Wildfire visualization - blend function

Hello everyone,

I am going through the Wildfire visualization script from Pierre Markuse and don’t fully understand the blend function.

function blend(bArr1, bArr2, opa1, opa2) {
return bArr1.map(function(num, index) {
return (num / 100 * opa1 + bArr2[index] / 100 * opa2);
});
}

Assuming that index is the index of the image bands (3 band image), then what is num? Any hint would be very much appreciated.

Thank in advance, Philipp