The algorithm at https://tc39.es/ecma402/#sup-array.prototype.tolocalestring requires looking up a "toLocaleString" property on each non-undefined non-null element of the receiver array, invoking it as a method, and (absent an exception) passing the result through ToString. However, behavior when that lookup does not return a method or when the method call returns undefined or null appears to not be covered by test262.

The following statement list should throw an exception:

delete Object.prototype.toLocaleString;
[{}].toLocaleString();

The following expression should evaluate to "undefined":

[{toLocaleString(){}}].toLocaleString()

The following expression should evaluate to "null":

[{toLocaleString(){return null}}].toLocaleString()

Every engine except JavaScriptCore seems to get this right, and bugs have now been reported there:

• https://bugs.webkit.org/show_bug.cgi?id=232723
• https://bugs.webkit.org/show_bug.cgi?id=232724