Can I override the Javascript Function object to log all function calls?

Can I override the behavior of the Function object so that I can inject behavior prior t every function call, and then carry on as normal? Specifically, (though the general idea is intriguing in itself) can I log to the console every function call without having to insert console.log statements everywhere? And then the normal behavior goes on?

I do recognize that this will likely have significant performance problems; I have no intention of having this run typically, even in my development environment. But if it works it seems an elegant solution to get a 1000 meter view on the running code. And I suspect that the answer will show me something deeper about javascript.

Answers:

Answer

Intercepting function calls

Many here have tried to override .call. Some have failed, some have succeeded. I'm responding to this old question, as it has been brought up at my workplace, with this post being used as reference.

There are only two function-call related functions available for us to modify: .call and .apply. I will demonstrate a successful override of both.

TL;DR: What OP is asking is not possible. Some of the success-reports in the answers are due to the console calling .call internally right before evaluation, not because of the call we want to intercept.

Overriding Function.prototype.call

This appears to be the first idea people come up with. Some have been more successful than others, but here is an implementation that works:

// Store the original
var origCall = Function.prototype.call;
Function.prototype.call = function () {
    // If console.log is allowed to stringify by itself, it will
    // call .call 9 gajillion times. Therefore, lets do it by ourselves.
    console.log("Calling",
                Function.prototype.toString.apply(this, []),
                "with:",
                Array.prototype.slice.apply(arguments, [1]).toString()
               );

    // A trace, for fun
   console.trace.apply(console, []);

   // The call. Apply is the only way we can pass all arguments, so don't touch that!
   origCall.apply(this, arguments);
};

This successfully intercepts Function.prototype.call

Lets take it for a spin, shall we?

// Some tests
console.log("1"); // Does not show up
console.log.apply(console,["2"]); // Does not show up
console.log.call(console, "3"); // BINGO!

It is important that this is not run from a console. The various browsers have all sorts of console tools that call .call themselves a lot, including once for every input, which might confuse a user in the moment. Another mistake is to just console.log arguments, which goes through the console api for stringification, which in turn cause an infinite loop.

Overriding Function.prototype.apply as well

Well, what about apply then? They're the only magic calling functions we have, so lets try that as well. Here goes a version that catches both:

// Store apply and call
var origApply = Function.prototype.apply;
var origCall = Function.prototype.call;

// We need to be able to apply the original functions, so we need
// to restore the apply locally on both, including the apply itself.
origApply.apply = origApply;
origCall.apply = origApply;

// Some utility functions we want to work
Function.prototype.toString.apply = origApply;
Array.prototype.slice.apply = origApply;
console.trace.apply = origApply;

function logCall(t, a) {
    // If console.log is allowed to stringify by itself, it will
    // call .call 9 gajillion times. Therefore, do it ourselves.
    console.log("Calling",
                Function.prototype.toString.apply(t, []),
                "with:",
                Array.prototype.slice.apply(a, [1]).toString()
               );
    console.trace.apply(console, []);
}

Function.prototype.call = function () {
   logCall(this, arguments);
   origCall.apply(this, arguments);
};

Function.prototype.apply = function () {
    logCall(this, arguments);
    origApply.apply(this, arguments);
}

... And lets try it out!

// Some tests
console.log("1"); // Passes by unseen
console.log.apply(console,["2"]); // Caught
console.log.call(console, "3"); // Caught

As you can see, the calling parenthesis go unnoticed.

Conclusion

Fortunately, calling parenthesis cannot be intercepted from JavaScript. But even if .call would intercept the parenthesis operator on function objects, how would we call the original without causing an infinite loop?

The only thing overriding .call/.apply does is to intercept explicit calls to those prototype functions. If the console is used with that hack in place, there will be lots and lots of spam. One must furthermore be very careful if it is used, as using the console API can quickly cause an infinite loop (console.log will use .call internally if one gives it an non-string).

