How to extend regexp object

Question

I created an class with which it is possible to get all start and end positions of groups within a regexp object (https://github.com/valorize/MultiRegExp2). I want to wrap the initial regexp by this new "class" and add a new method execForAllGroups. How can I do this / overwrite the old regular expression but still use all its functions like search, test etc.?

I have:

function MultiRegExp2(baseRegExp) {
    let filled = fillGroups(baseRegExp);
    this.regexp = filled.regexp;
    this.groupIndexMapper = filled.groupIndexMapper;
    this.previousGroupsForGroup = filled.previousGroupsForGroup;
}

MultiRegExp2.prototype = new RegExp();
MultiRegExp2.prototype.execForAllGroups = function(string) {
    let matches = RegExp.prototype.exec.call(this.regexp, string);
    ...

Edit: Thanks to T.J. Crowder I adapted the ES6 class syntax and extended RegExp:

class MultiRegExp extends RegExp {
    yourNiftyMethod() {
        console.log("This is your nifty method");
    }
}

But
let rex = new MultiRegExp(); // rex.constructor.name is RegExp not MultiRegExp
rex.yourNiftyMethod(); // returns: rex.yourNiftyMethod is not a function

When I extend from String or another Object it all works as expected.

T.J. Crowder · Accepted Answer · 2017-02-11 17:19:44Z

5

You have at least a couple of options. As I can see you're using ES2015 (aka ES6) features, the most obvious thing to do is to extend RegExp:

class MultiRegExp2 extends RegExp {
  yourNiftyMethod() {
    console.log("This is your nifty method");
  }
}

let rex = new MultiRegExp2(/\w+/); // or   = new MultiRegExp2("\\w+");
console.log(rex.test("testing"));  // "true"
rex.yourNiftyMethod();             // "This is your nifty method"

Alternately you could augment the built-in RegExp type by simply adding to RegExp.prototype:

RegExp.prototype.yourNiftyMethod = function() {
  console.log("This is your nifty method");
};

let rex = /\w+/;
console.log(rex.test("testing"));  // "true"
rex.yourNiftyMethod();             // "This is your nifty method"

Note that extending built-in prototypes is controversial, there are at least two camps, one saying "Never do that, you'll run into trouble" and another saying "This is what prototypes are for." From a pragmatic perspective, beware of naming conflicts — with other code also extending native prototypes, and with future additions to the base type as the language and its runtime evolve.

edited Feb 11, 2017 at 17:19

answered Feb 11, 2017 at 17:07

T.J. Crowder

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3 Comments

valoricDe Over a year ago

I like the idea of extending RegExp with a class but I get: Uncaught TypeError: rex.yourNiftyMethod is not a function

valoricDe Over a year ago

Given the article v8project.blogspot.de/2017/01/… Maybe it is better to just save the regexp as an property and create hand-trough methods? test(str) { return this.regex.test(str); }?

T.J. Crowder Over a year ago

@velop: Perhaps, or perhaps that's premature micro-optimization. :-) Interesting article, though. If hyper-performance is necessary, once Chrome 57 comes out, you might want to test it both ways. (This page suggests Chrome Canary may already be using that version of V8.)

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