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 cu

Answer 1

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;
    }
};

Answer 2

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.