js =+ 和 += 的区别

let x = 2 , z = 'a' , y;
    y =+ x;
    console.log(y);//2
    z += x;
    console.log(z);//'a2'

let x = '2', z = 'a', y;
    y =+ x; 
    z += x;
    console.log(y)//2
    console.log(z)//'a2'

let x = true, z='a', y;
    y =+ x; 
    z += x;
    console.log(y)//1
    console.log(z)//'atrue'

let x = 'a', z ='a', y;
    y =+ x; 
    z += x;
    console.log(y)//NAN
    console.log(z)//'aa'

表达式A=+B是先把B转换成数字，在赋值给A  
表达式A+=B是进行A = A+B,也就是JavaScript的加法运算