=== won't work even if the statement is true

Question

var age =prompt("What is your age?");

if (age === 21) {
 console.log("Happy 21st Birthday!");
}

When I write 21 in the prompt, it gives me an undefined, if I replace the === with == then it will work. Why? 21 is the same type and value as the 21 I write in the prompt

prompt will get a string and you are comparing number with a string. that's why your condition became false. == checks if the value of the variable is equal. === checks if the value and type are equal. — Harsh Patel, Oct 26 '17 at 09:03
Possible duplicate of [Difference between == and === in JavaScript](https://stackoverflow.com/questions/523643/difference-between-and-in-javascript) — Durga, Oct 26 '17 at 09:06

Zenoo · Accepted Answer · 2017-10-26T09:12:05.600

2

Your variable age gets a String from the prompt. For it to work you need to convert it to an int with the operator +:

If the use of the + operator feels strange to you in this case, you can always use the function parseInt() instead. It will achieve the same result.

var age = +prompt("What is your age?");
//        ^ Converts your String to an int
if (age === 21) {
 console.log("Happy 21st Birthday!");
}

edited Oct 26 '17 at 09:12

answered Oct 26 '17 at 09:04

Zenoo

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parseInt() might be better for a beginner rather than using short hand – Andrew Bone Oct 26 '17 at 09:06
2

I see many people not knowing about the shorthand, even advanced ones. I try to advertise it as much as possible :) – Zenoo Oct 26 '17 at 09:08
1

@AndrewBone I don't see a problem with it, its an answer at the end of the day and I managed to learn something new today. Thanks Zenoo. – Master Yoda Oct 26 '17 at 09:09
Good point. It might be worth editing your answer to have both, your call though – Andrew Bone Oct 26 '17 at 09:09
@AndrewBone I agree, was just about to suggest adding both methods and possibly a link to documentation for the shorthand. – Master Yoda Oct 26 '17 at 09:10
I edited my answer to fulfill both your requests. I can't seem to find any doc on the shorthand for some reason, though. – Zenoo Oct 26 '17 at 09:14

score 0 · Answer 2 · answered Oct 26 '17 at 09:05

The prompt() function returns a string. You check for an integer.

Basically == checks if the value of the variable is equal.

=== checks if the value and type is equal.

You can find more about it here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Equality_comparisons_and_sameness

score 0 · Answer 3 · answered Oct 26 '17 at 09:14

=== is check the value and type of variable and prompt return string value thats why === return false because prompt return string 21 and you compare with int 21 so return false

so below to way to get your output using == or string to int conversion

var age = prompt("What is your age?");

if (age == 21) {
  console.log("Happy 21st Birthday!");
}

//OR

if (parseInt(age) === 21) {
  console.log("Happy 21st Birthday!");
}

=== won't work even if the statement is true

3 Answers3