Question

Given this Javascript:

String.method('deentityify', function () {
    // The entity table.  It maps entity names to
    // characters.
    var entity = {
        quot: '"',
        lt: '<',
        gt: '>'
    };

    // Return the deentityify method
    return function () {

        return this.replace(/&([^&;]+);/g,
            function (a, b) {
                var r = entity[b];
                return typeof r === 'string' ? r : a;
            }
        );
    };
}());

document.writeln('&lt;&quot;&gt;'.deentityify());

What goes into the last function as a and b, and why?

I get that we’re splitting up the string I’m passing in into 3 groups, but I don’t understand why < is going into a and why just lt is going into b.

Answer 1

The first argument contains the whole match, all consecutive arguments are the matched groups. The last two arguments are the offset and full input string.

var input = '<">'
input.replace( /&([^&;]+);/g, function (a, b)

The pattern matches all occurrences of & + every non-& + ;.

a      b
<   lt 
" quot
>   gt

See also: MDN: String.replace with a function