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Exporting with Advanced minification

Ashley's avatar
Medal
Construct Team Founder
Published 21 Aug, 2019
915 words
~4-6 mins

When exporting your project, there is an option to Minify script. This compresses all the JavaScript code used both in the Construct engine and in your own scripts, and can help obfuscate the project to make it more difficult to reverse-engineer. However if you choose the Advanced mode, you may need to adjust how you write JavaScript code in Construct.

The mode None simply skips minifying scripts completely, so they are not altered. Simple mode eliminates whitespace and does simple adjustments like renaming local variables to shorter names. This does not affect how any of the code is run so is always safe to use. Advanced mode renames everything else, including object properties, class method names, and so on. This process is referred to as property mangling. It provides the best compression and obfuscation, but in some cases it can affect how the code is run. Therefore you have to be aware of how it works and write your code accordingly to safely use it.

How property mangling works

Property mangling renames object properties to shorter names. For example consider the following code:

const obj = {
    apples: 1,
    oranges: 2
};
console.log(obj.apples, obj.oranges);

This will log the numbers 1 and 2 as they correspond to the object properties apples and oranges. After advanced minification the properties are renamed to shorter names, e.g.:

const obj = {
    a: 1,
    b: 2
};
console.log(obj.a, obj.b);

This is shorter code (which is faster to load) and harder to understand (which is harder to reverse-engineer). It also works identically to how it did previously.

Avoiding renaming

In some cases there are specific names you don't want renamed. For example if you load an external library at runtime and call a method like this:

externalLibrary.doSomethingUseful();

Property renaming will then change the name of the function, e.g.:

externalLibrary.a();

This then breaks the code, since it switched to calling a function that doesn't exist. The problem is that the minification process doesn't know about code that comes from outside your own scripts, and ends up renaming things it shouldn't.

To avoid this, you can use the string property syntax instead, as shown below.

externalLibrary["doSomethingUseful"]();

Advanced minification never renames string properties, so the name doSomethingUseful is preserved even after minification, and the code continues to work.

To write a global name as a string property, access it as a property of globalThis, e.g.:

globalThis["myGlobalFunction"]();

Built-in names

Construct uses an internal list of built-in browser API names which the minifier uses to avoid renaming built-in functions and properties. For example the names console and log are on the list, so it never renames the properties in built-in calls like console.log("Hello").

In some cases a browser API may not be on the internal list, for example if it was only just added in the latest browser release. In this case you can access it with string properties to ensure it's not renamed.

Object and variable names from Construct

Some runtime APIs use the names of things from the Construct editor, such as runtime.objects.Sprite (which takes the name Sprite from the name of an object). These properties are all consistently renamed with everything else in your code, so you can access them normally, without the string syntax.

Don't mix property styles

The main mistake that breaks code with advanced minification is mixing normal properties (which are renamed) with string properties (which are not). For example consider the following code which will be broken:

const obj = {
    "apples": 1
};
console.log(obj.apples);

This uses both string syntax and normal syntax for the property name apples. Normally this code works (logging the number 1), but after advanced minification only one of the properties is renamed, resulting in code like this:

const obj = {
    "apples": 1
};
console.log(obj.a);

Notice how "apples" had its name preserved (because it uses string syntax), but obj.apples was renamed to obj.a. That property does not exist on the object, so now the code logs undefined instead of 1.

The solution is to always use the same syntax consistently. So long as a property name consistently never uses string syntax, or consistently always does use string syntax, it will work correctly. Code is only broken if the two types are mixed.

There are several less-obvious ways string properties can end up mixed up. For example Object.defineProperty() only takes a string of a property name. Since strings are never renamed, if you use this method, that property name must always be referred to with string syntax.

Conclusion

Script minification helps compress the size of the code and makes reverse engineering more difficult. Advanced minification provides the best compression and protection, but requires some care in writing code to ensure it won't be broken by property mangling. Often code will work just fine without any specific changes in advanced mode. The only cases to be aware of are:

  1. Calling external library functions (or new browser APIs that aren't on Construct's built-in names list)
  2. Mixing string property syntax with normal property syntax

If you make sure your code handles the above correctly, then you should be able to safely use advanced minification mode with your own JavaScript code in Construct.