Change the type of a variable in a statement of a line, change the type of variables that come and sequence, why?

Question

By declaring more than one variable of the same type on a line, I can subsequently perform arithmetic operations with these values. Ex.:

let num1 = 10, num2 = 5, num3= 10;

let resultado = num1 + num2 + num3;

console.log(resultado); //O resultado será 25

However, if I change the type of the first variable num1 to string and try to perform the same operation:

let num1 = "10", num2 = 5, num3 = 10;

let resultado = num1 + num2 + num3;

console.log(resultado); //O resultado será 10510

Why does this behavior occur? Why are all variables converted?

Answer 1

This is because when finding a text value, everything else is treated as text because the operating signal + becomes the concatenate operation.

But note that this only occurs when finding a text, see these examples that the operations before are performed, and the later ones (to the text) are all treated as text:

console.log(10+20+30+40);  // tudo somado
console.log(10+20+30+"40"); // 10+20+30 somado, depois concatenado
console.log(10+20+"30"+40); // 10+20 somado, depois concatenado
console.log(10+"20"+30+40); // tudo concatenado
console.log("10"+20+30+40); // tudo concatenado

Answer 2

Complementing the answer from Ricardo, the fact that variables are declared on the same line makes no difference. If so, it gives the same result:

let num1 = "10";
let num2 = 5;
let num3 = 10;

let resultado = num1 + num2 + num3;

console.log(resultado); // 10510

For what defines the + will be treated as "sum" or "concatenation" is the type of variables involved in the operation.

If you want to force num1 be a number, just use parseInt:

let num1 = "10";
let num2 = 5;
let num3 = 10;

let resultado = parseInt(num1) + num2 + num3;

console.log(resultado); // 25