where is wrong !!! ??? {javascript}

Question

/*global alert,prompt*/

function multiplicationnumber(a, b) {
"use strict";
return a * b;
}
function collectnumbers(a, b) {
"use strict";
return a + b;
}
function subtractnumbers(a, b) {
"use strict";
return a - b;
}
function divisionnumbers(a, b) {
"use strict";
return a / b;
}

var number1 = prompt("pleease enter the first number!!!"),
number2 = prompt("pleease enter the second number!!!"),
mark = prompt("please enter operation type");
if (mark === "+") {
alert(collectnumbers(number1, number2));
} else if (mark === "*") {
alert(multiplicationnumber(number1, number2));
} else if (mark === "-") {
alert(subtractnumbers(number1, number2));
} else if (mark === "/") {
alert(divisionnumbers(number1, number2));
}

for * and / and - operate normally
but for + the number 1 and number2 are string and do not combine but write together
where is wrong !!! ???

Answer 1

+ is also used to concatenate two strings together, so number1 and number2 aren't cast into integers like with the other operators.

Use parseInt() to parse a string into an int.

var number1 = parseInt(prompt("pleease enter the first number!!!"));
var number2 = parseInt(prompt("pleease enter the second number!!!"));

Answer 2

thank you zen :) it's right