Why does one of these calculations return NaN when the others work as expected?

Question

var fir=prompt("Enter first Number");
var sec=prompt("Enter second number");
var sum=Number(fir)+Number(sec);
alert("The sum is " + sum);
alert("The difference is " + fir-sec);
alert("The product is " + fir*sec);
alert("The division is " + fir/sec);

Now: suppose fir=2 and sec=1.

The output is:

3
NaN
2
2

Why is the difference NaN instead of 1?

Answer 1

* and / have higher operator precedence than + and -, just like with PEMDAS order of operations in standard math. Specifically, + and - have precedence 13, whereas * and / have precedence 14.

When an expression has + and -s only, they're evaluated in left-to-right order. So, your code is equivalent to:

alert(("The Difference is " + fir) - sec);
alert("The product is " + (fir*sec));
alert("The division is " + (fir/sec));

In the second and third alert, fir and sec get combined into a single numeric expression before being concatenated with the prior string.

In the first alert, "The Difference is " + fir get put together first, resulting in another string (a string + a number results in another string). So then you have

alert((someString) - sec);

But someString - sec doesn't make sense - you can't subtract something from a string, so the expression resolves to NaN.

Answer 2

You can do one thing

var fir=prompt("ENter 1st Number");
var sec=prompt("Enter second number");
var sum=Number(fir)+Number(sec);
var sub=Number(fir)-Number(sec);
alert("The Sum is " + sum);
alert("The Difference is " + sub);
alert("The product is " + fir*sec);
alert("The division is " + fir/sec);

Usually, the plus operator + sums numbers.

But, if the binary + is applied to strings, it merges (concatenates) them:

let s = "my" + "string"; alert(s); // mystring Note that if one of the operands is a string, the other one is converted to a string too.

For example:

alert( '1' + 2 ); // "12"
alert( 2 + '1' ); // "21"

See, it doesn’t matter whether the first operand is a string or the second one. The rule is simple: if either operand is a string, the other one is converted into a string as well.

However, note that operations run from left to right. If there are two numbers followed by a string, the numbers will be added before being converted to a string:

alert(2 + 2 + '1' ); // "41" and not "221"

String concatenation and conversion is a special feature of the binary plus +. Other arithmetic operators work only with numbers and always convert their operands to numbers.

For instance, subtraction and division:

alert( 2 - '1' ); // 1
alert( '6' / '2' ); // 3

reference https://javascript.info/operators