JS Headers iteration wat

headers-wat.js

headers = new Headers([["a","A"],["b","B"]]);
headers.forEach((_,k) => headers.delete(k));
console.log([...headers].length);

// expected: 0
// actual: 1 (Chrome 114)
// actual: 1 (Bun.js v0.6.13)
// actual: 1 (Safari 16.4)
// actual: 0 (Node v18.12.1)

Seems like a peculiarity of how browsers implement the Headers class

If you replace Headers with Map, it all works as expected

headers = new Headers([["a","A"],["b","B"]]);
headers.forEach((_,k) => headers.delete(k));
console.log([...headers].length);

// actual: 0

This doesn't work as expected:

headers = new Headers([["a","A"],["b","B"]]);
for (let k of headers.keys()) { headers.delete(k); }
console.log([...headers].length);

This does:

headers = new Headers([["a","A"],["b","B"]]);
for (let k of [...headers.keys()]) { headers.delete(k); }
console.log([...headers].length);

// actual: 1 (Node v18.14.2)

See this test case in Web Platform Test (WPT) web-platform-tests/wpt@a31d3ba