You may place your functions names in an Array object and then call the array key respective to each function to execute it, DEMO:
function captchaTest(msg){ let x = Math.floor(Math.random()*(21-1)) +1; let y = Math.floor(Math.random()*(11-1)) +1; let sum = function(){ return x+y; } let sub = function(){ if (y > x){ let m = y; y = x; x = m; console.log(x,y,m,'--') } return x-y; } let mul = function(){ return x*y; } let OParr = [sum(), sub(), mul()]; let OP = Math.floor(Math.random()*OParr.length); let s = OParr[OP]; //!!! HERE !!! is the call as array element switch(OP){ case 0: opra = '＋'; break; case 1: opra = '━'; break; default: opra = '✖'; } let msg2 = 'Answer the following question to continue:' let p = prompt(msg+''+msg2+'\n'+'What is the result of '+x+opra+y+' ?','') console.log(s,p,OP) if (s == p){ alert ('Wow, Correct Answer!') return true; } else{ alert('Sorry, the answer is not correct!') return false; }}