XS: String search methods ignore an omitted search argument

[d01c2]

Build environment: macOS 26.5.2

Moddable SDK version:

• Installed via jsvu, so there is no SDK source tree / git describe.
• Engine reports: XS 17.9.3, slot 32 bytes, ID 4 bytes

Target device: desktop (host xs)

Description
String.prototype.indexOf reports no match when the searchString argument is omitted. The same behavior occurs with String.prototype.lastIndexOf, String.prototype.includes, String.prototype.startsWith, and String.prototype.endsWith.

An omitted argument supplies undefined. Each of these methods begins with Let searchStr be ? ToString(searchString), and ToString converts undefined to the String "undefined", so they should search for the six-character string undefined and behave as if undefined were passed explicitly.

Steps to Reproduce

$ xst -e "print(String('undefined'.indexOf()))"
-1

All five, on the receiver "undefined":

$ xst -e "for (const f of ['indexOf','lastIndexOf','includes','startsWith','endsWith']) print(f + ': ' + String('undefined'[f]()));"
indexOf: -1
lastIndexOf: -1
includes: false
startsWith: false
endsWith: false

Expected behavior
0, 0, true, true, true.

For comparison, V8 (15.6.20):

$ v8 -e "for (const f of ['indexOf','lastIndexOf','includes','startsWith','endsWith']) print(f + ': ' + String('undefined'[f]()));"
indexOf: 0
lastIndexOf: 0
includes: true
startsWith: true
endsWith: true

Other information
XS already produces the expected result when undefined is passed explicitly:

$ xst -e "for (const f of ['indexOf','lastIndexOf','includes','startsWith','endsWith']) print(f + ': ' + String('undefined'[f](undefined)));"
indexOf: 0
lastIndexOf: 0
includes: true
startsWith: true
endsWith: true

Neighbouring String.prototype methods that also apply ToString to an optional argument are unaffected: search, split, replace, concat, at, padStart, and localeCompare all behave the same whether the argument is omitted or explicitly undefined.