Question

我有一个长度约为1000的数组。为什么map函数在某些索引中返回undefined？有没有办法只返回满足这个条件的数组？我想要返回一个值为＆gt;的数组。 0

 var total_percents = cars.map(function(element) {
        var savings_percent = Number(element[0].getAttribute("percent-savings"));
        if (savings_percent > 0)
            return savings_percent;
});

Answer 1

您需要在映射后过滤值，您可以使用filter数组方法对car => car > 0

这样的谓词进行过滤

var total_percents = cars.map((element) => Number(element[0].getAttribute("percent-savings"))).filter(car => car > 0)

您也可以使用reduce方法同时合并两个操作：

＆＃13;

var total_percents =
  cars.reduce((acc, element) => {
    const percentSavings = Number(element[0].getAttribute("percent-savings"));

    if (percentSavings > 0) {
      acc.push(percentSavings);
    }

    return acc;
  }, [])

＆＃13;

Answer 2

不确定您尝试完成的任务，但是：

尝试这样的方法来返回所有pctSavings的总和：

const totalPercents = cars.reduce((sum, el) => {
    const pctSavings = +(el[0].getAttribute("percent-savings"));
    if (pctSavings > 0) sum += pctSavings;
    return sum;
}, 0);

要返回pctSavings数组，只需执行以下操作：

const totalPercents = cars.reduce((arr, el) => {
    const pctSavings = +(el[0].getAttribute("percent-savings"));
    if (pctSavings > 0) arr.push(pctSavings);
    return arr;
}, []);

要获得max pctSavings，请执行以下操作：

let maxPctSavings = 0;
cars.forEach(el => {
    const pctSavings = +(el[0].getAttribute("percent-savings"));
    if (pctSavings > maxPctSavings) maxPctSavings = pctSavings
});
console.log(maxPctSavings) // this is your answer

Answer 3

使用reduce来过滤和映射值（应该比单独的filter和map更快且使用更少的内存）

var total_percents = cars.reduce(
  function(out, element) {
    var savings_percent = Number(element[0].getAttribute("percent-savings"));
    if (savings_percent > 0) {
      // Only add to the out-array if value is greater than 0.
      out.push(savings_percent);
    }
    return out;
  },
  [] // start with empty array
);

total_percents将是一个数组，其值大于0.

从另一个答案的评论中读到，你想要的是最大的节省。然后我会这样做：

var max_savings = cars.reduce(
  function(value, element) {
    var savings_percent = Number(element[0].getAttribute("percent-savings"));
    return Math.max(savings_percent, value)
  },
  0 // start with no savings
);

为什么JS Map函数返回未定义？

3 个答案: