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Live Development: Research for live JavaScript

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dangoor edited this page Apr 30, 2013 · 12 revisions

Live JS With Chrome: What You Can/Can't Do

There are specific things that you can and can't do reliably when modifying JavaScript code using Chrome's debugging features. I'll run through specific cases to describe how they work/don't work. First, a couple of high-level notes:

  • It does not appear that you can modify JS that is in the HTML file. This is not tested at the API level, but it does not work within the Chrome Dev Tools UI.
  • Since JavaScript is an imperative language, the only changes that can reasonably be expected to have an effect are changes to functions that will be called again.
  • When doing Live JavaScript editing, the live editing icon has the disconnected appearance.
  • Using TodoMVC as an example, the footer that displays the number of todo items tends to blink in a way that it does not when you're not doing live JS editing.

The following sections will describe specific kinds of cases and how they perform.

The Setup

Use the dangoor/live-js-research branch of Brackets. All this branch does is turn on the live JS feature.

index.html:

<!DOCTYPE html>
<html>
    <head>
        <script src="jquery.min.js"></script>
        <script src="testing.js"></script>
    </head>
    <body>
        <h1>Hello</h1>
        <div onclick="logIt()">Logger</div>
        <div onclick="clearConsole()">Clear Console</div>
        <div id="console">
        </div>
    </body>
</html>

testing.js:

var startTime = new Date().getTime();

function log(message) {
    var difference = parseInt((new Date().getTime() - startTime) / 1000);
    $("#console").append($("<div>" + difference + " " + message + "</div>"));
}

function clearConsole() {
    $("#console").html("");
}

Simple Top-Level Functions

This is the simplest case. Imagine the following code attached to a click handler:

function logIt() {
    log("global message");
}

You can modify that line of code, add additional lines, add code that creates local variables, access global variables. In short, functions of this sort can be modified without difficulty.

Top-Level Function with a Closure

function logIt() {
    var i = 1;
    log("global message " + i++);
    setTimeout(function () {
        log("timed message " + i);
    }, 1000);
}

As with the case above, this one appears to work just fine. In fact, this case was derived from the previous one without reloading, so adding a closure is fine.

Object Construction

function Thing(name) {
    this.name = name;
}

function logIt() {
    var t = new Thing("Alfred");
    setTimeout(function () {
        log("timed message " + t.name);
    }, 15);
}

In my file, I already had the Thing constructor above in the same file as the logIt function from the previous case. I was able to modify logIt into the form above without reloading.

I just also confirmed that random whitespace changes between these functions has no effect.

We now reach our first unexpected behavior. I altered the code to appear as below:

function Thing(name, age) {
    this.name = name;
    this.age = age;
}

function logIt() {
    var t = new Thing("Alfredbot", 21);
    setTimeout(function () {
        log(t.name + " is " + t.age + " years old");
    }, 15);
}

When run, logIt displays "Alfredbot is undefined years old". If I reload the page (just by saving the JS file), this works as expected. I can alter the age provided in logIt. I also changed the Thing constructor to set this.age = age * 2;, and that also worked. I added this.doubleAge = age * 2; and changed logIt to display t.doubleAge instead of t.age, and that also worked.

I was able to change the Thing constructor's arguments to be n, a and that change also behaved just fine.

The first concrete rule:

You cannot add a new argument to a function.

Adding Function Arguments, Part 2

We start with our logging function:

function log(message) {
    var difference = parseInt((new Date().getTime() - startTime) / 1000);
    $("#console").append($("<div>" + difference + " " + message + "</div>"));
}

You can add text to the appended log message. Make edits to get the following (add num argument and add num to the message:

function log(message, num) {
    var difference = parseInt((new Date().getTime() - startTime) / 1000);
    $("#console").append($("<div>" + difference + " " + message + num + "</div>"));
}

Also modify logIt to pass in a num argument. I observed in making these changes that:

  • Adding the argument did not work (as expected from the previous case)
  • Removing the added code did not put the log function back into a state where live changes could be made
  • Changes could still be made to logIt, even though log would no longer accept changes

Removing an Argument

Save the file with the modified log function above. After the reload, it should log with the num argument just fine. Remove the num argument and the + num in the log function.

Removing a function argument also seems problematic

Renaming a Function

Put the log function back to its original state. Rename the function to logger, but don't change the call to it from the logIt function. Clicking on Logger on the test page still logs, despite the fact that the name is theoretically different.

Renaming a function has no effect

Adding a Function

Reload the page and then add a function called doubleIt and call that function from logIt. Here's the code we end up with:

function doubleIt(a, b) {
    return (a + b) * 2;
}

function logIt() {
    var t = new Thing("Alfredbot", 22, "Sword");
    log(t.name + " is " + t.doubleAge + " years old and his weapon of choice is " + t.np + " " + doubleIt(4, 1));
}

Not surprisingly, logIt is now broken because it does not see the doubleIt function. This is not a surprise, because the doubleIt function declaration has not been executed.

You cannot add a new function in a scope that has already been executed

Adding a Function by Executing Another

For the next cases, we'll start walking through prototype manipulation. We'll redo the setup to give us another function we can execute to work with.

Here's index.html:

<!DOCTYPE html>
<html>
    <head>
        <script src="jquery.min.js"></script>
        <script src="testing.js"></script>
    </head>
    <body>
        <h1>Hello</h1>
        <div class="resetter" onclick="resetThing()">Reset Thing</div>
        <div onclick="logIt()">Logger</div>
        <div onclick="clearConsole()">Clear Console</div>
        <div id="console">
        </div>
    </body>
</html>

And testing.js:

function Thing(name, age) {
    this.name = name;
    this.age = age;
}

var startTime = new Date().getTime();

function log(message) {
    var difference = parseInt((new Date().getTime() - startTime) / 1000);
    $("#console").append($("<div>" + difference + " " + message + "</div>"));
}

function logIt() {
    var t = new Thing("Alfredbot", 22);
    log(t.name + " is " + t.age + " ");
}


function resetThing() {
    log("resetting");
}

function clearConsole() {
    $("#console").html("");
}

resetThing();

Be sure to reload after these changes have been made. In resetThing, add the doubleIt function to window (without a reload):

function resetThing() {
    log("resetting");
    
    window.doubleIt = function(a, b) {
        return (a + b) * 2;
    }
}

and call doubleIt from logIt:

    log(t.name + " is " + t.doubleAge + " years old and his weapon of choice is " + t.np + " " + doubleIt(4, 5));

Click Reset Thing on the page, then click Logger. You'll see that doubleIt was successfully added to the page.

Modifying a Function by Re-Running Code

Changing the doubleIt function works just fine, as you'd expect. What if we want to change the function signature? Add a c parameter to doubleIt, and also add a third number to logIt.

doubleIt's declaration now looks like this:

    window.doubleIt = function(a, b, c) {
        return (a + b + c) * 2;
    }

Click Reset Thing and then Logger. The new definition of doubleIt is in place.

Adding to an Object Prototype

Remove doubleIt and the reference to it in logIt. Reload the page.

Add the following to resetThing:

    Thing.prototype.doubleAge = function() {
        return this.age * 2;
    }

and call it from logIt:

    log(t.name + " is " + t.age + " " + t.doubleAge());

Click Reset Thing, Logger. Note that it doesn't log because it can't find t.doubleAge. Reload and confirm that logIt can then find t.doubleAge. Once you've done this, you can change doubleAge and it works fine.

Adding to an object prototype doesn't work!

Adding to an Object Prototype, Part 2

Add this to the resetThing function and change logIt to call this instead of doubleAge:

    Thing.prototype.tripleAge = function() {
        return this.age * 3;
    }

Click Reset/Logger. Note that adding this function worked where the previous one did not.

You can add to an object prototype in a function where you previously added to it

Adding to a New Prototype

Let's see how differently it behaves when you use a different style of manipulating an object prototype:

function Thing(name, age) {
    this.name = name;
    this.age = age;
}

var startTime = new Date().getTime();

function log(message) {
    var difference = parseInt((new Date().getTime() - startTime) / 1000);
    $("#console").append($("<div>" + difference + " " + message + "</div>"));
}

function logIt() {
    var t = new Thing("Alfredbot", 22);
    log(t.name + " is " + t.age + " ");
}


function resetThing() {
    log("resetting");
    
    Thing.prototype = {
    };
}

function clearConsole() {
    $("#console").html("");
}

resetThing();

Reload with that copy of testing.js. Now, add doubleAge in this style:

    Thing.prototype = {
        doubleAge: function() {
            return this.age * 2;
        }
    };

and call it from logIt. Reset/Logger.

That worked for me.

Adding to a New Prototype, Part 2

Add a tripleAge function just as we did doubleAge.

        tripleAge: function() {
            return this.age * 3;
        }

and call that from logIt. Click Reset/Logger. That works again.

Summary of the Rules

These are the boundaries we have seen of Chrome's live JavaScript swapping feature. The actions below do not work:

  • Altering code that has already been executed and doesn't run again
  • Adding a new argument to a function
  • Removing an argument from a function
  • Renaming a function
  • Adding a function in a scope that has already been executed
  • Adding to an object's prototype (unless you previously added to the prototype)

Limitations of this Research

This research looked at the usability boundaries of our existing JavaScript live development experimental code. It did not involve looking into Chrome's mechanism for this functionality to see if improvements have been made since the original experimental code was created, or to see if there are other ways to apply this feature of Chrome's.

Conclusion

Without a good way to predict whether a given function will be executed again, it is very hard to predict whether a live JS edit that the user makes will have any effect. Most well-written JavaScript code today, runs in some kind of closure (the module pattern, for example). Just being within a function body does not tell us much about when that function can safely be executed.

Chrome's script replacement is not enough to give us "live JavaScript development" that is comparable to what we give users for CSS and plan to give them for HTML. There are too many common cases that aren't handled.

Due to the very nature of JavaScript, "Live Development" for JavaScript cannot be as high fidelity as live development for CSS is. Proceeding from here means deciding which tradeoffs we're willing to make.

  1. Using Chrome's script replacement (as tested here), we could allow users to change certain bits of code without reloading. We could identify and warn the user about some changes that require a reload to take effect, but almost certainly not in all cases.
  2. We could do further research to try to remove some of the limitations discussed here by instrumenting JavaScript code or augmenting the runtime.
  3. We could do research into coding styles/frameworks for building JavaScript that are specifically suited to hot code replacement.
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