Uglifyjs: [ES8] async/await not supported also in harmony

[emidelgo]

Hello,

async/await are now supported by a large browser base by default ( http://caniuse.com/#search=await ), only Microsoft Edge requires a flag to be enabled.

But UglifyJS, also in harmony doesn't support this feature yet. I got this error:

Unexpected token: keyword (function)

on code as:

async function connect() { //async code }

I think it's a really important feature for javascript to be supported.

[braadworst]

+1 Same issue

[kzc]

It's a fair bit of work involved to add ES2017 async/await to uglify-es (a.k.a. harmony).

If someone is interested in implementing it, here's an outline:

• add an async boolean property to AST_Lambda
• create a new expression type AST_Await
• parser support
  
  
  
  • async is not a reserved word - it can also be an identifier in other contexts.
  
  
  • await identifier issues
  
  
  • function declaration
  
  
  • function expression
  
  
  • arrow function
  
  
  • concise methods in object literals
  
  
  • class method
    
    
    
    
    
    • static
    
    
    
    • regular
    
    
    
  
  
  • cannot combine async with setters, getters and generators
  
  
• transform support
• compress
  
  
  
  • must disable certain optimizations on async functions and await expressions
  
  
  • not likely any optimization opportunities
  
  
• output support
  
  
  
  • mirror of parser support
  
  
• tests
• mozilla conversion functions (optional, can be done later)

Resources:

• https://tc39.github.io/ecma262/#sec-async-function-definitions
• https://github.com/estree/estree/blob/master/es2017.md
• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function
• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/await
• https://github.com/ternjs/acorn/blob/master/test/tests-asyncawait.js

[kzc]

The proposed AST_Await class would be very similar to AST_Yield which was implemented in #1104. AST_Await would also have a single child expression, although the precedence of the await expression is different than yield, and the expression is mandatory in await.

No need for an async class - it would just be a new property on AST_Lambda and AST_ConciseMethod. It would be much like is_generator is for AST_Yield.

[kzc]

@alexlamsl I have a first cut implementation of async/await for uglify-es but I need master to be merged to harmony as a prerequisite for this patch. Need to disallow the inline optimization for async functions. The IIFE parameter substitution optimization also might not be safe for async functions - I have to think about that.

[alexlamsl]

@kzc cool - I'll make a release later on today then.

[kzc]

So far the patch covers most async/await use cases except for async arrow function expressions. Parsing them without backtracking is challenging because async is not a reserved word and code like the following is valid ES2017:

let x = 1, async = function async(){}; async (x); async (x) => await x + x;

[kzc]

@alexlamsl After you merge master to harmony I'm inclined to create a PR with the work I have so far. If you're inspired to get async arrow function expressions working at that point, feel free.

[alexlamsl]

Cool, and I think I'll get your PR in before cutting the harmony release.

[alexlamsl]

@kzc FYI, master now merged into harmony via #2094

[kzc]

Great - I'll rebase.

[kzc]

@alexlamsl I need some help getting the correct options for the following test to be inlined as just console.log("top"); so that I can test the async equivalent is not inlined:

    options = {
        inline: true,
        conditionals: true,
        evaluate: true,
        negate_iife: true,
        side_effects: true,
        toplevel: true,
        unused: true,
    }
    input: {
        function top() { console.log("top"); } top();

CLI works fine:

$ echo 'function top() { console.log("top"); } top();' | bin/uglifyjs -c toplevel
console.log("top");

[kzc]

Nevermind - it was reduce_vars: true that was needed.

[alexlamsl]

reduce_vars (marking top)
https://github.com/mishoo/UglifyJS2/blob/8af362ed57ab6d236f87abe76301f50928153661/lib/compress.js#L358

unused (function top() { console.log("top"); } top(); :arrow_right: (function() { console.log("top"); })();)
https://github.com/mishoo/UglifyJS2/blob/8af362ed57ab6d236f87abe76301f50928153661/lib/compress.js#L4069

inline ((function() { console.log("top"); })(); :arrow_right: void console.log("top");)
https://github.com/mishoo/UglifyJS2/blob/8af362ed57ab6d236f87abe76301f50928153661/lib/compress.js#L3390

side_effects (void console.log("top"); :arrow_right: console.log("top");)
https://github.com/mishoo/UglifyJS2/blob/8af362ed57ab6d236f87abe76301f50928153661/lib/compress.js#L2709

[kzc]

Man, it's hard to remember all the interactions between the options. I just cut/paste all the settings and removed them one by one. :-)

[alexlamsl]

@kzc whatever works best for you - I'm happy to be the clean-up crew :sunglasses:

[kzc]

2098 implements async/await - except for async arrow functions.

[kzc]

Now that #2098 has been merged to harmony, this issue can be closed in favor of #2102.

[kzc]

async support in [email protected]