Error when running example code for Javascript doc - String.match()

[MEF4232]

Was reading about String.match() and testing the example code snippets. The code snippet for Using named capturing groups does not work (see screenshot below)

[wbamberg]

This is interesting. The same example works in the browser console, but fails in the interactive example's console, in both Firefox and Chrome.

Firefox: "Error: t.constructor is undefined"
Chrome: "Error: Cannot read property 'name' of undefined"

I think it's very likely that this is due to a bug in the interactive examples code that implements console.log: formatOutput is trying to work out what kind of thing it's trying to log, and if it thinks it's an object, calling formatObject. In particular it is doing:

var objectName = input.constructor.name;

...so it's assuming that if it's being called, then the argument has a constructor property. But in the browser console:

const paragraph = 'The quick brown fox jumps over the lazy dog. It barked.';

const capturingRegex = /(?<animal>fox|cat) jumps over/;
const found = paragraph.match(capturingRegex);

console.log(typeof found.groups); // "object"
console.log(typeof found.groups.constructor) // "undefined"
console.log(typeof found.groups.constructor.name) // "TypeError: found.groups.constructor is undefined"

@schalkneethling , what do you reckon?

[schalkneethling]

@schalkneethling , what do you reckon?

Will take a look. Thanks for the heads up, @wbamberg

[schalkneethling]

@MEF4232 Thank you for reporting this. @wbamberg Thank you for digging in, that is indeed the problem. As this is a problem with interactive examples/Bob I am moving the issue to the mdn/bob repo

[schalkneethling]

@wbamberg Ok, so adding this solves the problem:

if (!objectName.constructor || !objectName.prototype) {
    var formattedChild = '';
    var start = true;
    for (var key in input) {
        if (start) {
            start = false;
        } else {
            formattedChild = formattedChild + ', ';
        }
        formattedChild =
            formattedChild + key + ': ' + this.formatOutput(input[key]);
    }
    return 'Object { ' + formattedChild + ' }';
}

What I am really curious about is what type of Object this is that has neither a constructor nor a prototype. According to the docs:

All objects (with the exception of objects created with Object.create(null)) will have a constructor property

Are there other instances of this other than on String.match?

[karlcow]

Just to remember for later for myself (but if it's interesting to others)

• Prototype methods, objects without proto
• createMatchResultTemplateObject with Indices has groups only property and createMatchResultTemplateObject

[moztcampbell]

Searching the ECMAScript spec for uses of OrdinaryObjectCreate(null) I find a few:

• @@unscopables properties such as [1,2,3][Symbol.unscopables]
• import.meta in modules
• The regexp named capture groups described in their issue

So it is very uncommon.

[schalkneethling]

Thank you for the extremely valuable info @karlcow and @moztcampbell ❤️