Since this answer existed before ES6, this answer is taken from ES5.

+'11'

Answer source: ECMAScript5.1 11.4.6

One dollar+operation converts its operation value into a number. Don’t ask why, it is defined in the specification.

1 + '11'

Answer source: ECMAScript5.1 11.6.1

Steps of addition operation:

1. Convert the left and right values to metatypes (such as strings and numbers) first; for example, Boolean will be converted to numbers, and objects will usually be converted to strings, etc.;

2. Add it up

   1. If either the lvalue or rvalue converted value is a string, both values will be converted into strings for splicing operation;

   2. Otherwise, convert both the lvalue and rvalue into numbers, and then perform addition operations on the numbers; (For example, the Boolean conversion element type is not a string, but is still a Boolean, so when judging this branch here, the Boolean Convert to number 1 or 0)

3. Return results.

javascriptis a weakly typed language, and that's for one reason. Furthermore, if+is put together with a number, it is considered a positive number. For example,var a = +11is equivalent tovar a = 11. In this way,+will only be used as the concatenation operator when concatenating strings. And+ '11'is not a concatenated string, because there is no concatenated string at all. Sojswill implicitly convert'11'to11.

I am also a beginner ofjs. This is my understanding. If it is wrong, you are welcome to correct me.

+'11' -----> Unary operator

The unary + operator converts its operand to Number type.

1+'11' ----> addition operator

If one of the operands is of string type, convert the other operand to a string and return the result of the concatenation of the two strings.

The

+of+'11'is the positive of the positive and negative, which is equivalent toNumber('11')

1 + '11'is the addition of addition and subtraction, calling the internaltoPrimitivemethod for comparison. When one of the parties is a string, it will try to call the other party'stoStringmethod