Can JavaScript functions be hooked?

Javascript functions can be hooked; hook is a method of dynamically modifying the function execution process or returning results. JavaScript, as a dynamic language, can also perform hook operations. Hooks are usually used to target certain parameters or variables. Listen, print output, replace and other operations.

The operating environment of this tutorial: Windows 10 system, JavaScript version 1.8.5, Dell G3 computer.

Can JavaScript functions be hooked?

Yes.

Hook is a method of dynamically modifying the execution process of a function or returning results, and is very widely used in practical applications. As a dynamic language, Javascript can also perform Hook operations. With the development of Javascript language, there are more and more Hook methods.

hook literally means hook. In the field of reverse engineering, it is usually used to listen, print output, replace, etc. for certain parameters and variables. operate.

The example is as follows:

Sample code

function add(a,b){
return a + b
}

hook code

This is a very simple addition function that can be obtained through Hook To find the values ​​of these two parameters, it is equivalent to adding a code console.log(a,b) before return, so that the values ​​​​of these two can be output for analysis. Then you can use the following method to rewrite the function, and this method is the decorator mode in javascript

let _add = add
add = function () {
  console.log('arguments', arguments)
  let result = _add.apply(this, arguments)
  console.log('result', result)
  return result // 如果不需要result 则可直接return _add()
}

Complete code

function add(a, b) {
  return a + b
}
 
let _add = add
add = function () {
  console.log('arguments', arguments)
  let result = _add.apply(this, arguments)
  console.log('result', result)
  return result 
}
 
add(1, 2)

Calling add(1,2) again will output the arguments parameters And result 3, a very simple HOOK is implemented.

However, this example may be too simple. What I want to express is that through Hook, we can locate the functions and variables we want to Hook, and through a series of operations (function copying, metaprogramming), as long as the function or variable is triggered By using (taking value, modifying) this variable, we can get the results we want (the results before and after (such as before encryption, after encryption)). This is our purpose.

The examples given in the book are intended to illustrate that if you want to add some functions to a certain original function (such as add here), but the original function may be written by other developers, then modify the original function directly. It may cause unknown BUG in itself, so you can use the above method to overwrite the original function without destroying the original function.

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