css - javaScript typeof operator

Question

I recently encountered a question from js. The question is like this.
var str = 'abc' ;
typeof (str );
At first I thought the value returned was a String type value! But the result is Number, which makes me confused!
The idea I understand is that first split str , that is, str = str 1; Isn't this just string splicing? What is returned is still str.
But when I print out str, it is of NaN type. typeof(NaN) is a Number type!

That is to say
console.log(str)
console.log(str = str 1)
is not equivalent!

Is that why? ?

Answer 1

Written in javascript advanced programming, ++ and -- are unary operators that increase and decrease. They can only operate on one data. They are different from the additive operator +.
He will first The operand is converted to Number type, and then added or subtracted by one. So str = str+1 is not equal to str++

Answer 2

Because when using str++, js has implicitly converted the string type of str into a numeric type

Answer 3

console.log(str++) ++ will try to convert str into a number. If the automatic conversion is unsuccessful, it will become NaN

console.log(str = str+1) +At this time, try to splice the string and become abc1

Answer 4

There is nothing in js that guarantees that a++ and a=a+1 are equivalent.

++ only has <number>overloads, + has <string, string> and <number, number>overloads. Even if behavior is not considered, types are no longer equivalent.

Other languages ​​may have it, and it should not be confused with JS.

Answer 5

First of all, your understanding is wrong. str++ is not completely equivalent to str+=1. Here it only needs to be understood as self-increment, but self-increment only applies to numbers, so str++ will perform implicit type conversion first, and the value of str is' abc' is NaN after conversion. The return value of typeof NaN is Number