format-number-with-si-prefix.js

function formatN (n) {
	var nn = n.toExponential(2).split(/e/);
	var u = Math.floor(+nn[1] / 3);
	return nn[0] * Math.pow(10, +nn[1] - u * 3) + ['p', 'n', 'u', 'm', '', 'k', 'M', 'G', 'T'][u+4];
}

var array = [1e7, 1e6, 2345, 100, 10, 1, 1e-2, 1e-3, 1e-5, 1e-6, 256e-9];

for (var i = 0, len = array.length; i < len; i++) {
	var n = array[i];
	var formatted = formatN(n);
	console.log(n, formatted);
}

gistfile1.txt

10000000 '10M'
1000000 '1M'
2345 '2.35k'
100 '100'
10 '10'
1 '1'
0.01 '10m'
0.001 '1m'
0.00001 '10u'
0.000001 '1u'
2.56e-7 '256n'