What are the limitations?

Admittedly, WebAssembly is not without its limitations. New features are being actively developed and the technology is constantly evolving, but the MVP functionality represents only a portion of WebAssembly's capabilities. In this section, we'll cover some of these limitations and how they impact the development process.

No garbage collection

WebAssembly supports a flat linear memory, which isn't a limitation per se, but requires some understanding of how to explicitly allocate memory to execute code. C and C++ were logical choices for the MVP because memory management is built into the language. The reason why some of the more popular high-level languages such as Java weren't included initially is due to something called garbage collection (GC).

GC is a form of automated memory management wherein memory occupied by objects that are no longer in use by the program is reclaimed automatically. GC is analogous to an automatic transmission on a car. It has been heavily optimized by skilled engineers to operate as efficiently as possible, but limits the amount of control the driver has. Manually allocating memory is like driving a car with a manual transmission. It affords greater control over speed and torque, but misuse or lack of experience can leave you stranded with a severely damaged car. Part of C and C++'s excellent performance and speed can be attributed to the manual allocation of memory.

GC languages allow you to program without having to worry about memory availability or allocation. JavaScript is an example of a GC language. The browser engine employs something called a mark-and-sweep algorithm to collect unreachable objects and free up the corresponding memory. Support for GC languages is currently being worked on in WebAssembly, but it's hard to say exactly when it will be completed.

No direct DOM access

WebAssembly is unable to access the DOM, so any DOM manipulation needs to be done indirectly through JavaScript or using a tool such as Emscripten. There are plans to add the ability to reference DOM and other Web API objects directly, but that's still in the proposal phase. DOM manipulation will likely go hand in hand with GC languages, since it will allow the seamless passing of objects between WebAssembly and JavaScript code.

No support in older browsers

Older browsers don't have the global WebAssembly object available to instantiate and load Wasm modules. There are experimental polyfills that utilize asm.js if the object isn't found, but the WebAssembly Working Group currently has no plans to create one. Since asm.js and WebAssembly are closely related, simply serving up an asm.js file if the WebAssembly object is unavailable will still offer performance gains while accommodating for backward compatibility. You can see which browsers currently support WebAssembly at https://caniuse.com/#feat=wasm.