[v8 engine] Functions with newline after return statement returns undefined

[t2ym]

[v8 engine] Arrow functions with newline after return statement returns undefined

Root Cause

• v8 has a bug that fails to parse newline after return in functions See the comment below

Reproducible Code Examples

(() => { return
1 })() // returns undefined

(function () { return
1})() // returns undefined

Side Effects in hooking

• original script - parentheses after return are hiding the issue
  • Works as expected

    () => { return (
    1) }

• hooked script - parentheses after return are omitted in code generation
  • This hooked function always return undefined

    (...args) =>
    (__hook__(() => {
    return 1;
    }, null, args, '../nl.js'));

Workaround

• Use compact=true option to eliminate extra whitespaces

[t2ym]

Root Cause

• This is by design for JavaScript automatic semicolon insertion.
• escodegen cannot properly handle redundant parentheses including a newline just after the left parenthesis after return statement

  original

  () => { return (
  1); }

  escodegen-generated (automatic semicolon insertion by JavaScript parser)

  () => { return
  1; }

Notes

• Can espree-generated ASTs retain redundant parentheses for escodegen?
• Can escodegen insert parentheses for return statement argument if the return token is followed by a newline?

Referenced Issue

https://github.com/Microsoft/TypeScript/issues/2704