Accessing line number in V8 JavaScript (Chrome & Node.js)

Question

JavaScript developers who have spent time in languages like C often miss the ability to use certain types of introspection, like logging line numbers, and what method the current method was invoked from. Well if you're using V8 (Chrome, Node.js) you can employ the following.

Answer 1

Object.defineProperty(global, '__stack', {
  get: function(){
    var orig = Error.prepareStackTrace;
    Error.prepareStackTrace = function(_, stack){ return stack; };
    var err = new Error;
    Error.captureStackTrace(err, arguments.callee);
    var stack = err.stack;
    Error.prepareStackTrace = orig;
    return stack;
  }
});

Object.defineProperty(global, '__line', {
  get: function(){
    return __stack[1].getLineNumber();
  }
});

console.log(__line);

The above will log 19.

Combined with arguments.callee.caller you can get closer to the type of useful logging you get in C via macros.

Answer 2

The problem with the accepted answer, IMO, is that when you want to print something you might be using a logger, and when that is the case, using the accepted solution will always print the same line :)

Some minor changes will help avoiding such a case!

In our case, we're using Winston for logging, so the code looks like this (pay attention to the code-comments below):

/**
 * Use CallSite to extract filename and number, for more info read: https://v8.dev/docs/stack-trace-api#customizing-stack-traces
 * @returns {string} filename and line number separated by a colon
 */
const getFileNameAndLineNumber = () => {
    const oldStackTrace = Error.prepareStackTrace;
    try {
        // eslint-disable-next-line handle-callback-err
        Error.prepareStackTrace = (err, structuredStackTrace) => structuredStackTrace;
        Error.captureStackTrace(this);
        // in this example I needed to "peel" the first CallSites in order to get to the caller we're looking for
        // in your code, the number of stacks depends on the levels of abstractions you're using
        // in my code I'm stripping frames that come from logger module and winston (node_module)
        const callSite = this.stack.find(line => line.getFileName().indexOf('/logger/') < 0 && line.getFileName().indexOf('/node_modules/') < 0);
        return callSite.getFileName() + ':' + callSite.getLineNumber();
    } finally {
        Error.prepareStackTrace = oldStackTrace;
    }
};