从 JavaScript 对象中的键中查找最大值 [重复]

Question

我有以下对象：

var questions = [{
    'question_id': 245,
    'question': 'Question 1',
    'ordinal': 1,
    'master_difficulty': 1,
    'overridden_difficulty': ''
},{
    'question_id': 34,
    'question': 'Question 2',
    'ordinal': 2,
    'master_difficulty': 1,
    'overridden_difficulty': ''
},{
    'question_id': 656,
    'question': 'Question 3',
    'ordinal': 3,
    'master_difficulty': 1,
    'overridden_difficulty': ''
},{
    'question_id': 86,
    'question': 'Question 4',
    'ordinal': 4,
    'master_difficulty': 1,
    'overridden_difficulty': ''
}]

我想从中得到最高的序数值。

例如var highestOrdinal = highestOrdinalFunction(questions);

highestOrdinalFunction 将返回最高序数。

所以在这种情况下它会返回4。

Math.max 可以为此工作吗？

Answer 1

您可以通过两种方式找到最大值。您可以传递一个函数来映射属性的值，也可以直接在项目上调用该属性。这些结果将是应用于 Math.max 计算的映射值，与项目列表相关。

Math.prototype.max

var questions = [
  {'question_id':245,'question':'Question 1','ordinal':1,'master_difficulty':1,'overridden_difficulty':''},
  {'question_id':34,'question':'Question 2','ordinal':2,'master_difficulty':1,'overridden_difficulty':''},
  {'question_id':656,'question':'Question 3','ordinal':3,'master_difficulty':1,'overridden_difficulty':''},
  {'question_id':86,'question':'Question 4','ordinal':4,'master_difficulty':1,'overridden_difficulty':''}
]

document.body.innerHTML =  'Via Function: ' + highestOrdinalValueFn(questions) + '\n';
document.body.innerHTML += 'Via Property: ' + highestOrdinalValueProp(questions);

// #1 : Via function
function highestOrdinalValueFn(items) {
  return max(items, function() {
    return this.ordinal;
  });
}
// #2 : Via property
function highestOrdinalValueProp(items) {
  return max(items, 'ordinal');
}

function max(list, prop) {
  return Math.max.apply(Math, list.map(function(item) {
    return isFunction(prop) ? prop.call(item) : item[prop];
  }));
}

function isFunction(fn) {
  return fn && {}.toString.call(fn) === '[object Function]';
}

body { white-space: pre; font-family: monospace; }

---

Array.prototype.reduce

var questions = [
  {'question_id':245,'question':'Question 1','ordinal':1,'master_difficulty':1,'overridden_difficulty':''},
  {'question_id':34,'question':'Question 2','ordinal':2,'master_difficulty':1,'overridden_difficulty':''},
  {'question_id':656,'question':'Question 3','ordinal':3,'master_difficulty':1,'overridden_difficulty':''},
  {'question_id':86,'question':'Question 4','ordinal':4,'master_difficulty':1,'overridden_difficulty':''}
]

document.body.innerHTML = highestValue(questions, 'ordinal');

function highestValue(items, prop) {
  return compare(items, function(curr, prev) {
    return maxNumericComparator(curr, prev, prop);
  })[prop];
}

function maxNumericComparator(curr, prev, prop) {
  if (curr && !prev) return -1;
  if (!curr && prev) return 1;
  if (curr[prop] && !prev[prop]) return -1;
  if (!curr[prop] && prev[prop]) return 1;
  if (curr[prop] > prev[prop]) return -1;
  if (curr[prop] < prev[prop]) return 1;
  return 0;
};

function compare(list, comparator) {
  return list.reduce(function(max, item) {
    return comparator(item, max) < 0 ? item : max;
  }, null);
}

Answer 2

我想这就是你要找的：

Math.max.apply(Math,questions.map(function(ques){return ques.ordinal;}))

请参阅Array.prototype.map 了解有关 map() 工作原理的更多信息。

Answer 3

Array#reduce (spec | MDN) 是这样做的惯用方式：

function highestOrdinalFunction() {
    return questions.reduce(function(previous, entry) {
        return previous === undefined || entry.ordinal > previous ? entry.ordinal : previous;
    }, undefined);
}

Array#reduce 反复调用回调，传入回调的前一个返回值，该返回值以reduce 的第二个参数为种子。这个东西通常被称为reduce操作的“累加器”。 reduce 的结果是累加器的最终值。

如果questions 为空，则上面将返回undefined。

Answer 4

我们可以按ordinal 值对该数组中的对象进行排序，然后返回第一个条目。

function highestOrdinalFunction( questions ) {
    return questions.sort( ( a, b ) => a.ordinal - b.ordinal )[ 0 ].ordinal;
}

在这种情况下，将Array.prototype.map 与Math.max() 结合起来可能会更好：

Math.max.apply( null, questions.map(q => q.ordinal) );