JS function to calculate complementary colour?

Parsed through http://design.geckotribe.com/colorwheel/

    // Complement
    temprgb={ r: 0, g: 0xff, b: 0xff }; // Cyan
    temphsv=RGB2HSV(temprgb);
    temphsv.hue=HueShift(temphsv.hue,180.0);
    temprgb=HSV2RGB(temphsv);
    console.log(temprgb); // Complement is red (0xff, 0, 0)
    
    function RGB2HSV(rgb) {
    	hsv = new Object();
    	max=max3(rgb.r,rgb.g,rgb.b);
    	dif=max-min3(rgb.r,rgb.g,rgb.b);
    	hsv.saturation=(max==0.0)?0:(100*dif/max);
    	if (hsv.saturation==0) hsv.hue=0;
     	else if (rgb.r==max) hsv.hue=60.0*(rgb.g-rgb.b)/dif;
    	else if (rgb.g==max) hsv.hue=120.0+60.0*(rgb.b-rgb.r)/dif;
    	else if (rgb.b==max) hsv.hue=240.0+60.0*(rgb.r-rgb.g)/dif;
    	if (hsv.hue<0.0) hsv.hue+=360.0;
    	hsv.value=Math.round(max*100/255);
    	hsv.hue=Math.round(hsv.hue);
    	hsv.saturation=Math.round(hsv.saturation);
    	return hsv;
    }
    
    // RGB2HSV and HSV2RGB are based on Color Match Remix [http://color.twysted.net/]
    // which is based on or copied from ColorMatch 5K [http://colormatch.dk/]
    function HSV2RGB(hsv) {
    	var rgb=new Object();
    	if (hsv.saturation==0) {
    		rgb.r=rgb.g=rgb.b=Math.round(hsv.value*2.55);
    	} else {
    		hsv.hue/=60;
    		hsv.saturation/=100;
    		hsv.value/=100;
    		i=Math.floor(hsv.hue);
    		f=hsv.hue-i;
    		p=hsv.value*(1-hsv.saturation);
    		q=hsv.value*(1-hsv.saturation*f);
    		t=hsv.value*(1-hsv.saturation*(1-f));
    		switch(i) {
    		case 0: rgb.r=hsv.value; rgb.g=t; rgb.b=p; break;
    		case 1: rgb.r=q; rgb.g=hsv.value; rgb.b=p; break;
    		case 2: rgb.r=p; rgb.g=hsv.value; rgb.b=t; break;
    		case 3: rgb.r=p; rgb.g=q; rgb.b=hsv.value; break;
    		case 4: rgb.r=t; rgb.g=p; rgb.b=hsv.value; break;
    		default: rgb.r=hsv.value; rgb.g=p; rgb.b=q;
    		}
    		rgb.r=Math.round(rgb.r*255);
    		rgb.g=Math.round(rgb.g*255);
    		rgb.b=Math.round(rgb.b*255);
    	}
    	return rgb;
    }

    //Adding HueShift via Jacob (see comments)
    function HueShift(h,s) { 
        h+=s; while (h>=360.0) h-=360.0; while (h<0.0) h+=360.0; return h; 
    }
    
    //min max via Hairgami_Master (see comments)
    function min3(a,b,c) { 
        return (a<b)?((a<c)?a:c):((b<c)?b:c); 
    } 
    function max3(a,b,c) { 
        return (a>b)?((a>c)?a:c):((b>c)?b:c); 
    }

I find that taking the bit-wise complement works well, and quickly.

var color = 0x320ae3;
var complement = 0xffffff ^ color;

I'm not sure if it's a perfect complement in the sense of "mixes together to form a 70% grey", however a 70% grey is "pure white" in terms of color timing in film. It occurred to me that XORing the RGB hex out of pure white might be a good first approximation. You could also try a darker grey to see how that works for you.

Again, this is a fast approximation and I make no guarantees that it'll be perfectly accurate.

See https://github.com/alfl/textful/blob/master/app.js#L38 for my implementation.

None of the other functions here worked out the box, so I made this one.

It takes a hex value, converts it to HSL, shifts the hue 180 degrees and converts back to Hex

/* hexToComplimentary : Converts hex value to HSL, shifts
 * hue by 180 degrees and then converts hex, giving complimentary color
 * as a hex value
 * @param  [String] hex : hex value  
 * @return [String] : complimentary color as hex value
 */
function hexToComplimentary(hex){

    // Convert hex to rgb
    // Credit to Denis http://stackoverflow.com/a/36253499/4939630
    var rgb = 'rgb(' + (hex = hex.replace('#', '')).match(new RegExp('(.{' + hex.length/3 + '})', 'g')).map(function(l) { return parseInt(hex.length%2 ? l+l : l, 16); }).join(',') + ')';

    // Get array of RGB values
    rgb = rgb.replace(/[^\d,]/g, '').split(',');

    var r = rgb[0], g = rgb[1], b = rgb[2];

    // Convert RGB to HSL
    // Adapted from answer by 0x000f http://stackoverflow.com/a/34946092/4939630
    r /= 255.0;
    g /= 255.0;
    b /= 255.0;
    var max = Math.max(r, g, b);
    var min = Math.min(r, g, b);
    var h, s, l = (max + min) / 2.0;

    if(max == min) {
        h = s = 0;  //achromatic
    } else {
        var d = max - min;
        s = (l > 0.5 ? d / (2.0 - max - min) : d / (max + min));

        if(max == r && g >= b) {
            h = 1.0472 * (g - b) / d ;
        } else if(max == r && g < b) {
            h = 1.0472 * (g - b) / d + 6.2832;
        } else if(max == g) {
            h = 1.0472 * (b - r) / d + 2.0944;
        } else if(max == b) {
            h = 1.0472 * (r - g) / d + 4.1888;
        }
    }

    h = h / 6.2832 * 360.0 + 0;

    // Shift hue to opposite side of wheel and convert to [0-1] value
    h+= 180;
    if (h > 360) { h -= 360; }
    h /= 360;

    // Convert h s and l values into r g and b values
    // Adapted from answer by Mohsen http://stackoverflow.com/a/9493060/4939630
    if(s === 0){
        r = g = b = l; // achromatic
    } else {
        var hue2rgb = function hue2rgb(p, q, t){
            if(t < 0) t += 1;
            if(t > 1) t -= 1;
            if(t < 1/6) return p + (q - p) * 6 * t;
            if(t < 1/2) return q;
            if(t < 2/3) return p + (q - p) * (2/3 - t) * 6;
            return p;
        };

        var q = l < 0.5 ? l * (1 + s) : l + s - l * s;
        var p = 2 * l - q;

        r = hue2rgb(p, q, h + 1/3);
        g = hue2rgb(p, q, h);
        b = hue2rgb(p, q, h - 1/3);
    }

    r = Math.round(r * 255);
    g = Math.round(g * 255); 
    b = Math.round(b * 255);

    // Convert r b and g values to hex
    rgb = b | (g << 8) | (r << 16); 
    return "#" + (0x1000000 | rgb).toString(16).substring(1);
}