Arithmetic Add Operator

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The add operator + is used in the following example:

var result = 1 + 2;

The add operator behaves differently for special values, as follows:

Operand	Result
If either operand is NaN	the result is NaN.
If Infinity is added to Infinity	the result is Infinity.
If -Infinity is added to -Infinity	the result is -Infinity.
If Infinity is added to -Infinity	the result is NaN.
If +0 is added to +0	the result is +0.
If -0 is added to +0	the result is +0.
If -0 is added to -0	the result is -0.
If both operands are strings	the second string is concatenated to the first.
If only one operand is a string	the other operand is converted to a string and the result is the concatenation of the two strings.
If either operand is an object, number, or Boolean	its toString() method is called to get a string value and then the previous rules regarding strings are applied.For undefined and null, the String() function is called to retrieve the values "undefined" and "null", respectively.

				var result1 = 5 + 5;     //two numbers 
				console.log(result1);           //10 
				var result2 = 5 + "5";   //a number and a string 
				console.log(result2);           //"55" 
				

Pay more attention to the following code

				var num1 = 5; 
				var num2 = 10; 
				var message = "The sum of 5 and 10 is " + num1 + num2; 
				console.log(message);    //"The sum of 5 and 10 is 510" 
				

Because each addition is done separately. The first combines a string with a number (5), which results in a string.

The second takes that result string and adds a number (10), which also results in a string.

To perform the arithmetic calculation and then append that to the string, add some parentheses like this:

				var num1 = 5; 
				var num2 = 10; 
				var message = "The sum of 5 and 10 is " + (num1 + num2); 
				console.log(message);    //"The sum of 5 and 10 is 15"
				

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