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Incorrect compile result of arrow function using implicit return (starts with optional chaining) and with line comments in between #41814

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QuincyHe opened this issue Dec 4, 2020 · 2 comments · Fixed by #42026
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@QuincyHe
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@QuincyHe QuincyHe commented Dec 4, 2020

TypeScript Version: Since v3.8.3

Search Terms:
Arrow function, implicit return, line comments, optional chaining
Code

// Target: ES5
// arrow function, implicit return on 2nd line that starts with optional chaining, and has a line comment in between the lines.
const test = (names: string[]) => 
    // testing 1 two
    names?.filter(x => x);

console.log(test(["a"]));

Expected behavior:
Something like:

"use strict";
var test = function (names) { // testing 1 two
return names === null || 
names === void 0 ? void 0 : 
names.filter(function (x) { return x; }); };
console.log(test(["a"]));

Actual behavior:

  1. the comment is being copied 3 times.
  2. the first line comment terminates the return, making this function always return undefined.
"use strict";
var test = function (names) { return 
// testing 1 two
names === null || 
// testing 1 two
names === void 0 ? void 0 : 
// testing 1 two
names.filter(function (x) { return x; }); };
console.log(test(["a"]));

Playground Link:
https://www.typescriptlang.org/play?target=1&ts=3.8.3#code/MYewdgzgLgBFCm0YF4YAowEMC2iBcM0ATgJZgDmA2gLoCUKAfDAFAxswD0HciUZ5MAIxwA7iFbssuCAH4AdADMSAGwRE0AD0YwNtANzNmoSCGXw5ykOTQJoaSgCJMDuvqA
Related Issues:

@RyanCavanaugh RyanCavanaugh added the Bug label Dec 4, 2020
@RyanCavanaugh RyanCavanaugh added this to the TypeScript 4.2.0 milestone Dec 4, 2020
@RyanCavanaugh
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@RyanCavanaugh RyanCavanaugh commented Dec 4, 2020

@rbuckton correctness-affecting emit bug here

@rbuckton
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@rbuckton rbuckton commented Dec 18, 2020

// @showEmit
// @target: es5
const test = (names: string[]) => 
    // single-line comment
    names?.filter(x => x);

Workbench Repro

@typescript-bot typescript-bot added the Has Repro label Dec 18, 2020
@typescript-bot typescript-bot added the Fix Available label Dec 18, 2020
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4 participants