Simple question of! NOT because it does not return what I think

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Simple question of! NOT because it does not return what I think

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#1 by (3 votes)
#2 by (1 votes)

1

Focusing on this part = > Number (! username) Because it does not return 'Please enter some value. and if it works with ! Number (userName) I do not understand well At all, when I put 0 in the prompt it is a zero string, which is by default true, and what I want is that if I put a 0 it rejects it and says' Please, enter some value I put it! front, okay, but I do not understand why it's put in front of the Number ???

let numeroUsuario, modulo;
for (var i = 0; i < 3; i++) {
  numeroUsuario = prompt('Introduce un numero');

  if (numeroUsuario === null) {
    alert('Por favor, introduce algún valor.');
  } else if(Number(!numeroUsuario) || isNaN(numeroUsuario) || numeroUsuario === '') {
    alert('Por favor, introduce un valor numérico válido.');
  } else {
    if (numeroUsuario % 2 === 0) {
      modulo = 'par';
      alert('El numero ' + numeroUsuario + ' es ' + modulo);
    } else {
      modulo = 'impar';
      alert('El numero ' + numeroUsuario + ' es ' + modulo);
    }
  }
}

javascript if

asked by fran 14.12.2017 в 00:17

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2 answers

1

prompt returns a string , not a Number . So at the moment of doing !"0" false.

Try converting the value to Number using parseInt() :

let numeroUsuario, modulo;
for (var i = 0; i < 3; i++) {
  numeroUsuario = prompt('Introduce un numero');

  if (numeroUsuario === null) {
    alert('Por favor, introduce algún valor.');
  } else if(Number(!parseInt(numeroUsuario)) || isNaN(numeroUsuario) || numeroUsuario === '') {
    alert('Por favor, introduce un valor numérico válido.');
  } else {
    if (numeroUsuario % 2 === 0) {
      modulo = 'par';
      alert('El numero ' + numeroUsuario + ' es ' + modulo);
    } else {
      modulo = 'impar';
      alert('El numero ' + numeroUsuario + ' es ' + modulo);
    }
  }
}

answered by 14.12.2017 в 00:45

Problems with calling an external function within another Ajax does not run correctly

score 3 · Accepted Answer

Why are you denying 0 as a boolean. You are denying !0 that is equal to true that can be converted to 1 . In javascript 1==true . Then you get Number(1) == true and enter as if it were a correct value but the value of numeroUsuario is still 0 .

You need to deny the result of Number(0)== false so that it becomes true and show you the error message. How are you doing? You are evaluating badly.

This part of here Number(0)== false is telling you that it is not a number. But you are converting the 0 into 1 with ! so that's where the error comes from. First you must evaluate that the data is a number, then deny it so that it enters the second part of your ifelse .

In your code I put some alerts so you can see how the script is evaluated and understand the error that is of application of concepts. Remember that in jscript it is not necessary to define types so a 1 and 0 can represent true and false , so it can be modified by ! .

I also give you an alert so you can see how a number is evaluated when using this same operator (when the number is different from 1)

It is important to understand this way of working in javascript. You can make conversions between types of data without realizing it and that is what you are doing. It's kind of intricate.

This happens divided in steps (xO)

Number(!0) becomes Number(true) because 0=false

Number(true) becomes Number(1) because 1=true

Number(1) becomes True

Do not crack your cracklings.

This is what you want to happen:

!Number(0) is evaluated and becomes !false

!false becomes true ;

Your cracklings are thundering.

  alert( "Esto es lo que quieres !Number(0) = " + !Number("0"));
alert( "Esto es lo que estas haciendo mal Number(!0) = " + Number(!0));
alert("Esto pasa con un string '0' = " + Boolean("0"));
alert( "Por que !0= " + !0);
alert(Number(!"0") +"--"+ Number(!0));

alert("Evaluando numeros: 2 " + Boolean(2));
alert("Evaluando numeros: 9 " + Boolean(9));
alert("Evaluando numeros: !9 " + (!9) + "Aquí paso una conversión de tipo sin que te dieras cuenta. ");


let numeroUsuario, modulo;
for (var i = 0; i < 3; i++) {
  numeroUsuario = prompt('Introduce un numero');

  if (numeroUsuario === null) {
    alert('Por favor, introduce algún valor.');
  } else if(!Number(numeroUsuario) || isNaN(numeroUsuario) || numeroUsuario === '') {
    alert('Por favor, introduce un valor numérico válido.');
  } else {
    if (numeroUsuario % 2 === 0) {
      modulo = 'par';
      alert('El numero ' + numeroUsuario + ' es ' + modulo);
    } else {
      modulo = 'impar';
      alert('El numero ' + numeroUsuario + ' es ' + modulo);
    }
  }
}