index.js

// Create a handy array function
function in_array( arr, obj ) {
    if (typeof arr.includes === 'function') {
        arr.includes(obj);
    }

    var len = arr.length;
    for ( var x = 0 ; x <= len ; x++ ) {
        if ( arr[x] == obj ) return true;
    }
    return false;
}

// Setup our groups
var group1 = '';
var group2 = '';
var group3 = '';
var group4 = '';
var group5 = '';
var randomArrayIndex = '';

// Some combinations are not allowed
var notAllowed = new Array(
    'GB',
    'BG',
    'NK',
    'KN',
    'TN',
    'NT',
    'ZZ'
);

// Some letters can't be first and some can't be second
var firstAllowed = new Array(
    'A',
    'B',
    'C',
    'E',
    'G',
    'H',
    'J',
    'K',
    'L',
    'M',
    'N',
    'O',
    'P',
    'R',
    'S',
    'T',
    'W',
    'X',
    'Y',
    'Z'
);

var secondAllowed = new Array(
    'A',
    'B',
    'C',
    'E',
    'G',
    'H',
    'J',
    'K',
    'L',
    'M',
    'N',
    'P',
    'R',
    'S',
    'T',
    'W',
    'X',
    'Y',
    'Z'
);

// Only certain characters can show up last
var lastAllowed = new Array(
    'A',
    'B',
    'C',
    'D',
    ''
);

exports.generate = function (style) {
    // Set a default value for style and make sure it is valid
    var style = (style == null || (style !== 1 && style !== 2)) ? 1 : style;

    // Generate group1
    while( group1 == '' || in_array(notAllowed, group1) ) {
        // First letter
        randomArrayIndex = Math.floor(Math.random() * firstAllowed.length);
        group1 = firstAllowed[ randomArrayIndex ];
    
        // Second letter
        randomArrayIndex = Math.floor(Math.random() * secondAllowed.length);
        group1 = group1 + secondAllowed[ randomArrayIndex ];
    }

    // Generate group2
    group2 = ''+Math.floor(Math.random()*10)+Math.floor(Math.random()*10);

    // Generate group3
    group3 = ''+Math.floor(Math.random()*10)+Math.floor(Math.random()*10);

    // Generate group4
    group4 = ''+Math.floor(Math.random()*10)+Math.floor(Math.random()*10);

    // Generate group5
    randomArrayIndex = Math.floor(Math.random() * lastAllowed.length);
    group5 = lastAllowed[ randomArrayIndex ];

    if( style == 1 ){
        var nino = group1+group2+group3+group4+group5;
    } else {
        var nino = group1+" "+group2+" "+group3+" "+group4;
        if( group5 != '' ){
            nino = nino + " " + group5;
        }
    }
    
    return nino;
}

exports.validate = function (nino) {
        var regex = /^[A-CEGHJ-PR-TW-Z][A-CEGHJ-NPR-TW-Z] ?[0-9]{2} ?[0-9]{2} ?[0-9]{2} ?[ABCD]?/;

    if (nino.match(regex)) {
        regex = /^(GB)|^(BG)|^(NK)|^(KN)|^(TN)|^(NT)|^(ZZ)/;
        if ( !nino.match(regex) ) {
            return true;
        }
    }

    return false;
}