Answer

I am getting SOME results and no page crashes with the following :

(function () {
  var 
    origCall = Function.prototype.call,
    log = document.getElementById ('call_log');  

  // Override call only if call_log element is present    
  log && (Function.prototype.call = function (self) {
    var r = (typeof self === 'string' ? '"' + self + '"' : self) + '.' + this + ' ('; 
    for (var i = 1; i < arguments.length; i++) r += (i > 1 ? ', ' : '') + arguments[i];  
    log.innerHTML += r + ')<br/>';



    this.apply (self, Array.prototype.slice.apply (arguments, [1]));
  });
}) ();

Only tested in Chrome version 9.xxx.

It is certainly not logging all function calls, but it is logging some! I suspect only actual calls to 'call' intself are being processed

Answer

Only a quick test, but it seems to work for me. It may not be useful this way, but I'm basically restoring the prototype whilst in my replacement's body and then "unrestoring" it before exiting.

This example simply logs all function calls - though there may be some fatal flaw I've yet to detect; doing this over a coffee break

implementation

callLog = [];

/* set up an override for the Function call prototype
 * @param func the new function wrapper
 */
function registerOverride(func) {
   oldCall = Function.prototype.call;
   Function.prototype.call = func;
}

/* restore you to your regular programming 
 */
function removeOverride() {
   Function.prototype.call = oldCall;
}

/* a simple example override
 * nb: if you use this from the node.js REPL you'll get a lot of buffer spam
 *     as every keypress is processed through a function
 * Any useful logging would ideally compact these calls
 */
function myCall() { 
   // first restore the normal call functionality
   Function.prototype.call = oldCall;

   // gather the data we wish to log
   var entry = {this:this, name:this.name, args:{}};
   for (var key in arguments) {
     if (arguments.hasOwnProperty(key)) {
      entry.args[key] = arguments[key];
     }
   }
   callLog.push(entry);

   // call the original (I may be doing this part naughtily, not a js guru)
   this(arguments);

   // put our override back in power
   Function.prototype.call = myCall;
}

usage

I've had some issues including calls to this in one big paste, so here's what I was typing into the REPL in order to test the above functions:

/* example usage
 * (only tested through the node.js REPL)
 */
registerOverride(myCall);
console.log("hello, world!");
removeOverride(myCall);
console.log(callLog);
Answer

You can override Function.prototype.call, just make sure to only apply functions within your override.

window.callLog = [];
Function.prototype.call = function() {
    Array.prototype.push.apply(window.callLog, [[this, arguments]]);
    return this.apply(arguments[0], Array.prototype.slice.apply(arguments,[1]));
};
Answer

I found it easiest to instrument the file, using an automatic process. I built this little tool to make it easier for myself. Perhaps somebody else will find it useful. It's basically awk, but easier for a Javascript programmer to use.

