Difference between += and =+ in javascript

Question

I want to know why after running the third line of code the result of a is 5?

a = 10;
b = 5;
a =+ b;

Answer 1

Awkward formatting:

a =+ b;

is equivalent to:

a = +b;

And +b is just a fancy way of casting b to number, like here:

var str = "123";
var num = +str;

You probably wanted:

a += b;

being equivalent to:

a = a + b;