Hi everyone I am doing a js code Debug in Chrome dev tools using Breakpoints etc.
So, there is this annoying function I don't want to be executed, to invalidate it, I set a breakpoint on it, so that every time the function is called I change the parameters used by this function to values which wont work for this last one.
I was wondering if there was a better way to do this(i.E: a script in the Dev Tools?)
The only way I can see to execute code automatically when the breakpoint is hit is with a Conditional Breakpoint.
Usually the condition expression will just return the value of a condition, but I think you could have the expression reassign the parameters and return false (so it won't actually stop).
Right-click on the line, select Add conditional breakpoint, and set the condition expression to something like:
param1 = value1, param2 = value2, ..., false
Related
I am currently editing a theme code in which the following javascript snippet occurs:
...
void 0 === Cookies.get("Key") && e("#modal").fadeIn(),
e("#modal .newsletter-close a").on("click", function (e) {
e.preventDefault();
}),
...
First of all, I don't understand the first line.
void boolean && function
Secondly, I don't understand that the functions in the snippet are separated with commas even though these functions are all inside another one and not in an object.
I hope someone can explain what is happening there (not specifically in this snippet, but generally for this spelling) or give me a keyword to google for
void forces the return of undefined. This is rarely desirable, however it also forces the line after it to execute, and then throws away the result of that execution. Most often you'll see it used to stop a link from going to it's href. Instead, it'll have something like href="javascript:void(performAction());". In which case, clicking the link will cause the performAction() function to fire and the whole thing returns undefined and the anchor doesn't go anywhere.
The double ampersand is a logical operator, and is saying if the cookie portion is true, and the modal portion is true, then the next thing.
The comma operator acts as a shorthand break between statements.
This is a shorthand way of saying, immediately execute this shorthand if, if the Key cookie is equal to 0 (likely means not set), and the modal is faded in, then attach the click event to the close button of the modal.
More than likely, this modal is dynamically created, and the click event isn't attached when the $(document).ready() function fires, because the modal doesn't exist yet. This is a way to allow the modal to have a click event in the shortest amount of code they could think of.
More here:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Comma_Operator
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/void
I want to modify the output of the functions (just say RANDOM examples, apologies for any code mistakes):
ng-if=!pfile.isgame
ng-if=! pfile.examplefile
-from false to true before it even has the page has any chance to drop any code on the page. How can I make it so I can append code to the page to the very beginning of the page to force every output of these particular functions to go true, on a live page?
This is definitely possible, I'm not sure where the function would be however the elements you can actually see the arguments on the page and it doesn't not look server sided at all, its just how its done. I read many articles but it many of them have not really helped me.
I am aware of Event Listener Breakpoints, its just the problem if I'm choosing the right one.
Thank you and I really appreciate it just if you can please dum down the explanation for me as even though I do understand HTML and JavaScript to an OK standard, I am still a massive beginner. This is something I always wanted to try out.
Hopefully I have understood your question correctly. There are a couple of options and the answer will depend on whether the functions are declarations or expressions.
If they are declarations, they get hoisted to the top on first pass, so that by the time your code begins execution, the function already exists and you can overwrite it early on.
If it's a function expression, you have to wait until the function expression has been created.
Example 1 (Function Declaration):
I have a function declaration on my page, which returns true if there is a remainder in the calculation, otherwise false. I execute it on page load. The output is false here:
function hasRemainder(first, second) {
return (first % second != 0);
}
console.log(hasRemainder(10, 5));
false
I have now added the Script First Statement breakpoint in DevTools, so that the debugger breaks before any script is run:
I re-open the page and the execution pauses. I now run the following code in the Console tab to override the hasRemainder function so that it always returns true:
hasRemainder = function() {
return true;
}
Finally, I click Play to continue execution. You can long click to select Long Resume, which skips breakpoints for 500ms so that you don't get caught for very single breakpoint thereafter.
true
The output this time is true as you would expect.
Example 2 (Function Expression):
We can't rely on the early breakpoint this time because the function won't exist yet. We need to add the breakpoint just after the function expression has been created.
Search for the functions using Cmd+Opt+F (Mac) or Ctrl+Shift+F (Windows).
When you are in the file with the function expression, put a breakpoint at the end of the function. When the debugger pauses, run the overriding function into the Console, and then press play to continue execution.
This is one of the first time's I'm looking at javascript, so please excuse the newbish question.
I'm trying to read the code for a specific function on a website that is of interest to me. I didn't write anything for the website, so cannot really comment on the general structure. This is almost like reverse engineering. Where it's called (in a js/main.js) looks like:
$(document).ready(function() {
$('#search').funcA();
From what I understand this is saying from the file/class or whatever that comesf rom the id search, call funcA. My questions is: how do I see the file that is called with #search?
funcA is almost certainly a jQuery plugin (or part of jQuery itself). The first thing I would try in your situation is searching for "jQuery funcA" on Google.
