function makeArray(array) { var log = []; Object.setPrototypeOf(array, new Proxy(Array.prototype, new Proxy({ has(t, pk) { log.push(`Has:${String(pk)}`); return Reflect.has(t, pk); }, }, { get(t, pk, r) { if (pk in t) return Reflect.get(t, pk, r); throw new Error(`Unexpected trap "${pk}" called`); } }))); return {array, log}; } var {array, log} = makeArray([1, null, 3]); Array.prototype.indexOf.call(array, 100, { valueOf() { array.length = 0; return 0; } }); assertEqArray(log, ["Has:0", "Has:1", "Has:2"]); var {array, log} = makeArray([5, undefined, 7]); Array.prototype.lastIndexOf.call(array, 100, { valueOf() { array.length = 0; return 2; } }); assertEqArray(log, ["Has:2", "Has:1", "Has:0"]); if (typeof reportCompare === "function") reportCompare(true, true);