纯javascript中的延迟分配

在这个问题中,我遇到了以下简化问题:

我们从一个带有value属性的Objects数组开始.我们想要为每个值计算它的总和的百分比,并将其作为属性添加到结构中.要做到这一点,我们需要知道值的总和,但这个总和不是事先计算的.

//Original data structure
[
  { "value" : 123456 },
  { "value" : 12146  }
]

//Becomes
[
  { 
    "value" : 123456,
    "perc"  : 0.9104
  },
  {
    "value" : 12146 ,
    "perc"  : 0.0896
  }
]

一个简单且可能最具可读性的解决方案是两次遍历数据结构.首先我们计算总和,然后计算百分比并将其添加到数据结构中.

var i;
var sum = 0;
for( i = 0; i < data.length; i++ ) {
  sum += data[i].value;
}
for( i = 0; i < data.length; i++ ) {
  data[i].perc = data[i].value / sum;
}

我们是否可以只通过数据结构一次,并以某种方式告诉只有在知道整个总和后才能评估百分比表达式？

我主要对解决纯javascript问题的答案感兴趣.那就是:没有任何库.

具有自修改代码的解决方案.

它将f计算功能移动到迭代结束,然后通过链接函数分配单个项目的百分比.

var data = [
        { "value": 123456 },
        { "value": 12146 },
    ];

data.reduceRight(function (f, a, i) { // reduceRight, i=0 is at the end of reduce required
    var t = f;                        // temporary save previous value or function
    f = function (s) {                // get a new function with sum as parameter
        a.perc = a.value / s;         // do the needed calc with sum at the end of reduce
        t && t(s);                    // test & call the old func with sum as parameter
    };
    f.s = (t.s || 0) + a.value;       // add value to sum and save sum in a property of f
    i || f(f.s);                      // at the last iteration call f with sum as parameter
    return f;                         // return the function
}, 0);                                // start w/ a value w/ a prop (undef/null don't work)

document.write('
' + JSON.stringify(data, 0, 4) + '
');

function add_percentage(arr) {
  var sum = 0;
  arr.forEach(e => {
    sum += e.value;
    Object.defineProperty(e, "perc", {
       get: function() { return this.value / sum; }
    });
  });
}

function add_percentage(arr) {
  var sum = 0;
  arr.forEach(e => {
    sum += e.value;
    setTimeout(() => e.perc = e.value / sum);
  });
}

var sum = (data[0] ? data[0].value : 0) +
          (data[1] ? data[1].value : 0) +
          (data[2] ? data[2].value : 0) +
          ...
          (data[50] ? data[50].value : 0);

for( i = 0; i < data.length; i++ ) {
   data[i].perc = data[i].value / sum;
}

var sum = data.reduce(function(output,item){
   return output+item.value;
},0);
for( i = 0; i < data.length; i++ ) {
  data[i].perc = data[i].value / sum;
}

var data = [
  { "value" : 123456 },
  { "value" : 12146  }
]
var sum = 0;
var rej = null;
var res = null;
var def = new Promise(function(resolve,reject){
    rej = reject;
    res = resolve;
});
function perc(total){
    this.perc = this.value/total;
}

for( i = 0; i < data.length; i++ ) {
  def.then(perc.bind(data[i]));
  sum+=data[i].value;      
}
res(sum);