How to monitor the memory usage of Node.js?

The built-in process module has a method memoryUsage that offers insight in the memory usage of the current Node.js process. Here is an example from in Node v0.12.2 on a 64-bit system:

$ node --expose-gc
> process.memoryUsage();  // Initial usage
{ rss: 19853312, heapTotal: 9751808, heapUsed: 4535648 }
> gc();                   // Force a GC for the baseline.
undefined
> process.memoryUsage();  // Baseline memory usage.
{ rss: 22269952, heapTotal: 11803648, heapUsed: 4530208 }
> var a = new Array(1e7); // Allocate memory for 10m items in an array
undefined
> process.memoryUsage();  // Memory after allocating so many items
{ rss: 102535168, heapTotal: 91823104, heapUsed: 85246576 }
> a = null;               // Allow the array to be garbage-collected
null
> gc();                   // Force GC (requires node --expose-gc)
undefined
> process.memoryUsage();  // Memory usage after GC
{ rss: 23293952, heapTotal: 11803648, heapUsed: 4528072 }
> process.memoryUsage();  // Memory usage after idling
{ rss: 23293952, heapTotal: 11803648, heapUsed: 4753376 }

In this simple example, you can see that allocating an array of 10M elements consumers approximately 80MB (take a look at heapUsed).
If you look at V8's source code (Array::New, Heap::AllocateRawFixedArray, FixedArray::SizeFor), then you'll see that the memory used by an array is a fixed value plus the length multiplied by the size of a pointer. The latter is 8 bytes on a 64-bit system, which confirms that observed memory difference of 8 x 10 = 80MB makes sense.

You can use node.js process.memoryUsage():

const formatMemoryUsage = (data) => `${Math.round(data / 1024 / 1024 * 100) / 100} MB`;

const memoryData = process.memoryUsage();

const memoryUsage = {
  rss: `${formatMemoryUsage(memoryData.rss)} -> Resident Set Size - total memory allocated for the process execution`,
  heapTotal: `${formatMemoryUsage(memoryData.heapTotal)} -> total size of the allocated heap`,
  heapUsed: `${formatMemoryUsage(memoryData.heapUsed)} -> actual memory used during the execution`,
  external: `${formatMemoryUsage(memoryData.external)} -> V8 external memory`,
};

console.log(memoryUsage);

/*
{
  "rss": "177.54 MB -> Resident Set Size - total memory allocated for the process execution",
  "heapTotal": "102.3 MB -> total size of the allocated heap",
  "heapUsed": "94.3 MB -> actual memory used during the execution",
  "external": "3.03 MB -> V8 external memory"
}
*/

Also, if you'd like to know global memory rather than node process':

var os = require('os');

os.freemem();
os.totalmem();

See documentation

If you are using express.js framework then you can use express-status-monitor. Its very easy to integrate and it provides CPU usage, memory usage, response time etc in graphical format.

On Linux/Unix (note: Mac OS is a Unix) use top and press M (Shift+M) to sort processes by memory usage.

On Windows use the Task Manager.