How to get console.log line numbers shown in Nodejs?

Question

Got an old application, that prints out quite a lot of messages using console.log, but I just can not find in which files and lines console.log is

Answer 1

All solutions to this question so far rely on splitting and matching the stack trace as a string, which will break in (the unlikely) case the format of that string is changed in the future. Inspired by this gist on GitHub and the other answers here, I want to provide my own solution:

'use strict';

const path = require('path');

['debug', 'log', 'warn', 'error'].forEach((methodName) => {
    const originalLoggingMethod = console[methodName];
    console[methodName] = (firstArgument, ...otherArguments) => {
        const originalPrepareStackTrace = Error.prepareStackTrace;
        Error.prepareStackTrace = (_, stack) => stack;
        const callee = new Error().stack[1];
        Error.prepareStackTrace = originalPrepareStackTrace;
        const relativeFileName = path.relative(process.cwd(), callee.getFileName());
        const prefix = `${relativeFileName}:${callee.getLineNumber()}:`;
        if (typeof firstArgument === 'string') {
            originalLoggingMethod(prefix + ' ' + firstArgument, ...otherArguments);
        } else {
            originalLoggingMethod(prefix, firstArgument, ...otherArguments);
        }
    };
});

// Tests:
console.log('%s %d', 'hi', 42);
console.log({ a: 'foo', b: 'bar'});

Unlike the other solutions, this script

• outputs no additional lines and
• handles string substitutions correctly.

You can color the prefix with chalk or color.js, but I didn't want to introduce dependencies for this here.

The above script uses the V8 API to customize stack traces. The callee is a CallSite object with the following methods in case you want to customize the prefix:

• getThis: returns the value of this
• getTypeName: returns the type of this as a string. This is the name of the function stored in the constructor field of this, if available, otherwise the object’s [[Class]] internal property.
• getFunction: returns the current function
• getFunctionName: returns the name of the current function, typically its name property. If a name property is not available an attempt is made to infer a name from the function’s context.
• getMethodName: returns the name of the property of this or one of its prototypes that holds the current function
• getFileName: if this function was defined in a script returns the name of the script
• getLineNumber: if this function was defined in a script returns the current line number
• getColumnNumber: if this function was defined in a script returns the current column number
• getEvalOrigin: if this function was created using a call to eval returns a string representing the location where eval was called
• isToplevel: is this a top-level invocation, that is, is this the global object?
• isEval: does this call take place in code defined by a call to eval?
• isNative: is this call in native V8 code?
• isConstructor: is this a constructor call?
• isAsync: is this an async call (i.e. await or Promise.all())?
• isPromiseAll: is this an async call to Promise.all()?
• getPromiseIndex: returns the index of the promise element that was followed in Promise.all() for async stack traces, or null if the CallSite is not a Promise.all() call.

This answer is a cross-post of an answer I just gave to a similar question as more people might find this page.