code: function mean( arr ) {
  	var len = arr.length,
  		delta,
  		mu,
  		i;
  
  	if ( !len ) {
  		return null;
  	}
  	mu = 0;
  	for ( i = 0; i < len; i++ ) {
  		delta = arr[ i ] - mu;
  		mu += delta / (i+1);
  	}
  	return mu;
  }
docs: FUNCTION: mean( arr )
  Computes the arithmetic mean of a numeric array.
  
  @param {Number[]|Int8Array|Uint8Array|Uint8ClampedArray|Int16Array|Uint16Array|Int32Array|Uint32Array|Float32Array|Float64Array} arr - input array
  @returns {Number|Null} arithmetic mean or null