JavaScript: How To Intercept Function and Method Calls

JavaScript:  How To Intercept Function and Method Calls

Intercepting function or method invocation in JavaScript. Monkey-patching or proxies. Let’s talk about that.

The other day I had an object and needed to know when any of its methods were invoked. There were many methods on that object, and these methods were invoked throughout a lot of files. So it seemed impractical to add an extra line of code after every invocation to get a notification of the invocation.

The problem

Let’s consider the following object:

let myObj = {
  multiply(x, y) {
    return x * y;
  squared(x) {
    return x ** x;

Now every time you call myObj.multiply(x, y) or myObj.squared(x) you want to know that a method was invoked on the myObj object. And in the ideal world, you would like to know which method it was and what arguments it was invoked with. Something like this.

interceptMethodCalls(myObj, (fnName, fnArgs) => { ... });


“Monkey patching is a technique to add, modify, or suppress the default behavior of a piece of code at runtime without changing its original source code.”

So monkey-patching is basically a dynamic replacement of attributes at runtime. How can we go about our problem with monkey-patching?

function interceptMethodCalls(obj, fn) {
  Object.keys(obj).forEach(key => { // (A)
    const prop = obj[key];
    if (typeof prop === 'function') { // (B)
      const origProp = prop;
      obj[key] = (...args) => { // (C)
        fn(key, args); // (D)
        return Reflect.apply(origProp, obj, args); // (E)

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