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JavaScript JavaScript Functions Arrow Functions Function Challenge

Posted by Daniel Hardeman
Daniel Hardeman
1,089 Points

Why does my alert dialog not trigger when the user enters a non-number value?

Here is my code:

/**

  • Returns a random number between 2 user-defined numbers
  • @param {number} lower - the lowest number value.
  • @param {number} upper - the highest number value.
  • @return {number} The random number value. */

function getRandomNumber( lower, upper ) { const lowerInput = prompt(Enter a minimum number:); const upperInput = prompt(Enter a maximum number:); lower = parseFloat(lowerInput); upper = parseFloat(upperInput); const main = document.querySelector('main'); const randomNumberGen = Math.floor(Math.random() * ( upper - lower + 1)) + lower;

if ( lower !== isNaN(lower) || upper !== isNaN(upper))
{
    return main.innerHTML = `<h1>Random Number Generator:</h1>
                             <h3>Choose a minimum and maximum number and the Random Number Generator will randomly select a number between the two.</h3>
                             <p>${randomNumberGen} is a random number between ${lower} and ${upper}.</p>`;
}
else
{
    alert('You must enter two numbers. Please reload and try again.')
}

}

getRandomNumber();

3 Answers

boi
boi
14,241 Points
boi
boi
14,241 Points

Your conditional statement (if ()) is wrong.

Your conditional statement is not doing what you might expect, instead what it's actually doing is this:

If a string is passed.

if ( NaN !== true || NaN !== true)

If a number is passed say 5 and 10.

if ( 5 !== false || 10 !== false )

In both cases, the if condition is coming out TRUE. So in reality the else never gets executed.

Michael Kobela
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Michael Kobela
Full Stack JavaScript Techdegree Graduate 19,570 Points
Michael Kobela
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Michael Kobela
Full Stack JavaScript Techdegree Graduate 19,570 Points 
let lower = Nan;
if (lower !== isNaN(lower))

If lower is a Nan, the if equates to if( Nan !== true ) so the if executes because it's true, Nan is not equal to true.

Anna Silvester
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Anna Silvester
Full Stack JavaScript Techdegree Student 3,870 Points
Anna Silvester
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Anna Silvester
Full Stack JavaScript Techdegree Student 3,870 Points

Would this work?
If either upper or lower are not numbers then the condition is true. Otherwise they are both numbers and the else statement will execute.

The value of isNaN(lower) is either true or false so can be written this way.

if ( isNaN(lower) || isNaN(upper))
{
       alert('You must enter two numbers. Please reload and try again.')
}
else
{
       return main.innerHTML = `<h1>Random Number Generator:</h1>
                             <h3>Choose a minimum and maximum number and the Random Number Generator will randomly select a number between the two.</h3>
                             <p>${randomNumberGen} is a random number between ${lower} and ${upper}.</p>`;
}