json 对象到数组，node.js答案

【问题标题】：json object to array, node.jsjson 对象到数组，node.js
【发布时间】：2020-03-22 08:35:45
【问题描述】：

我有一个 ETHBTC.json 文件：

[
{
    "open": "0.06353900",
    "high": "0.06354800",
    "low": "0.06341700",
    "close": "0.06347300",
    "volume": "335.48500000",
    "timestamp": 1521640800000
},
{
    "open": "0.06347300",
    "high": "0.06365400",
    "low": "0.06344000",
    "close": "0.06357500",
    "volume": "461.02800000",
    "timestamp": 1521641100000
},
{
    "open": "0.06349500",
    "high": "0.06360400",
    "low": "0.06341400",
    "close": "0.06352300",
    "volume": "495.50600000",
    "timestamp": 1521641400000
}
]

我正在尝试遍历数据并将所有“低”值保存到一个数组中

const fs = require('fs');
var data = fs.readFileSync('charts/ETHBTC.json');
var CurrentPair = JSON.parse(data);


var lowTotal = [];

for(item in CurrentPair){
lowTotal[item] = item.low;
console.log(lowTotal[item]); //put this just to check if working.
}

控制台日志给了我未定义的值。任何帮助都会很棒，谢谢

【问题讨论】：

标签： javascript json node.js

【解决方案1】：

运算符in 返回对象的属性，在您的情况下返回该数组的索引。

var array = [{    "open": "0.06353900",    "high": "0.06354800",    "low": "0.06341700",    "close": "0.06347300",    "volume": "335.48500000",    "timestamp": 1521640800000},{    "open": "0.06347300",    "high": "0.06365400",    "low": "0.06344000",    "close": "0.06357500",    "volume": "461.02800000",    "timestamp": 1521641100000},{    "open": "0.06349500",    "high": "0.06360400",    "low": "0.06341400",    "close": "0.06352300",    "volume": "495.50600000",    "timestamp": 1521641400000}];

var lowTotal = [];
for(item in array){
  lowTotal.push(array[item].low);
}

console.log(lowTotal);

可能你想使用的是for-of-loop

这种for-loop 循环遍历数组，每次迭代都返回一个特定的元素/对象。

var array = [{    "open": "0.06353900",    "high": "0.06354800",    "low": "0.06341700",    "close": "0.06347300",    "volume": "335.48500000",    "timestamp": 1521640800000},{    "open": "0.06347300",    "high": "0.06365400",    "low": "0.06344000",    "close": "0.06357500",    "volume": "461.02800000",    "timestamp": 1521641100000},{    "open": "0.06349500",    "high": "0.06360400",    "low": "0.06341400",    "close": "0.06352300",    "volume": "495.50600000",    "timestamp": 1521641400000}];

var lowTotal = [];
for(item of array){
  lowTotal.push(item.low);
}

console.log(lowTotal);

您可以使用reduce 或简单的forEach。

var array = [{    "open": "0.06353900",    "high": "0.06354800",    "low": "0.06341700",    "close": "0.06347300",    "volume": "335.48500000",    "timestamp": 1521640800000},{    "open": "0.06347300",    "high": "0.06365400",    "low": "0.06344000",    "close": "0.06357500",    "volume": "461.02800000",    "timestamp": 1521641100000},{    "open": "0.06349500",    "high": "0.06360400",    "low": "0.06341400",    "close": "0.06352300",    "volume": "495.50600000",    "timestamp": 1521641400000}];

var result = array.reduce((a, {low}) => [...a, low], []);

console.log(result);

【讨论】：

【解决方案2】：

x in y 循环遍历对象的属性名称。

您试图将x 视为属性的值，而不是名称。

为此，您需要x of y。

for(item of CurrentPair){
    lowTotal[item] = item.low;
    console.log(lowTotal[item]); //put this just to check if working.
}

不过，使用map 会更惯用。

var lowTotal = CurrentPair.map(item => item.low);
console.log(lowTotal);

旁白：约定，在 JavaScript 中，为构造函数保留以大写字母开头的变量名。不要将名称 CurrentPair 用于普通的旧数组。

【讨论】：

也可以使用Object.values(CurrentPair)

【解决方案3】：

JavaScript 中的数组不支持for ... in 循环。

你想要的是

for (const item of CurrentPair) { ... }

或

for (let i = 0; i < CurrentPair.length; i++) {
    const item = CurrentPair[i];
}

或

CurrentPair.forEach(item => { ... });

【讨论】：

它受支持，但它并没有像 OP 认为的那样做。
没错，我的措辞不是最好的。

【解决方案4】：

你几乎做对了。但是在for ... in 循环中，您将在对象内收到name 或index 属性。所以你需要使用CurrentPair来获取你想要的信息。

var data = '[{"open": "0.06353900","high": "0.06354800","low": "0.06341700","close": "0.06347300","volume": "335.48500000","timestamp": 1521640800000},{"open": "0.06347300","high": "0.06365400","low": "0.06344000","close": "0.06357500","volume": "461.02800000","timestamp": 1521641100000},{"open": "0.06349500","high": "0.06360400","low": "0.06341400","close": "0.06352300","volume": "495.50600000","timestamp": 1521641400000}]';

var CurrentPair = JSON.parse(data);
var lowTotal = [];

for (var item in CurrentPair) {
  lowTotal.push(CurrentPair[item].low);
}

console.log(lowTotal);

因为CurrentPair 是array，您也可以简单地使用forEach：

var data = '[{"open": "0.06353900","high": "0.06354800","low": "0.06341700","close": "0.06347300","volume": "335.48500000","timestamp": 1521640800000},{"open": "0.06347300","high": "0.06365400","low": "0.06344000","close": "0.06357500","volume": "461.02800000","timestamp": 1521641100000},{"open": "0.06349500","high": "0.06360400","low": "0.06341400","close": "0.06352300","volume": "495.50600000","timestamp": 1521641400000}]';

var CurrentPair = JSON.parse(data);
var lowTotal = [];

CurrentPair.forEach(function(item) {
  lowTotal.push(item.low);
});

console.log(lowTotal);

【讨论】：

【解决方案5】：

item 是数组索引，而不是数组值。在lowTotal[item] 中使用它时正确使用它，但在尝试读取low 属性时却没有。应该是：

for (item in CurrentPair) {
    lowTotal[item] = CurrentPair[item].low;
}

但是，请参阅Why is using "for...in" with array iteration a bad idea?

【讨论】：

【解决方案6】：

//I don't know what u are exactly trying to do. This may help


//push all low values in an array

const lowTotal =CurrentPair.map((pair)=>pair.low);    

 or 

//push all low values as an array of object

const lowTotal =CurrentPair.map((pair)=> {return {low:pair.low}});


//if you want to sum all low values then follow this approach

const lowTotal =CurrentPair.map((pair)=>pair.low).reduce((pre,nex)=>Number(pre)+Number(nex));

or

const lowTotal=CurrentPair.map(pair=>{return{low:pair.low}}).reduce((pre,nex)=> {return {low:Number(pre.low)+ Number(nex.low)}})

【讨论】：