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JavaScript

var SI_POSTFIXES = ["", "k", "M", "G", "T", "P", "E"];

function abbreviateNumber(number) {

    // what tier? (determines SI prefix)
    var tier = Math.log10(Math.abs(number)) / 3 | 0;

    // if zero, we don't need a prefix
    if(tier == 0) return number;

    // get postfix and determine scale
    var postfix = SI_POSTFIXES[tier];
    var scale = Math.pow(10, tier * 3);

    // scale the number
    var scaled = number / scale;

    // format number and add postfix as suffix
    var formatted = scaled.toFixed(1) + '';
    
    // remove '.0' case
    if (/\.0$/.test(formatted))
    	formatted = formatted.substr(0, formatted.length - 2);
    
    return formatted + postfix;
}

var tests = [0, 999, -999, 100000, 
-100000, 100000000, -100000000, 2345.2342, -2345.2342];

tests.forEach(test => { console.log(abbreviateNumber(test)); })