Why so many nested Object.assign calls?

[ackvf]

While verifying a certain spread pattern, I noticed the nested object.assign transpiled code and was wondering if that's necessary. The flattened object.assign produces just the same output.

TypeScript Version: 3.9.2 in REPL

Search Terms:
nested object.assign, object spread
Code

source

let aname = "John"
let xname = ""
let yname = xname
let zname
let active = true

const isDefined = (value: any) => typeof value !== 'undefined'

const result = {
    ...(aname && { aname }),
    ...(xname && { xname }),
    ...(isDefined(yname) && { yname }),
    ...(isDefined(zname) && { zname }),
    ...(isDefined(active) && { active })
}

console.log(result)

const result2 = Object.assign({}, (aname && { aname }), (xname && { xname }), (isDefined(yname) && { yname }), (isDefined(zname) && { zname }), (isDefined(active) && { active }));

console.log(result2)

result

"use strict";
let aname = "John";
let xname = "";
let yname = xname;
let zname;
let active = true;
const isDefined = (value) => typeof value !== 'undefined';
const result = Object.assign(Object.assign(Object.assign(Object.assign(Object.assign({}, (aname && { aname })), (xname && { xname })), (isDefined(yname) && { yname })), (isDefined(zname) && { zname })), (isDefined(active) && { active }));
console.log(result);
const result2 = Object.assign({}, (aname && { aname }), (xname && { xname }), (isDefined(yname) && { yname }), (isDefined(zname) && { zname }), (isDefined(active) && { active }));
console.log(result2);

output

{aname: "John", yname: "", active: true}
{aname: "John", yname: "", active: true}

Expected behavior:

Actual behavior:

Playground Link:
https://www.typescriptlang.org/play/#code/DYUwLgBAhgdlC2IIF4ICIBSB7AFjNAsAFCiQAeciK6hJ4EAnpUqhQiMaRAF7Of1QAxmACWANxYQwAJwCuHIsUFYYAZ0gjVAERAAzETBAATagAoxUYPIBc0GAwCUKAHxSGABxBZdEC1aQAhMioAOSyMEZ6BsYhxEoq6hDSIKqywJCoAN7EELkQAHSFprDsEABkZRCZdqUAvg4ANDl5hfmmbFQVVRAdSPVNRHkFRZo6+oZGpkzsTl3V01T9zbmtpqNRE6a8M+WV1duLjcvDbevjxsXC4iCze9BXEhD1xLVxRMpqWKD5wFgA5qZkql0g43h9EkC0mAAEzUADyACMAFYgYT5KCqVQiP4wUyZWoNCDFZi7bolQ6E9okuY9En9IlnaKTBY3UnzOmNBnaDYXA6sml8p6ctbc86TISiCS3MkPPoOBwAbjBCS+IB+-0BKSh0IcQA

[jcalz]

I assume this is working as intended because of the issues mentioned in #32514. Everything's fine until your spreadable things have side effects, and then 🤮.

[ackvf]

@jcalz wow. Thank you