What's wrong with javascript ternary operator in my app?

Question

I'm writing a simple discount app with prompt input. The problem is in my ternary operator - it doesn't count the newPrice as well as show that input is invalid. Can you tell me what I'm doing wrong?

I if the input is wrong I want to assign newPrice to null and alert 'Invalid data'

My code:

const price = prompt('Please, enter the price:');
const discount = prompt('Please, enter the discount amount:');

const newPrice = ((9999999 > price > 0) && (99 > discount > 0)) ? (price - price * discount / 100) : (null, alert('Invalid data'));
console.log(newPrice)

probably an operator precedence thing. when in doubt, put more parenthesis around things — user6713871
– user6713871, Commented Feb 5, 2019 at 6:58
please check your conditions and if you are having lot of decision making then why can't you go with if-else conditions — Adesh Kumar
– Adesh Kumar, Commented Feb 5, 2019 at 7:19

Tope · Accepted Answer · 2019-02-05 07:06:37Z

1

Try separating out the code statements to

make sure new price is actually null. Not undefined. (per your question)
the a > b > c stuff does not work afaik in JS. Please refactor that syntax as i did below or similar.

const price = prompt('Please, enter the price:');
const discount = prompt('Please, enter the discount amount:');

const newPrice = (price && (9999999 > price) && discount && (99 > discount)) ? (price - price * discount / 100) : null;
console.log(newPrice);
if(!newPrice){alert('Invalid data');}

answered Feb 5, 2019 at 7:06

Tope

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Comments

Jack Bashford · Accepted Answer · 2019-02-05 07:01:15Z

0

First, convert your inputs to numbers, and second, refactor your conditions to be valid (you can't have x > y > z IIRC):

const price = parseInt(prompt('Please, enter the price:'));
const discount = parseInt(prompt('Please, enter the discount amount:'));

const newPrice = (((9999999 > price) && price) && ((99 > discount) && discount)) ? (price - price * discount / 100) : (null, alert('Invalid data'));
console.log(newPrice);

answered Feb 5, 2019 at 7:01

Jack Bashford

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Comments

Dhananjai Pai · Accepted Answer · 2019-02-05 07:13:55Z

0

The prompt may be reading the value as a string. Convert it to number using parseInt or unary +

const price = parseInt(prompt('Please, enter the price:'));
const discount = parseInt(prompt('Please, enter the discount amount:'));

const newPrice = ((price && price > 0 && price < 9999999) && (discount && discount >0 && discount < 99 )) ? (price - price * discount / 100) : (null, alert('Invalid data'));
console.log(newPrice)

edited Feb 5, 2019 at 7:13

answered Feb 5, 2019 at 6:58

Dhananjai Pai

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10 Comments

user6713871 Over a year ago

string and number comparison works fine when the string only contains numbers

Teemu Over a year ago

@ChrisRollins It doesn't, try console.log('10' > '9');.

Tope Over a year ago

moot. good question though. the input prompt is actually taking the number as a number (you can run the code snippet here in SO) and doing the substraction math correctly.

Leocete Over a year ago

so it's not necessary to add + or parseInt before prompt?

Teemu Over a year ago

@ChrisRollins Yeah, but in general your code should not rely on automatic type coercion.

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Adesh Kumar · Accepted Answer · 2019-02-05 07:36:29Z

0

Here is the example of doing by both method if-else and ternary operator (?:). But I would recommend going with if-else if you have much decision makings.

var price = prompt('Please, enter the price:');
var discount = prompt('Please, enter the discount amount:');
var newPrice;
if((price>0 && price < 9999999)&&(discount>0 && discount < 99)){
newPrice=price - (price * discount) / 100;
}else{
newPrice='null';
alert('Invalid data');
}
var newPrice1 = ((9999999 > price && price > 0) && (99 > discount && discount > 0)) ? (price - (price * discount) / 100) : ('null', alert('Invalid data'));
console.log(newPrice);
console.log(newPrice1);

edited Feb 5, 2019 at 7:36

answered Feb 5, 2019 at 7:28

Adesh Kumar

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