Comment and assertion changes transpilation of return statement

[ei-pi]

🔎 Search Terms

transpile, output, return statement

🕗 Version & Regression Information

• This changed between versions 4.5.5 and 4.6.0. Versions 4.6.1, 4.6.2, 4.7.4, 4.8.4, 4.9.5, 5.0.4, 5.1.6, 5.2.2, 5.3.3, 5.4.5, 5.5.4, 5.6.3, 5.7.0-beta, and 5.7.0-dev.20241020 (aka the ones accessible from the TS version dropdown on the Playground, along with 4.6.1) have all been explicitly tested on the Playground and all exhibit the behaviour—presumably, the versions in between do too.

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/CYUwxgNghgTiAEYD2A7AzgF3kgRgKwC54AlcJGYAHkxgEsUBzAGnihQE8A+AKG4Ho+8FEkRIAtmJAosbYEJEByKGgXwAvNgDWUdtwBmAVxRgMtVKwAUASngBvbvEfw4GAzBTwLDpz9x4A2gBEUDhggQC63o42GgCMANzcAL68hsam5jjWdlHOIK7unrk+AtgoEOysohJSWBpI2ro+vvhBIWGRPjHwCcm8pagVrB5KKupaOvpGJmYeYNn2Pi5uHl7NTn5toRGsaEIGYjggMLndvSlT6bPwwAu5y4Vr6-ClAO60GAAW8F8IyDXSeAAangowULA+8BwcCgmjQxQ2rWC23Cu32h2Op3G524QA

💻 Code

declare let x: number | undefined

// no comment and no 'as' = okay
function a() {
    return (
        x
    ) = 1;
}

function b() {
    return (
        // only a comment = okay
        x
    ) = 1;
}

// only an 'as' = okay
function c() {
    return (
        x as number
    ) = 1;
}

function d() {
    return (
        // with the comment + 'as', it breaks
        x as number
    ) = 1;
}

which transpiles to:

"use strict";
// no comment and no 'as' = okay
function a() {
    return (x) = 1;
}
function b() {
    return (
    // only a comment = okay
    x) = 1;
}
// only an 'as' = okay
function c() {
    return x = 1;
}
function d() {
    return 
    // with the comment + 'as', it breaks
    x = 1;
}

🙁 Actual behavior

The issue is function d—this function will always return undefined at runtime

🙂 Expected behavior

As with versions prior to 4.6.0, function d should be transpiled as:

function d() {
    return (
    // with the comment + 'as', it breaks
    x) = 1;
}

Additional information about the issue

No response

[MartinJohns]

Related: #56591 and #59237.