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Accepted
David Willlock
Asked: 2020-05-15 21:30:41 +0800 CST 2020-05-15 21:30:41 +0800 CST

Why, when evaluating a non-boolean value in JavaScript within an if, does it return true or false?

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You see, I was practicing conditionals and using the short circuit operator to assign a value to a variable, I didn't quite understand why values ​​are considered true or false even though they are not booleans.

try this example

let hola,hola2="Hola mundo"
hola = hola2 || "Adios"

In this case the value I got was the value of hello2 as it was considered true.

While in this other example using &&

let hola,hola2="Hola mundo"
hola = hola2 && "Adios"

I got the second value which in this case is "Goodbye"

From what I learned about the short-circuit operators when using them to assign a value to a variable, it is that in the case of using the operator || if the first value is true it will assign the first value to the variable and if it is false it will assign the second value. In the case of the && operator, if the first value is true, the second value is assigned to the variable and if not, the first value is assigned.

I did another test doing this example with if:

let persona="Hola mundo";

if(persona){
 console.log("Hola mundo");
}

persona=null;

if(persona){
 console.log("No es null o undefined");
}else{
 console.log("Es null o undefined");
}

My question is, why are my variables considered true or false if they are not boolean? true value when they have a stored value and false when they are undefined or null. Is it proper to language to do that?

javascript

1 Answers

  1. Best Answer

    In Javascript there is a concept we call Truthy :

    In JavaScript, a true value is a value that is considered true when evaluated in a Boolean context. All values ​​are true unless set to false (that is, except false, 0, "", null, undefined, and NaN).

    Using Javascript 's type coercion (forcing the interpreter to change the types of variables to operate on them), you can check this with simple tests:

    console.log('Booleano', true);

//Usamos la doble negación para forzar la transformación a boolean
console.log('Cadena vacía', !!''); 
console.log('cero', !!0);
console.log('números distintos de cero', !!-1, !!3);
console.log('Cadena no vacía',!!'hola');
console.log('Null', !!null);
console.log('Array vacío y con elementos',!![],!![1]);
console.log('Cualquier objeto',!!{});

    This feature is useful for quick checks on whether a field has been filled:

    if (input.value) {
  // Entramos si el input no es una cadena vacía
}

    or to check that an attribute exists:

    if (obj.prop1) {
  // si prop1 es null o undefined no entramos
}

    But you have to be careful if the value is a number, because if it prop1is 0 it can be a correct value, in that case you would have to do something like

    if (typeof obj.prop !== 'number')

    Another utility is to take default values: imagine that you have to check if an attribute exists and, if it doesn't, assign a value to it:

    function concatena (str1, str2) {
  return str1 + str2;
}

console.log(concatena('ho','la'));
console.log('Segundo parámetro omitido', concatena('ho'));

function concatena2 (str1, str2) {
  
  str2 = str2 || ''; //si el segundo parámetro de la funciónn es "Falsy", asumimos una cadena vacía
  return str1 + str2;
}

console.log('Segundo parámetro omitido', concatena2('ho'));

    Conditionals can be created, albeit quite ugly:

    a && f(a);

    This causes it to be executed f(a)only if it aexists and has a value of truthy , because the operation &&is short- circuited (the second part is not executed if the first part is not fulfilled)

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