t9.js

var lookups = {
	1: [''],
	2: ['a', 'b', 'c'],
	3: ['d', 'e', 'f'],
	4: ['g', 'h', 'i'],
	5: ['j', 'k', 'l'],
	6: ['m', 'n', 'o'],
	7: ['p', 'q', 'r', 's'],
	8: ['t', 'u', 'v'],
	9: ['w', 'x', 'y', 'z'],
	0: [' '],
};

var Tel = {
	getCharacters: function(n) {
		return lookups[n] || [];
	}
};

console.log(JSON.stringify(t9('5674'), null, 2));

function t9(numbers) {
    return t9helper([], numbers.split(''));
}

function t9helper(seed, numbers) {
    if (numbers.length === 0) {
    	return seed.join('');
    }
    var letters = Tel.getCharacters(numbers[0]);
    return letters.map(l => {
        return t9helper(seed.concat(l), numbers.slice(1));
    }).reduce((acc,n) => acc.concat(n), []);     
}