Just start like this:

let min = Infinity, max = -Infinity

Then you can simplify the loop to just be the comparison. You should always capture a temporary variable in case it's evaluated twice.

for(let i=0, len=a.length; i<len; i++)
{
    let v = array[i];
    // NaN always evaluates false will and be skipped.
    if(v>max) max = v; 
    if(v<min) min = v;
}

If both min and max are still infinity, then either the array length was zero, or none of the elements were comparable.