javascript 如何在javascript中找到地图的大小
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How to find the size of map in javascript
提问by Ratnesh
I want to print the size of map in javascript. I used the command map.sizebut the answer is coming 0 which is not correct.
我想在 javascript 中打印地图的大小。我使用了命令，map.size但答案是 0，这是不正确的。
Here is my source code:
这是我的源代码：
<div id="demo" ></div>
<script>
function add(test, key, value) {
if (!test[key])
test[key] = [value];
else
test[key].push(value);
}
var mapp = new Map();
var i,j,k=0;
for (i=0;i<5;i++)
{
for (j=0;j<i;j++)
add(mapp,i,j);
}
document.getElementById('demo').innerHTML="Map size are: "+ mapp.size;
</script>
Output: Map size are: 0
输出： Map size are: 0
However, when I am printing the map entries:
但是，当我打印地图条目时：
for (var m in mapp)
{
for (k=0;k<mapp[m].length;k++)
document.write(mapp[m][k]+" ");
document.write("<br>");
}
I can see the output:
我可以看到输出：
0
0 1
0 1 2
0 1 2 3
Please tell me how to find the size of map and why the above code mapp.sizeis not working.
请告诉我如何找到地图的大小以及为什么上面的代码mapp.size不起作用。
回答by zero298
You are treating Maplike an array and using []to access key/value items of it. You need to use the actual methods of Map. Instead of [], use .get()and .set().
您Map像对待数组一样对待并使用它[]来访问它的键/值项。您需要使用 的实际方法Map。而不是[]，使用.get()和.set()。
Right now, you are setting the actual propertiesof the Mapwhich are not the same as the actual tracked iterable k/v items stored by the data structure.
现在，您正在设置 的实际属性，Map这些属性与数据结构存储的实际跟踪可迭代 k/v 项不同。
// Incorrect
const m1 = new Map();
m1["foo"] = "bar";
console.log(m1.length); // undefined
console.log(m1.size); // 0
// Correct
const m2 = new Map();
m2.set("foo", "bar");
console.log(m2.length); // undefined
console.log(m2.size); // 1回答by Prasann
You are treating Maplike an Array. You should use getand setmethods on map to use the inbuilt functions and not in the array fashion [ ]. Javascript cannot calculate the size of the map if you are not using it correctly
你Map就像一个Array. 您应该在地图上使用get和set方法来使用内置函数而不是数组方式 []。如果您没有正确使用它，Javascript 将无法计算地图的大小
If you can't change the code, then you can have your own function to calculate the size
如果你不能改变代码，那么你可以有自己的函数来计算大小
function getMapSize(x) {
var len = 0;
for (var count in x) {
len++;
}
return len;
}
alert(getMapSize(mapp));
回答by forrestg
Comment to poster's code, the right call should look like samples in this tutorial: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map
评论海报的代码，正确的调用应该类似于本教程中的示例：https: //developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map
Depends on data structure, for array like object, we can get map size by:
取决于数据结构，对于像对象这样的数组，我们可以通过以下方式获得地图大小：
Object.getOwnPropertyNames(responseJson.result.priceMap).length;
var obj = { 0: 'a', 1: 'b', 2: 'c' };
console.log(Object.getOwnPropertyNames(obj).length);
// logs 3
回答by Jagannath Rao
Refer How to check if a Map or Set is empty?.
It works with empty map as well as map with values.
它适用于空地图以及带值的地图。
var list = {}; console.log(Object.keys(list).length );
变量列表 = {}; console.log(Object.keys(list).length);
result = 0
结果 = 0
list = {"key":"hello"}; console.log(Object.keys(list).length );
list = {"key":"你好"}; console.log(Object.keys(list).length);
