mongo/jstests/core/query/undefined_array_elements.js

let t = db[jsTestName()];

/* eslint-disable no-sparse-arrays */
let s0 = [, , 3, , , 6];
t.coll.insert({mys: s0});

let cur = t.coll.find();
let doc = cur.next();
assert.eq(6, doc["mys"].length);
assert.eq(undefined, doc["mys"][0]);
assert.eq(undefined, doc["mys"][1]);
assert.eq(3, doc["mys"][2]);
assert.eq(undefined, doc["mys"][3]);
assert.eq(undefined, doc["mys"][4]);
assert.eq(6, doc["mys"][5]);