How a string(like "123") is converting to a number if I add + before it but not after it?

Question

When I add a + before a number in quotes like +"123", it is converting to typeof number but if I add like "123"+, it is waiting for next operands. Why? Why in the first case it is converting to a number?

Answer 1

In the first case, you use an Unary plus +

The unary plus operator precedes its operand and evaluates to its operand but attempts to convert it into a number, if it isn't already. 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.

in the second you are using just an Addition

The addition operator produces the sum of numeric operands or string concatenation.

Answer 2

As you have + and - symbols(as unary operators) to numbers prepended only not appended. In the same way, when you add some + or - to strings, same applies and are if able to convert to numbers they will be converted to numbers. But when you append that, it is treated as binary operator. So, concatenates.