [ { name: 'John', occurences: 2 }, { name: 'Tom', occurences: 1 } ]