Arrow function with comment and type assertion will be compiled into unexpected ES5 script

[jet2jet]

TypeScript Version: 2.8.1, 2.9.0-dev.20180510

Search Terms:

• arrow
• return undefined

Code

const pickupNumbers = (a: any[]) => (
    // filter by numbers
    a.filter((x) => (typeof x === 'number')) as number[]
);
console.log(pickupNumbers([0, 1, 2, '3', '4', 5, null]));

Expected behavior:

Array [0, 1, 2, 5] will be logged

Actual behavior:

undefined will be logged

Playground Link:

https://www.typescriptlang.org/play/index.html#src=const%20pickupNumbers%20%3D%20(a%3A%20any%5B%5D)%20%3D%3E%20(%0D%0A%20%20%20%20%2F%2F%20filter%20by%20numbers%0D%0A%20%20%20%20a.filter((x)%20%3D%3E%20(typeof%20x%20%3D%3D%3D%20'number'))%20as%20number%5B%5D%0D%0A)%3B%0D%0Aconsole.log(pickupNumbers(%5B0%2C%201%2C%202%2C%20'3'%2C%20'4'%2C%205%2C%20null%5D))%3B%0D%0A

Details:

The above TS code will be compiled to the following (target: es5):

var pickupNumbers = function (a) { return 
// filter by numbers
a.filter(function (x) { return (typeof x === 'number'); }); };
console.log(pickupNumbers([0, 1, 2, '3', '4', 5, null]));

return in the first line is treated as returning nothing, so undefined will be returned. The parentheses around a.filter(...) are dropped when both comment and type assertion are written inside the parentheses.

Workaround:

1. Remove comment before compiling
2. Remove type assertion, or move as number[] after the parentheses (or put <number[]> before the parentheses)
3. Use block statement instead of expression
4. (In this case) remove the parentheses around a.filter(...) from TS code

[DanielRosenwasser]

Make sure this works for appropriate productions noted in https://tc39.github.io/ecma262/#sec-rules-of-automatic-semicolon-insertion

[Jessidhia]

The input source code is correct, it's using arrow function implicit return. But I would've written the filter as .filter((x): x is number => typeof x === 'number') instead of using an as assertion ;)

It's the generated code that's apparently generating the ASI trap.

[jet2jet]

Thanks!