function mskunary2d( arrays, shape, fcn ) {
	var S0;
	var S1;
	var i0;
	var i1;
	var x0;
	var y0;
	var m0;
	var x;
	var y;
	var m;

	S0 = shape[ 1 ];
	S1 = shape[ 0 ];
	if ( S0 <= 0 || S1 <= 0 ) {
		return;
	}
	x = arrays[ 0 ];
	y = arrays[ 2 ];
	m = arrays[ 1 ];
	for ( i1 = 0; i1 < S1; i1++ ) {
		x0 = x[ i1 ];
		y0 = y[ i1 ];
		m0 = m[ i1 ];
		for ( i0 = 0; i0 < S0; i0++ ) {
			if ( m0[ i0 ] === 0 ) {
				y0[ i0 ] = fcn( x0[ i0 ] );
			}
		}
	}
}
Applies a unary callback to elements in a two-dimensional nested input array according to elements in a two-dimensional nested mask array and assigns results to elements in a two-dimensional nested output array.

## Notes

-   The function assumes that the input and output arrays have the same shape.

@param {ArrayLikeObject<Array<Collection>>} arrays - array-like object containing one input nested array, an input nested mask array, and one output nested array
@param {NonNegativeIntegerArray} shape - array shape
@param {Callback} fcn - unary callback
@returns {void}

@example
var ones2d = require( '@stdlib/array/base/ones2d' );
var zeros2d = require( '@stdlib/array/base/zeros2d' );

function scale( x ) {
    return x * 10.0;
}

var shape = [ 2, 2 ];

var x = ones2d( shape );
var y = zeros2d( shape );

var mask = [ [ 0, 1 ], [ 0, 0 ] ];

mskunary2d( [ x, mask, y ], shape, scale );

console.log( y );
// => [ [ 10.0, 0.0 ], [ 10.0, 10.0 ] ]