JavaScript 调试问答：如何 `console.log()` 映射？

Question

有人告诉我 Map 比 JS Array [] 更好用。
现在，我正在调试一些正在解析的数据。

const Csv2Map_Promise = (async (csvFilepath) => {
  return new Promise((resolve) => {
    const csvData = new Map();
    createReadStream(csvFilepath)
      .pipe(parse({ delimiter: ',' }))
      .on('data', (csvRow) => { csvData.set(csvRow[0], { 'value 1': 1, 'value 2': 2, 'value 3': 3, 'value 4': 4 }); })
      .on('end', () => { resolve(csvData); });
  });
})

const csvFile1_Map = Csv2Map_Promise('csvFile1.csv');

csvFile1_Map.then(()=>{console.log(csvFile1_Map);})

我正在调试的文件非常大。
我怎样才能 console.log 只是一些数据？
（勾选 10 或 100 returns）

P.S。我知道我可以转换回数组，
但是有更好的方法吗？还是我在自欺欺人？

Answer 1

由于 Map 实例是一个迭代器，您可以循环遍历它并跟踪要打印的项目数：

let myMap = new Map();

for (let i = 1; i < 10; i++) {
  myMap.set(i, i * 10);
}

console.log("Full list of contacts:")
for (let [key, value] of myMap) {
  console.log(key + ' = ' + value)
};

console.log("3 items of contacts:")
let limit = 3
for (let [key, value] of myMap) {
  if (limit <= 0) {
    break
  }

  console.log(key + ' = ' + value)

  limit--
}

更糟糕的想法是简化代码，但将其转换为数组。为什么更糟？因为你需要将整个数组加载到内存中（消除了迭代器的优势），然后才取出它的一部分：

let myMap = new Map();

for (let i = 1; i < 10; i++) {
  myMap.set(i, i * 10);
}

console.log("Full list of contacts:");
for (let [key, value] of myMap) {
  console.log(key + " = " + value);
}

console.log("3 items of contacts:");
for (let [key, value] of Array.from(myMap.entries()).slice(0, 3)) {
  console.log(key + " = " + value);
}

对于您的代码，这种遍历地图的循环将发生在 .then 块内：

const makeMapPromise = (size) =>
  new Promise((resolve) => {
    let myMap = new Map();

    // I am creating a map of numbers as I cannot replicate your code
    // but it does not matter. What metters is that this code
    // populates the map somehow with some values
    for (let i = 1; i < size; i++) {
      myMap.set(i, i * 10);
    }

    resolve(myMap);
  });

const myMapPromise = makeMapPromise(10);

myMapPromise.then((myMap) => {
  console.log("Full list of contacts:");
  for (let [key, value] of myMap) {
    console.log(key + " = " + value);
  }

  console.log("3 items of contacts:");
  const arr = Array.from(myMap.entries()) // converting to an array
  const sliceOf3 = arr.slice(0, 3) // reslicing the array to get only first 3 elements
  for (let [key, value] of sliceOf3) {
    console.log(key + " = " + value);
  }
});

JavaScript 调试问答：如何 `console.log()` 映射？

JavaScript Debug Q&A: How do I `console.log()` Map?

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