数组对象

Question

我需要在数组中求和一些对象值。有些可以是int，有些可以是string，即：

JavaScript的：

let array = [
 {quantity: 1, amount: "24.99"}
 {quantity: 5, amount: "4.99"},
]

挖掘Stack Overflow我发现this method（我正在使用React）：

Array.prototype.sum = function (prop) {
    var total = 0
    for ( var i = 0, _len = this.length; i < _len; i++ ) {
        total += this[i][prop]
    }
    return total
};

let totalQuantity = array.sum("quantity");
console.log(totalQuantity);

虽然效果很好，但我需要对字符串amount执行相同的操作。由于我需要将amount转换为float，因此上述操作无效。 React抱怨Component's children should not be mutated.

不是JS忍者，我认为这会有些神奇：

Array.prototype.sum = function (prop) {
    var newProp = parseFloat(prop);
    var total = 0
    for ( var i = 0, _len = this.length; i < _len; i++ ) {
        total += this[i][newProp] // Surely this is wrong :(
    }
    return total
};

任何干净的方法来实现这一目标？

我需要这个：

let totalAmount = array.sum("amount");

Answer 1

定义一个通用sum函数，与

一样简单

let sum = a => a.reduce((x, y) => x + y);

并将其应用于从源数组中选取的值列表：

let array = [
 {quantity: 1, amount: "24.99"},
 {quantity: 5, amount: "4.99"}
];
  
let sum = a => a.reduce((x, y) => x + y);
  
let totalAmount = sum(array.map(x => Number(x.amount)));
  
console.log(totalAmount.toFixed(2))
  
  

Answer 2

请尝试：

Array.prototype.sum = function (prop) {
    var total = 0
    for ( var i = 0, _len = this.length; i < _len; i++ ) {
        total += parseFloat(this[i][prop]) // Surely this will work :)
    }
    return total
};

Answer 3

const array = [
 {quantity: 1, amount: "24.99"},
 {quantity: 5, amount: "4.99"}
]
  
Array.prototype.sum = function(key) {
  return this.reduce(function(total, item) {
    return total + parseFloat(item[key]);
  }, 0);
}

// weird javascript math ...
console.log(array.sum("amount"));

// workaround
console.log(array.sum("amount").toFixed(2));

这项工作很好;）

Answer 4

我经常使用reduce()方法来处理这类情况。这是一个小小的演示：http://codepen.io/PiotrBerebecki/pen/zKEQgL

let array = [
 {quantity: 1, amount: "24.99"},
 {quantity: 5, amount: "4.99"}
]

function sumProperty(arr, type) {
  return arr.reduce((total, obj) => {
    if (typeof obj[type] === 'string') {
      return total + Number(obj[type]);
    }
    return total + obj[type];
  }, 0);
}

let totalAmount = ( sumProperty(array, 'amount') ).toFixed(2); 
console.log(  totalAmount  ); // 29.98

let totalQuantity = sumProperty(array, 'quantity'); 
console.log(  totalQuantity  ); // 6