How does using + as a casting mechanism work?

Question

I have seen code where strings were casted to numbers using the plus operator.

This would look something like:

var x ="5",y;
y = +x;
console.log(typeof y) //number

How does this work?

Answer 1

In fact, there are two + operators : the binary + operator and this one : the Unary + operator.

See how it's described in the MDN :

• (Unary Plus)

The unary plus operator precedes its operand and evaluates to its operand but attempts to converts it into a number, if it isn't already. For example, y = +x takes the value of x and assigns that to y; that is, if x were 3, y would get the value 3 and x would retain the value 3; but if x were the string "3", y would also get the value 3. Although unary negation (-) also can convert non-numbers, unary plus is the fastest and preferred way of converting something into a number, because it does not perform any other operations on the number. It can convert string representations of integers and floats, as well as the non-string values true, false, and null. Integers in both decimal and hexadecimal ("0x"-prefixed) formats are supported. Negative numbers are supported (though not for hex). If it cannot parse a particular value, it will evaluate to NaN.

Answer 2

var x = "5",
    y;

You're declaring two variables named respectively x and y. The former is set to hold the string "5", the latter holds the undefined value(since it's declared but not defined). Then you're setting y to be the conversion in Number type of the string "5"(via the unary operator +), which is 5. So you're getting typeof y to be number.

The + operator is the unary operator. It evaluates an object trying to convert it into a number.