Minifying return await removes await which produces a different program

[jsoldi]

Uglify version 3.14.2

JavaScript input

window.fail = async function() { 
    throw new Error("Fail") 
}; 

window.test = async function() { 
    try { 
        return await fail(); 
    } 
    catch(e) { 
        console.log("Failed: " + e.message); 
        return 0; 
    } 
}

The uglifyjs CLI command executed or minify() options used.

Just the default options. This is my whole code:

import UglifyJS from 'uglify-js'
var res = UglifyJS.minify('window.fail = async function() { throw new Error("Fail") }; window.test = async function() { try { return await fail(); } catch(e) { console.log("Failed: " + e.message); return 0; } }');
console.log(res.code);

JavaScript output (beautified) or error produced.

window.fail = async function() {
    throw new Error("Fail")
}, 
window.test = async function() {
    try {
        return fail()
    } catch (n) {
        return console.log("Failed " + n.message), 0
    }
};

Uglify removes the await in the test function, which changes the program. The original test function logs Failed Fail and returns 0 without actually throwing, while the minified version throws Fail.

[kzc]

Test reformatted to run under NodeJS:

$ cat a.js
async function fail() { 
    throw new Error("PASS");
} 
async function test() { 
    try { 
        return await fail(); 
    } catch (e) { 
        console.log(e.message);
    } 
}
test();

$ cat a.js | node
PASS

$ cat a.js | uglify-js -bc | node
[stdin]:2
    throw new Error("PASS");
          ^

Error: PASS
    at fail ([stdin]:2:11)
    at test ([stdin]:7:16)
    at [stdin]:13:1

[kzc]

Another variation of the bug:

$ cat a2.js 
function fail() { 
    return new Promise((s, f) => f(new Error("PASS")));
} 
async function test() { 
    try { 
        return await fail(); 
    } catch (e) { 
        console.log(e.message);
    } 
}
test();

$ cat a2.js | node
PASS

$ cat a2.js | uglify-js -bc | node
[stdin]:2
    return new Promise((s, f) => f(new Error("PASS")));
                                   ^

Error: PASS
    at [stdin]:2:36
    at new Promise (<anonymous>)
    at fail ([stdin]:2:12)
    at test ([stdin]:7:16)
    at [stdin]:13:1

[alexlamsl]

Thanks @jsoldi for the report and @kzc for narrowing it down − investigating.

[alexlamsl]

Patch released in uglify-js@3.14.3