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limitations.md

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Limitations

The goal of esdown is to provide fast and inconspicuous compilation of ES6+ code to efficient ES5 code. There are some aspects of ES6 which are difficult to translate while maintaining these priorities. Keep these limitations in mind when coding with ES6 and esdown.

Runtime Library

In order to keep the translated code clean and similar in appearance to the original source code, esdown uses a small (9K unminified) runtime library to support certain features. The features which require runtime support are:

  • Classes
  • Computed property names
  • Destructuring
  • Spread
  • Async functions
  • Async generator functions

There are a couple of options for managing this depedency:

  1. Embed the runtime library in the output code using the --runtime flag.
  2. Install the esdown-runtime NPM package into your project as a dependency.

For-Of Statements

For-of statements requires the presence of Symbol.iterator. In environments which do not provide Symbol.iterator an ES6 polyfill library can be used. The polyfill library used by esdown is called esdown-polyfill and can be installed with NPM.

Map and Set Polyfills

For efficiency reasons, the Map and Set polyfills that are provided with esdown only accept strings and numbers as keys.

Classes

Subclassing is not allowed by esdown. In ES6 classes, the super class is initialized by calling super(...args), which is similar to constructing the superclass using new. In order to set the prototype chain correctly, an ES6 engine will send a hidden parameter called new.target to the superclass constructor. Unfortunately, we can't simulate new.target for arbitrary superclasses.

The typical method of simulating super(...args) is to simply call the superclass's constructor as a regular function, similar to how programmers have implemented "subsclassing" in ES5. However, this method will result in code that may break when the superclass is an actual ES6 class, because ES6 classes will throw an error when called as a function.

Block-Scoped Variables

Temporal dead zones of block-scoped variables are not enforced by esdown.

In ES6, if you attempt to access the value of a block-scoped variable before it has been initialized, a runtime error will be thrown.

f(); // ReferenceError: x is not defined
let x;
function f() {
    x = 0;
}

This is called a "temporal dead zone". A temporal dead zone is very difficult to implement efficiently with ES5 code, because in the general case it requires run-time initialization checks for each variable.