Whether or not it is actually part of jQuery, you can see the source for that function by running:
$('#search').funcA
in a REPL, such as your browser's console, or:
console.log( $('#search').funcA );
as long as the toString function for that function hasn't been overwritten and it is not a reference to a native function.
funcA appears to be defined as a jQuery method; try
console.log($.fn.funcA)
Open javascript console in the same browser window (I used chrome) that is displaying the page that contains that code. Then just execute this line:
> $('#search').funcA
You should see the body of funcA. Random example output when I did $("#myownid").show:
function funcA (a,b,c){var d,e;if(a||a===0)return
this.animate(cu("show",3),a,b,c);for(var
g=0,h=this.length;g
...
If you manage to see the body of the function, you should be able to infer likely sources (or post them here and we should be able to point you further)
The console.log suggestions here are nice use of Function.prototype.toString (and in some browsers some console magic), but I'd use debugger instead. Chrome has quite nice debugging tools for stuff like this and the debugger statement will get you there with ease.
var test = $('#search').funcA;
debugger;
Open the console and start investigating. When the execution of your code hits that breakpoint, you'll see handy tools like this
Right-clicking test there should also give you the option to "Show function definition" which will show you where the function was actually defined as source code.
And if you want to investigate even further from there, you can always set similar breakpoints right from the Chrome dev console.
Short version: Open the console and run $("#search") it will return a jquery object containing the dom node that has an id of search.
Long version:
$("something")
Is jquery (a java script library) for select elements by css selector returning a jquery object.
https://learn.jquery.com/using-jquery-core/selecting-elements/
$(document).ready(function() {
Is jquery for when my document (basically the page) is ready for me to muck with run this anonymous function.
https://learn.jquery.com/using-jquery-core/document-ready/
$('#search').funcA();
Selects a set of elements, in this case the single element with id "search" and then run funcA on each of them using the element as the scope. So it would run funcA on the element with ID "search" with the search node being the value of the special scope variable (scope is referenced through the key word "this", it can get rather complex).
So in essence what your seeing is:
When my document is ready find the search element and run my function funcA on it.
I have what looks like to me to be a simple variable assignment not working.
This code is in jQuery, for the context see here.
I'm calling:
$('#foo').on('someEvent', eventHandlerFn);
And I get this issue within the jQuery on function. Here's the starting point:
As you can see from the console below the code, selector is set the my eventHandlerFn and the fn variable is undefined. This is as expected.
On line 3509, the value of selector is assinged to fn. So, the value of fn should be same as the value of selector, no??
See below - selector is defined, as expected, but fn is still undefined. Why?
The end result is that my event handler is never registered.
The code runs well as shown in the following two screens (the issue is on how chrome sets the context to the console)
It looks like console has access to the variable at definition time (in this case the passed parameters) and not the live values as you run the code
Before the swap
After the swap
I'm not seeing any problem with this jsFiddle. Feel free to edit the jsFiddle to get it to look more like your code.
Can you try putting in console.log(fn); after line 3510 and rerunning? Maybe it's just a problem with the debugger?
This seems to be an issue with the debugger in Chrome - either a material problem or just a nuance of the debugger that I don't understand. fn does have a value toward the end of the call, but not where the breakpoint is.
Is there a way to make the following work?
function TimerEvent()
{
TIMER_OBJ = setTimeout('Ajaxsessioncheck();', '<%=Timer%>');
}
I am calling this function in the onload event but it is not calling the Ajaxsessioncheck function when the time has elapsed in Firefox. In IE and Chrome it works fine.
thanks for all for ur time.. i changed the code as sent timer as integer now i have a different problem. In the Ajaxsessioncheck() function i wil call a JSP page from i am not getting Response in Firefox.
You've specified '<%=Timer%>' as a string (denoted by the single quotes), where it should be an integer, like so: <%=Timer%>
You should also specify the first argument as a function reference rather than a string, so your final output would be:
setTimeout(Ajaxsessioncheck, <%=Timer%>);
you shouldn't pass the second parameter as string.
TIMER_OBJ = setTimeout('Ajaxsessioncheck();', <%=Timer%>);
should work fine. but to be even more correct, you should also avoid passing the first parameter as string, because otherwise is gets evaluated - a hidden execution of eval happens, and eval is evil. therefore, this is what you want:
TIMER_OBJ = setTimeout(Ajaxsessioncheck, <%=Timer%>);
PS. declaring a variable without using keyword var causes it to leak to the global scope. I'm not sure if you're aware of this fact.
'<%=Timer%>' is a string - it should be an int in milliseconds.
Almost all questions starting with X does not work in Y comes down to differences in browser implementation. Similar to
document.getElementById does not work in firefox and the element has a name but no ID. Works in IE but not in Fx