Why is the else executed when entering zero, but the conversion from the if condition is performed?

<!DOCTYPE html>
<html lang="en">
<head>
 <meta charset="UTF-8">
 <meta name="viewport" content="width=device-width, initial-scale=1.0">
 <title>JS test</title>
</head>
<body>
  <script type="text/javascript">

    let num = prompt('введите число:', 0);

    if ( num = Number(num) ){
      alert('in if');
      if (num > 0) {
        alert(typeof(num));
        alert('1');
      }else if (num < 0){
        alert(typeof(num));
        alert('-1');
      }else if(num == 0){
        alert('0');
      }
    }else{
      alert('in else');
      alert(typeof(num));
      alert('Это не число!');
    }

  </script>
</body>
</html>

JavaScript Anonymous, Mar 22, 2019

3 Answers

0
The first part should be replaced with
```
let num = Number(prompt('введите число:', 0));

if (!isNan(num ))
```
and now there is assignment instead of comparison
Juliette Keller

0
if (num = Number (num)) {}

here comes the assignment where the result is num = 0

if (0) condition is false, we fall into else
Nathan Holder

0
Because zero is a lie in the condition.

There are 7 false values in JS: false , null , 0 , 0n , "" , NaN , undefined .

You just have to remember it.

Specifically your condition:

if (num = Number (num)) {

you can try replacing with:

if ((num = Number (num)) || num === 0) {

Then it will go to else only in case of an error.

Of course, this is far from the only way and not the most beautiful one. But the essence of the error, I hope, is clear.
Anonymous

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