Generate a writing prompt

PromptGenerator.js

function PromptGenerator() {
    var self = this;
    
    function pos(a) {
        if (Array.isArray(a)) {
            if (!('d' in pos)) pos.d = {};
            for (var i = 0, p = null; i < a.length; i++) {
                (pos.d[p] ? pos.d[p] : pos.d[p] = []).push(p = a[i]);
            }
        } else if ("number" == typeof a) {
            for (var i = 0, p = null, o = []; i < a; i++) {
                if (i > 0 && p == null) {
                    o.pop();
                    break;
                }
                o.push(p = pos.d[p][Math.floor(Math.random() * pos.d[p].length)]);
            }
            if (o[o.length - 1] == null) o.pop();
            return o;
        }
    };

    self.words = {
        // Nouns
        n: ['hacker','assasin','warrior','lieutenant','vilan','perp','monster','demi god'],
        // Proper Nouns
        N: ['Molly','Jim','Henry','Harry','John','Tim','Sally','Susan','Ted','James'],
        // adjectives
        a: ['blue','beautiful','green','lovely','red','terrible','fantastic','stupid','dumb','big','small'],
        // verbs
        v: ['died','killed','yelled','cried','spoke','laughed','collapsed','lost','hated','loved','shot','slashed','cut'],
        // adverbs
        av: ['quickly','sharply','awkwardly','quietly','slowly','terribly','perfectly'],
        // articles
        t: ['the','a'],
    };

    // n: noun, N: proper noun, v: verb, a: adjective
    pos(['t', 'n',null]); // articles can precede nouns
    pos(['t', 'n','v']); // verbs can follow nouns
    pos(['N', 'v',null]); // verbs can also follow proper nouns 
    pos(['a', 'n',null]); // adjectives can precede nouns
    pos(['t', 'a','n',null]); // articles can precede adjectives
    pos(['n', 'v','av',null]); // verbs can be followed by adverbs

    pos(['t', 'n', 'v']); // articles can precede nouns
    pos(['t', 'n', 'v']); // articles can precede nouns
    pos(['t', 'n', 'v']); // verbs can follow nouns
    pos(['t', 'n', 'v']); // verbs can follow nouns
    pos(['t', 'n', 'v', 'av', null]); // verbs can be followed by adverbs
    pos(['N', 'v']); // verbs can also follow proper nouns 
    pos(['N', 'v']); // verbs can also follow proper nouns 


    self.randomWord = function (type) {
        if (type in self.words) {
            return self.words[type][Math.floor(Math.random() * self.words[type].length)];
        } else {
            throw new Error('That word type does not exist:', type);
        }
    };

    self.generatePrompt = function(length) {
        let sentence = pos(length);
        for (let i = 0; i < sentence.length; i++) {
            sentence[i] = self.randomWord(sentence[i]);
        }
        return sentence.join(' ');
    }
    
}

// var pg = new PromptGenerator();
// pg.generatePrompt(5);