What is the "groups" property in the result of a Regex?

Question

When searching for a certain occurrence in a string with Regex without the flag g (of global), the result comes, in addition to the array with the occurrence found, other properties such as index, input and groups:

var string = "abc";
var re = string.match(/b/);
console.log("Ocorrência:", re);
console.log("Index:", re.index);
console.log("Input:", re.input);
console.log("Groups:", re.groups);

My question is: what is and what is this property for groups?

I thought it would be some captured group, but when I put the b within parentheses to create a group, even though the property groups returns undefined:

var string = "abc";
var re = string.match(/(b)/);
console.log("Ocorrência:", re);
console.log("Index:", re.index);
console.log("Input:", re.input);
console.log("Groups:", re.groups);

Answer 1

According to the documentation, the property groups contains the named capturing groups (capture groups with name):

An array of named capturing groups or undefined if no named capturing groups Were defined.

In free translation:

An array of capture groups with name or undefined if no named capture group is defined.

A simple capture group (with only parentheses) has no name, so it is not returned in groups. To have a group with name, just use (?<Nome>:

var string = "abc";
var re = string.match(/(?<letra>b)/);
console.log("Ocorrência:", re);
console.log("Index:", re.index);
console.log("Input:", re.input);
console.log("Groups:", re.groups);

In the above case, the group is called "letter", and its value is the letter "b".

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Remembering that named capturing groups is a resource that was added in Ecmascript 2018.