conditional expression in while-loop does not work as expected

Question

I give conditional test to my while-loop but it doesn't seem working. I think it's because of increment operator. but I couldn't find out why

const nums = [3, 5, 15, 7, 5];
let n, i = 0;
while ((n = nums[i]) < 10, i++ < nums.length) {
  console.log(`Number less than 10: ${n}.`);
  };

expected [3, 5, 7, 5]
actual result [3, 5, 15, 7, 5]
I don't know why 15 came out.

I want to know why while-loop works like this.

Update:

This problem is from the book 'learning javascript 3rd' and , comma operator doesn't work like I thought it should.

the comma operator doesn't do what you seem to think. Did you mean the logical "and" operator (&&) instead? — Robin Zigmond
– Robin Zigmond, Commented Feb 15, 2019 at 10:13

mplungjan · Accepted Answer · 2019-02-15 10:57:59Z

2

The while is shortcut when you get to 15 if you correct the , to an &&

The comma operator returns the result of the i++ < nums.length

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Comma_Operator

You really want to look into ES5/ES6 and filter

let smallNums = [3, 5, 15, 7, 5].filter((n) => n<10)
console.log(smallNums)

Same without the ES6 arrow:

let smallNums = [3, 5, 15, 7, 5].filter(function(n) { return n<10; })
console.log(smallNums)

edited Feb 15, 2019 at 10:57

answered Feb 15, 2019 at 10:28

mplungjan

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3 Comments

fian Elf Over a year ago

!!The comma operator returns the result of the i++ < nums.length!!

Robin Zigmond Over a year ago

just to clarify, .filter is in ES5. The arrow function is ES6 (but can easily be replaced by an ordinary anonymous function declaration if you have to support very old browsers like IE).

mplungjan Over a year ago

@RobinZigmond Updated

holydragon · Accepted Answer · 2019-02-15 10:19:55Z

2

Here. You should make a condition inside the while loop since if the condition is false then the whole loop will be terminated.

const nums = [3, 5, 15, 7, 5];
let n, i = 0;
while (i < nums.length) {
  if ((n = nums[i++]) < 10) {
    console.log(`Number less than 10: ${n}.`);
  }
};

answered Feb 15, 2019 at 10:19

holydragon

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2 Comments

fian Elf Over a year ago

thanks very much! could you tell me how my code works??? i dont know why conditional expression doesn't catch the number more than 10

R3tep Over a year ago

Note : you need to add a break into if statement to get the same of behaviour of while loop condition

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