How to see variable id in nodejs

Node.js is a JavaScript runtime environment based on the Chrome V8 engine. It can run JavaScript code on the server side and is widely used in web applications, command line tools, server-side scripts and other fields. In Node.js, dealing with variables is very important, and variable references are the cornerstone of the program. In this article, we will take a deep dive into how to view variable ids in Node.js.

id is the unique identifier of a variable. In Node.js, id is assigned to different types of data structures, including functions, objects, arrays, etc. Each variable has its own address in memory. In Node.js, we can use typeof and the object inspection function Object.prototype.toString.call() to check the variable type. However, this doesn't tell us the id of the variable.

Node.js provides another way to view variable ids, which is to use the process object. The process object provides a lot of useful information and tools, including obtaining process IDs, command line parameters, environment variables, etc. In Node.js, we can view the id of the variable through the process object. To do this, we need to use an API provided by V8: v8.getHeapStatistics().

v8.getHeapStatistics() is used to obtain the statistical information of the V8 memory heap. This is a JavaScript object that contains a lot of information about memory usage, as follows:

{
  total_heap_size: 14773248,         // V8内存堆的总大小
  total_heap_size_executable: 4194304,// 可执行代码的大小
  total_physical_size: 14773248,     // 内存堆所占的总物理大小（包括使用中和空闲的部分）
  total_available_size: 12734672,    // 内存堆的总剩余空间
  used_heap_size: 8172856,           // V8内存堆中已被使用的大小
  heap_size_limit: 1535115264,       // V8内存堆的最大限制容量
  malloced_memory: 8192,             // 通过malloc分配的内存大小
  peak_malloced_memory: 58368,       // 通过malloc分配的内存峰值
  does_zap_garbage: 0                // 标识是否启用了zapping垃圾回收
}

We can Get the id of the variable by using the v8.getHeapStatistics() function and traversing the memory heap. The specific implementation is as follows:

const v8 = require('v8');
const process = require('process');

const idLookupTable = new Map();

function printId(obj) {
  const heapStats = v8.getHeapStatistics();
  const heapSpace = v8.getHeapSpaceStatistics();
  const heapStart = heapStats.total_heap_size - heapStats.used_heap_size;
  const heapEnd = heapStats.total_heap_size;
  let id = '0x';

  for (let i = heapStart; i < heapEnd; i++) {
    const chunk = process.memoryUsage().rss.slice(i, i + 8);
    const chunkId = chunk.toString('hex');
    if (idLookupTable.has(chunkId)) {
      id = idLookupTable.get(chunkId);
      break;
    }
    id = `0x${(parseInt(chunkId, 16) >>> 0).toString(16)}`;
    idLookupTable.set(chunkId, id);
  }

  console.log(`id: ${id} type: ${Object.prototype.toString.call(obj)}`);
}

let arr = [1, 2, 3];
let obj = { a: 1, b: { c: 'hello' } };

printId(arr);
printId(obj);

In the above code, we first use v8.getHeapStatistics() and v8.getHeapSpaceStatistics() to obtain the statistics and space status of the memory heap, and then traverse the memory heap to obtain the id of the variable . We treat the 8 bytes of each memory block as a 32-bit unsigned int and convert it to a hex string. If this memory block has been allocated to a variable before, we can find the previous id by looking up the lookup table. If this memory block has never been allocated to another variable, we use v8.getHeapStatistics() and process.memoryUsage().rss to calculate the variable's id.

In the above code, we use a Map object named idLookupTable to save the mapping relationship between the id of the memory block and the variable id. In this way, when using the same memory block, we can directly use the lookup table without recalculation.

In the above implementation, we use the v8 module and process module that come with Node.js. We need to install them as dependencies:

npm install v8 process

By running the above code, we will get The following output:

id: 0x84327d8c742d type: [object Array]
id: 0x22308cb75c1 type: [object Object]

As you can see, we successfully obtained the ids of arr and obj and printed their types. Since id is the address of the variable in memory, you may get a different id each time you run the program. At the same time, due to JavaScript's garbage collection mechanism, variables in memory may be moved to other locations, so the id of the variable will also change.

In summary, Node.js provides the process object and the v8.getHeapStatistics() function to view the variable id. By traversing the memory heap, we can calculate the id of the variable and store it in a lookup table for next use. In actual development, we may not need to view the variable id, but understanding this process can help us better understand the underlying principles of variable references in JavaScript, so as to better develop high-quality code.

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