您如何在Javascript函数式编程中做一个简单的计数器？

Question

I am redoing my coding to better fit functional programming and I can not find a good solution to a simple counter without mutation.

const countRows = (element) => (obj) => {
var count = 0;
  Object.entries(obj).forEach(([key,value]) => {
    if (value.nodeName == element) {
      count = count + 1;
    };
  });
  return count;
};

I have been trying to figure it out using a helper function and recursion but nothing seems to be working out unless I use some type of mutation. I feel like this is something that should be super simple but I just cant wrap my head around how this is suppose to work. At least I have contained the mutation to within the function and not a global variable.

Answer 1

这应该工作

Object.keys(obj).filter(key=>obj[key].nodeName == element).length

Answer 2

The accepted solution will make an array of the macthes for the sole purpose to count the elements. You can easily make a counting version of filter where the extra allocations doesn't happen:

function countIf (coll, predicate, context) {
  return coll.reduce((matches, value) => predicate.call(context, value) ? matches + 1 : matches, 0);
}

It is compatible with the underscore line of higher order functions AND it even works with Backbone collections. Here is hwo I would have implemented countRows :

const countRows = (element) => (obj) => 
  countIf(
    Object.keys(obj), 
    value => obj[value].nodeName == element);

Answer 3

I have been trying to figure it out using a helper function and recursion

I would like to give you a recursive solution , even if @Orbis has already given you a very good solution.

Steps

• check if object has entries
  • object has entries, check if object value is equal provided x
    • remove first object entry and increment the counter
    • else remove only the first object entry
• else return the counter which is by default 0

Code

"Longer" Version

 const isEqualObjectValue = ([key, value], x) => value === x const countRows = (objectEntries, x, counter = 0) => { if (objectEntries.length > 0) { if(isEqualObjectValue(objectEntries[0], x)) { return countRows(objectEntries.slice(1), x, counter + 1) } else { return countRows(objectEntries.slice(1), x, counter) } } else { return counter } } const test = { a: ':o', b: 'x', c: ':o', d: ':o', e: 'x' } console.log('how many \\':o\\' can you find:', countRows(Object.entries(test), ':o')) 

"Shorter" Version

 const isEqualObjectValue = ([key, value], x) => value === x const countRows = (objectEntries, x, counter = 0) => objectEntries.length > 0 ? isEqualObjectValue(objectEntries[0], x) ? countRows(objectEntries.slice(1), x, counter + 1) : countRows(objectEntries.slice(1), x, counter) : counter const test = { a: ':o', b: 'x', c: ':o', d: ':o', e: 'x' } console.log('how many \\':o\\' can you find:', countRows(Object.entries(test), ':o'))