### Summary `qs.stringify` with `arrayFormat: 'comma'` throws `TypeError: Cannot convert object to primitive value` when an array element cannot be...

Summary qs.stringify with arrayFormat: 'comma' throws TypeError: Cannot convert object to primitive value when an array element cannot be coerced to a string, such as an object with an own non-callable toString ({ "toString": "x" }) or a null-prototype object. The comma format joins array elements with Array.prototype.join and, under encodeValuesOnly, first passes each element through String() in utils.encode; neither guards against an element whose string coercion fails. ### Details In lib/stringify.js, the comma branch collapses the array with obj.join(','). For an element whose own toString is not callable, ToPrimitive skips it, falls through to Object.prototype.valueOf (which returns the object itself), and throws. Under encodeValuesOnly: true, the same element first reaches String(str) in utils.encode (lib/utils.js) and throws there. Nested arrays ({ "a": [[{ "toString": "x" }]] }) throw the same way, through Array.prototype.toString. The indices, brackets, and repeat formats walk into object elements instead of coercing them, and are not affected. Such elements can come from untrusted input. JSON.parse produces { "a": [{ "toString": "x" }] } with no qs option involved. qs.parse("a[0][toString]=x", { allowPrototypes: true }) keeps the toString key, and qs.parse("a[][b]=c", { plainObjects: true }) produces a null-prototype element; the default parse options drop toString and produce ordinary objects…