Can I put a data breakpoint which triggers if any variable is assigned to a string containing a certain substring?

For example, I want to reverse-engineer how a URL containing &ctoken= is constructed. It's done with plicated JavaScript where the goal is to obfuscate it.

If I could tell the JS VM to monitor all string variables and break when a certain substring appears on any variable, this would help me a lot.

Is this possible?

Can I put a data breakpoint which triggers if any variable is assigned to a string containing a certain substring?

For example, I want to reverse-engineer how a URL containing &ctoken= is constructed. It's done with plicated JavaScript where the goal is to obfuscate it.

If I could tell the JS VM to monitor all string variables and break when a certain substring appears on any variable, this would help me a lot.

Is this possible?

• javascript
• debugging
• breakpoints

Share Improve this question Follow asked Nov 12, 2017 at 19:31 sashoalmsashoalm 79.9k136136 gold badges478478 silver badges822822 bronze badges 5

• I understand the technical requirement. Can you also write the business requirement. – Subir Kumar Sao Commented Nov 15, 2017 at 9:06
• Depending on your IDE you can put conditional breakpoints which only cause the code to stop executing if a certain condition is met. In your case you could do something along the lines of: myString.search("&ctoken=") !== -1 – Thomas Cook Commented Nov 15, 2017 at 9:16
• As I understand, you want to create global conditional breakpoint, but unfortunately it's unpossible. Description of possible types of breakpoints (Chrome) – RQman Commented Nov 15, 2017 at 9:43
• @ThomasCook Yes, only with the caveat that it should work for any string variable, not just for a particular variable with a particular name. That is, the condition is "if at any point of execution, any variable at all holds a string that contains a certain substring, then break execution". – sashoalm Commented Nov 15, 2017 at 14:01
• A partial solution can be override che String constructor method and set breakpoint on it. This cannot work for assignment var str = "Hello" but works with var str = new String("Hello"); – Matteo Gaggiano Commented Nov 22, 2017 at 9:04

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2 Answers 2

Sorted by: Reset to default 7 +50

Before I start - as of my knowledge this is not possible.

What you'd need (even before creating the debugging feature) is the raw string types already boxed to String the native built-in object and String then already proxied.

Some more explanation:

only having

const s = "hi"

is not yet an instance of String - the built-in native object, which is supplied by the ECMAScript implementation to your scope - but a raw type. Such raw types are nothing more than pointers to a raw data memory reference. I even assume there are built in pools like in Java to optimize cases like

const s = "hi"
const x = new String("hi")

to be the same memory reference of the data object. but the later of course would be boxed by String. http://bespin.cz/~ondras/html/classv8_1_1String.html

On raw types we couldn't - even if we wanted to - add a subscriber.

for example then:

s.charAt(i)

will autobox s to its wrapper String.

to observe every raw type would mean that we'd have to box all raw strings to String which wouldn't be a good thing for performance at all.

not only that but also the implementation of String itself would have to allow us to add a subscriber and therefore be proxied already.

in JS such proxy would look like this (to make it more understandable what I mean by proxied):

var proxiedString = new Proxy(String, {
  defineProperty(target, propKey, propDesc) {
    console.log('defined a new string')
  },
  set(obj, prop, value) {
    console.log('set a new value to a string')
  }
});

proxiedString.x = 'newPropValue'

and that again I guess - wouldn't be good for performance.

1. You can use condition breakpoints at browser devTools, by right click with a menu.
2. If you can write a js somewhere in a page, you can do this:

.

    if(window.location.pathname.indexOf("&ctoken=") > -1){ 
       debugger;// browser will put breakpoint automaticaly here, if condition  is trully.
       console.dir(window.location);
    }