// This tool reads a file and builds a buffer of say ten lines.  
// When a line falls off the end of the buffer, it gets written to the output file. 
// When a line is read from the input file, it gets written to the first line of the buffer. 
// After each occurrence of a line being read from the input file and/or written to the output 
// file, a routine is given control.  The routine has the option of operating on the buffer.  
// It can insert a line before or after a line that is there, based on the lines surrounding. 
// 
// The immediate case is that if I have a set of lines like this: 
// 
//             getNum: function (a, c) {
//                 console.log(`getNum: function (a, c) {`);
//                 console.log(`arguments.callee = ${arguments.callee.toString().substr(0,100)}`);
//                 console.log(`arguments.length = ${arguments.length}`);
//                 for (var i = 0; i < arguments.length; i++) { console.log(`arguments[${i}] = ${arguments[i] ? arguments[i].toString().substr(0,100) : 'falsey'}`); }
//                 var d = b.isStrNum(a) ? (c && b.isString(c) ? RegExp(c) : b.getNumRegx).exec(a) : null;
//                 return d ? d[0] : null
//             },
//             compareNums: function (a, c, d) {
//                 console.log(`arguments.callee = ${arguments.callee.toString().substr(0,100)}`);
// 
// I want to change that to a set of lines like this: 
// 
//             getNum: function (a, c) {
//                 console.log(`getNum: function (a, c) {`);
//                 console.log(`arguments.callee = ${arguments.callee.toString().substr(0,100)}`);
//                 console.log(`arguments.length = ${arguments.length}`);
//                 for (var i = 0; i < arguments.length; i++) { console.log(`arguments[${i}] = ${arguments[i] ? arguments[i].toString().substr(0,100) : 'falsey'}`); }
//                 var d = b.isStrNum(a) ? (c && b.isString(c) ? RegExp(c) : b.getNumRegx).exec(a) : null;
//                 return d ? d[0] : null
//             },
//             compareNums: function (a, c, d) {
//                 console.log(`compareNums: function (a, c, d) {`);
//                 console.log(`arguments.callee = ${arguments.callee.toString().substr(0,100)}`);
// 
// We are trying to figure out how a set of functions work, and I want each function to report 
// its name when we enter it.
// 
// To save time, options and the function that is called on each cycle appear at the beginning 
// of this file.  Ideally, they would be --something options on the command line. 


const readline = require('readline');


//------------------------------------------------------------------------------------------------

// Here are the things that would properly be options on the command line.  Put here for 
// speed of building the tool. 

const frameSize = 10;
const shouldReportFrame = false;

function reportFrame() {
    for (i = frame.length - 1; i >= 0; i--) {
        console.error(`${i}.  ${frame[i]}`);  // Using the error stream because the stdout stream may have been coopted. 
    }
}

function processFrame() {
    // console.log(`********  ${frame[0]}`);
    // if (frame[0].search('console.log(\`arguments.callee = \$\{arguments.callee.toString().substr(0,100)\}\`);') !== -1) {
    // if (frame[0].search('arguments.callee') !== -1) {
    // if (frame[0].search(/console.log\(`arguments.callee = \$\{arguments.callee.toString\(\).substr\(0,100\)\}`\);/) !== -1) {
    var matchArray = frame[0].match(/([ \t]*)console.log\(`arguments.callee = \$\{arguments.callee.toString\(\).substr\(0,100\)\}`\);/);
    if (matchArray) {
        // console.log('********  Matched');
        frame.splice(1, 0, `${matchArray[1]}console.log('${frame[1]}');`);
    }
}

//------------------------------------------------------------------------------------------------


var i;
var frame = [];

const rl = readline.createInterface({
    input: process.stdin
});

rl.on('line', line => {
    if (frame.length > frameSize - 1) {
        for (i = frame.length - 1; i > frameSize - 2; i--) {
            process.stdout.write(`${frame[i]}\n`);
        }
    }
    frame.splice(frameSize - 1, frame.length - frameSize + 1);
    frame.splice(0, 0, line);
    if (shouldReportFrame) reportFrame();
    processFrame();
    // process.stdout.write(`${line}\n`);  // readline gives us the line with the newline stripped off
});

rl.on('close', () => {
    for (i = frame.length - 1; i > -1; i--) {
        process.stdout.write(`${frame[i]}\n`);
    }
});


// Notes
// 
// We are not going to control the writing to the output stream.  In particular, we are not 
// going to listen for drain events.  Nodejs' buffering may get overwhelmed. 
// 
Answer

The obvious answer is something like the following:

var origCall = Function.prototype.call;
Function.prototype.call = function (thisArg) {
    console.log("calling a function");

    var args = Array.prototype.slice.call(arguments, 1);
    origCall.apply(thisArg, args);
};

But this actually immediately enters an infinite loop, because the very act of calling console.log executes a function call, which calls console.log, which executes a function call, which calls console.log, which...

Point being, I'm not sure this is possible.

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