Question

Given the following code:

String.method('deentityify', function () {
    var entity = {
        quot: '"',
        lt: '<',
        gt: '>'
    };

    return function () {
        return this.replace(/&([^&;]);/g,
            function (a, b) {
                var r = entity[b];
                return typeof r === 'string' ? r : a;
            }
        );
    };
}());
document.write('deentityify: ' + '&lt;&quot;&gt;'.deentityify() + '<br>');

Regarding the

function (a, b) {
    var r = entity[b];
    return typeof r === 'string' ? r : a;
}

How come the anonymous function get the parameter value a, b? Of course I have tried, the output is right. Can anyone can help me?

Answer 1

The function is actually an argument to the ‘replace’ call. The regex matches are passed into the function as parameters.
To write the code another way, it would look:

function match(a, b) {
    var r = entity[b];
    return typeof r === 'string' ? r : a;
}

var result = this.replace(/&([^&;]);/g, match)

The names of the parameters (a & b) are inconsequential and could be anything you like.
The first parameter will be the matched value and the subsequent parameters will be the values of the matched groups. So for clarity the function could be written as:

function matchFn(match, group1, group2, group3) {
    var r = entity[group1];
    return typeof r === 'string' ? r : match;
}

To quote MDN

A function to be invoked to create the new substring (to put
in place of the substring received from parameter #1). The arguments
supplied to this function are described in the “Specifying a function
as a parameter” section below.