Javascript - Operator Add Operator

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Introduction

The add operator + adds two value together: 

var result = 1 + 2;

If the two operands are numbers, they perform an arithmetic add and return the result according to the following rules:

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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