Addition

The addition operator (\+) adds numbers.

var a = 9, b = 3, c = a + b;

c will now be 12

This operand can also be used multiple times in a single assignment:

var a = 9,
    b = 3,
    c = 8,
    d = a + b + c;

d will now be 20.

Both operands are converted to primitive types. Then, if either one is a string, they’re both converted to strings and concatenated. Otherwise, they’re both converted to numbers and added.

null + null;      // 0
null + undefined; // NaN
null + {};        // "null[object Object]"
null + '';        // "null"

If the operands are a string and a number, the number is converted to a string and then they’re concatenated, which may lead to unexpected results when working with strings that look numeric.

"123" + 1;        // "1231" (not 124)

If a boolean value is given in place of any of the number values, the boolean value is converted to a number (0 for false, 1 for true) before the sum is calculated:

true + 1;         // 2
false + 5;        // 5
null + 1;         // 1
undefined + 1;    // NaN

If a boolean value is given alongside a string value, the boolean value is converted to a string instead:

true + "1";        // "true1"
false + "bar";     // "falsebar"