Javascript code to an SI 'order of magnitude' prefix to a displayed value. For example, given 1000 and 'Hz', you will get 1kHz. For decimal number less than 1, big.js is needed to avoid errors. Note, I looked at using a mathematical approach to get the index, but it worked out to be about the same speed, so I opted for readability

SI Order of Magnitude.js

//ref: https://en.wikipedia.org/wiki/Metric_prefix

function valueToMagnitude(value, unit, fixedPlaces) { 
    var unitExponents = [
        [0,''],
        [3,'k'],
        [6,'M'],
        [9,'G'],
        [12,'T'],
        [15,'P'],
        [18,'E'],
        [21,'Y']
    ];
    
    // Note, that we may need to use big.js for other high  precision
    // decimal values. At this point I haven't had a need, so wasn't
    // fussed to fix this up. An exercise for the reader, or me if I
    // make the time
    if ((Math.abs(value) > 0 && Math.abs(value) < 1)) {
        return valueToMagnitudeVerySmall(value, unit, fixedPlaces);
    }
    
    var sign = (value<0?-1:1);
    value = Math.abs(value);
    var i = 0; idx=0;
    for (i=unitExponents.length-1; i >=0; i--) {
        if (value >= Math.pow(10, unitExponents[i][0])) {
            idx = i;
            break;
        }
    } 
    value = (value / Math.pow(10, unitExponents[idx][0]));
        
    if (fixedPlaces !== undefined) {
        value = Math.tofixed(value,fixedPlaces);
    }
  
    value = value * sign;
  
    return value + ' ' + unitExponents[idx][1] + unit;    
}

// Requires use of big.js
function valueToMagnitudeVerySmall(value, unit, fixedPlaces) { 
    var unitExponents = [
        [-12,'p'],
        [-9,'n'],
        [-6,'μ'],
        [-3,'m'],
        [0,''],
        [3,'k'],
        [6,'M'],
        [9,'G'],
        [12,'T'],
        [15,'P'],
        [18,'E'],
        [21,'Y']        
    ];  
    var valueBG = new Big(value);
    var sign = (valueBG.lt(0)?-1:1);
    valueBG = valueBG.abs(valueBG);
      
    var i=0; idx=unitExponents.length-1; match = false;
    for (i=0; i < unitExponents.length; i++) {
        if (valueBG.lte(new Big(10).pow(unitExponents[i][0]))) {
           idx = i;
           break;
        }
    }   
      
    valueBG = valueBG.div(new Big(10).pow(unitExponents[idx][0]));

    //  This will break with values smaller than the units we support      
    if (fixedPlaces !== undefined) {
       valueBG = value.tofixed(fixedPlaces);
    }
      valueBG = valueBG.times(sign);
    return  valueBG + ' ' + unitExponents[idx][1] + unit;  
}