hello world in JavaScript

iterators.js

const Filtered = Symbol('Filtered');
const Break = Symbol('Break');

class CharIterator {
    constructor(string) {
        this.string = string;
        this.index = 0;
    }
    
    next() {
        if (this.index < this.string.length) {
            return this.string[this.index++];
        }
        
        return Break;
    }
    
    map(fn) {
        return new Map(this, fn);
    }
    
    filter(fn) {
        return new Filter(this, fn);
    }
    
    collect() {
        return collect(this);
    }
}

class Split {
    constructor(string, delimiter) {
        this.iterator = new CharIterator(string);
        this.delimiter = delimiter;
        this.delimiterIndex = 0;
    }
    
    next() {
        const index = this.iterator.index;
        const char = this.iterator.next();
        
        if (char === Break) {
            return Break;
        }
        
        while (true) {
            const char = this.iterator.next();
            
            if (char === Filtered) {
                continue;
            }
            
            if (char === Break) {
                return this.iterator.string.slice(index, this.iterator.index);
            }
            
            if (char === this.delimiter.charAt(this.delimiterIndex)) {
                this.delimiterIndex++;
                
                if (this.delimiterIndex === this.delimiter.length) {
                    return this.iterator.string.slice(index, this.iterator.index - this.delimiter.length);
                }
            } else {
                this.delimiterIndex = 0;
            }
        }
    }
    
    map(fn) {
        return new Map(this, fn);
    }
    
    filter(fn) {
        return new Filter(this, fn);
    }
    
    collect() {
        return collect(this);
    }
}

class Map {
    constructor(iterator, fn) {
        this.iterator = iterator;
        this.fn = fn;
    }
    
    next() {
        const value = this.iterator.next();
        
        if (isControl(value)) {
            return value;
        }
        
        return this.fn(value);
    }
    
    map(fn) {
        return new Map(this, fn);
    }
    
    filter(fn) {
        return new Filter(this, fn);
    }
    
    collect() {
        return collect(this);
    }
}

class Filter {
    constructor(iterator, fn) {
        this.iterator = iterator;
        this.fn = fn;
    }
    
    next() {
        let value = this.iterator.next();
        
        if (isControl(value)) {
            return value;
        }
        
        if (!this.fn(value)) {
            return Filtered;
        }
        
        return value;
    }
    
    map(fn) {
        return new Map(this, fn);
    }
    
    filter(fn) {
        return new Filter(this, fn);
    }
    
    collect() {
        return collect(this);
    }
}

function isControl(value) {
    return value === Filtered || value === Break;
}

function collect(iterator) {
    let result = [];
    
    while (true) {
        let value = iterator.next();
        
        if (value === Break) {
            break;
        }
        
        if (value === Filtered) {
            continue;
        }
        
        result.push(value);
    }
    
    return result;
}

String.prototype.split = function(delimiter) {
    return new Split(this, delimiter);
};

let iterated = 'hello_ _world_ _how_ _are_ _you'
    .split('_ _')
    .map(value => value.toUpperCase())
    .filter(value => value.length > 3)
    .map(value => value.toLowerCase())
    .collect()
    .join(' ')
    .split(' ')
    .collect()
    .join(' ');

console.log(iterated);