Why is the ternary operator not working in javascript?

Question:

I am working on nodejs (although my question is valid for javascript in general) and I came across something that I don't understand.

It is related to the ternary operator which, as I understand it , works like this:

var numero = 5;
var positivo = numero >= 0 ? true : false; 

If the condition ( numero >=0 ) is fulfilled, the final result will be the first value that appears after said condition, in this case true . Otherwise, the final result will be the second value, in this case false .

So when printing:

console.log(positivo); //imprime true

If I change the variable numero , I can get a false value:

var numero = -1;
var positivo = numero >= 0 ? true : false; 
console.log(positivo); //imprime false

So far so good, but trying to isolate my problem I did this little code snippet:

var isValid = false;
var message;

if(isValid){
  message = 'es válida';
}else {
  message = 'no es válida';
}

console.log("Ésta condición " + message);
console.log("Ésta condición " + isValid ? 'es válida' : 'no es válida');

According to my logic, since in my code the variable isValid is false, both messages should print:

Ésta condición no es válida

However when I run my program the result is this:

Ésta condición no es válida
es válida

As you can see, using the ternary operator has ignored for some reason that my isValid variable was set to false , does anyone have any idea why this happens? And if it was an expected behavior (which I doubt) I'd like to know why.

Answer:

You must use parentheses:

console.log("Ésta condición " + (isValid ? 'es válida' : 'no es válida'));

The + operator takes precedence. What the compiler understands is the following:

console.log(("Ésta condición " + isValid) ? 'es válida' : 'no es válida');

Then print:

es valida

Since "Ésta condición " + isValid evaluates to true .


By the way, this order of precedence applies to most languages ​​(honestly, I don't know if there are any that are the other way around), including JavaScript.

This is the complete table of precedence of operators in JavaScript (see in MDN ).

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