const numBeats = Math.ceil(Math.random()*9);

let resultString = '';
const instructionPosibilities = [ 'n', 'p', 'm', 'b' ]

for(let i = 1; i <= numBeats; i++){
	const instructionIndex = Math.floor(Math.random()*instructionPosibilities.length);

	resultString += instructionPosibilities[instructionIndex] + ' ';
}

console.log(resultString);