Why javascript treats 0 equal to empty string?

Question

Possible Duplicate:
Implied string comparison, 0='', but 1='1'

Executing the following case in javascript, I get 0 equal to '' (an empty string)

var a   =   0;
var b   =   '';//empty string
if(a==b){
    console.log('equal');//this is printed in console
}else{
    console.log('not equal');
}

Could anyone guide that what is the reason behind this?

(typeof a == typeof b && a == b) to check if they are identical (a === b) to check if they are the same if a and b points to the same object in memory — DevWL, Mar 19 '16 at 23:43

raina77ow · Accepted Answer · 2012-09-14T10:01:07.623

51

Quoting the doc (MDN):

Equal (==)

If the two operands are not of the same type, JavaScript converts the operands then applies strict comparison. If either operand is a number or a boolean, the operands are converted to numbers if possible; else if either operand is a string, the other operand is converted to a string if possible.

As a operand type here is Number, b gets converted to Number as well. And Number('') evaluates to 0.

This can be quite surprising sometimes. Consider this, for example:

console.log(0 == '0');  // true
console.log(0 == '');   // true
console.log('' == '0'); // O'RLY?

... or this:

console.log(false == undefined); // false
console.log(false == null);      // false
console.log(null == undefined);  // fal.... NO WAIT!

...and that's exactly why it's almost always recommended to use === (strict equality) operator instead.

edited Sep 14 '12 at 10:01

answered Sep 14 '12 at 09:50

raina77ow

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Thanks for the details shared. type comparison definitely should be the preferred choice. But in our case, we had a challenge that this comparison was taking place in a function receiving values as arguments and user could pass 1 as '1' too which is permissible in javscript. In such a case, if we use type comparison then '1' would not be equal to 1. Due to this, we had to avoid type comparison. Don't know what can be the safest method for this? – netemp Sep 14 '12 at 10:16
1

@netemp I'd suggest using `parseInt(arg, 10)` construct. When `arg` is a string, it will be _parsed_ into number (and it's more consistent then `Number(arg)`), and when `arg` is a number, it'll get back to you unharmed. ) – raina77ow Sep 14 '12 at 10:20
Thankyou. Wasn't aware the type conversion was achieved using Number (which unlike parseInt does not return NaN in the case of '' or ' '). Answer also helped explain this for me, why: '' == 0 && ' ' == 0 // true. Yet: !!' ' == !!0 // false. With the latter the type conversion is done to each argument first (rather than the arguments either side of ==) therefore ' ' evaluates to truthy. – Moika Turns Mar 03 '17 at 14:02

Praveen Kumar Purushothaman · Answer 2 · 2012-09-14T09:58:35.257

5

0, "" (Empty String), false all technically have the same value, when typecasted. If you need to strictly treat them, you can use ===.

It is the same with similar programming languages, like PHP.

var a = 0;
var b = ''; //empty string
if(a == b){
    console.log('equal'); //this is printed in console
}else{
    console.log('not equal');
}

To make a strict comparison:

if(a === b){
    console.log('equal');
}else{
    console.log('not equal'); //this is printed in console
}

edited Sep 14 '12 at 09:58

answered Sep 14 '12 at 09:51

Praveen Kumar Purushothaman

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1

"It is the same with any programming language." Not true. BTW, I am not downvoter. – Leri Sep 14 '12 at 09:56
1

`0 null "" are all same` is a funny statement. And so is `It is the same with any programming language`. That said, I'd say your answer is correct, but inaccurate. – Michael Krelin - hacker Sep 14 '12 at 09:56
1

Well, I have updated the answer. – Praveen Kumar Purushothaman Sep 14 '12 at 09:58
Also the implication that 0==null & ''==null is not correct – Alex K. Sep 14 '12 at 09:59
@AlexK. Corrected. :) Should have been `false` not `null`. Thanks. – Praveen Kumar Purushothaman Sep 14 '12 at 10:01
Can the downvoter remove the downvote? – Praveen Kumar Purushothaman Sep 14 '12 at 10:02
I'm not the one, but your answer is still a bit incorrect. See, you get the same value - 0 - when casting `0`, `""` and `false` to `Number`. But it's the same with string `'0'` (and `' 0 '`, and even `' \r\n '`, for example), first. And casting `0` and `false` to `String` will give you totally different results. So no, they do not have the same _value_. – raina77ow Sep 14 '12 at 10:14
1

I (the downvoter if I didn't make it clear last time) removed the downvote. Though, you should be more specific about what you typecast it to and I think saying that something is similar to PHP may be offending :) – Michael Krelin - hacker Sep 14 '12 at 10:19
@MichaelKrelin-hacker Sure thanks. Will take care. And sorry if I had been rude. – Praveen Kumar Purushothaman Sep 14 '12 at 10:22

score 2 · Answer 3 · answered Sep 14 '12 at 09:50

== operator in javascript don't compare types so 0=='' === true (because as a number string '' evaluates to 0) or 0==false === true (because bool false evaluates to 0) to compare types to you can use === operator.

Here you'll find useful information about comparison.

score 1 · Answer 4 · answered Sep 14 '12 at 09:50

1

== does typecasting. Always use ===.

In your example, the empty string of b is being converted to 0. So both a and b are the same.

answered Sep 14 '12 at 09:50

Geuis

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score 1 · Answer 5 · answered Sep 14 '12 at 09:51

1

Because empty string represented as number is zero. If you compare apples and oranges you should think of how your particular orange would look if it were an apple.

answered Sep 14 '12 at 09:51

Michael Krelin - hacker

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score 1 · Answer 6 · answered Sep 14 '12 at 09:51

1

Because == checks value equality so false, 0 and empty strings are equals. Use identity check ===.

answered Sep 14 '12 at 09:51

kbec

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score 0 · Answer 7 · answered Sep 14 '12 at 09:50

0

Javascript automatically converts variables of different types for comparison. That's a common feature in non-strictly typed languages.

If you need to compare variables and check the type, use the === operator.

answered Sep 14 '12 at 09:50

chiborg

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score 0 · Answer 8 · answered Sep 14 '12 at 09:50

In most programming language(including javascript) ""(string), 0(integer), \x0(null) losely mean the same thing: "empty". What's happening is your javascript engine finds "" == 0 false, due to the == it converts 0 to an integer. Again this is false, so it converts 0 to null which is false, so then it converts 0 to an empty string. (Not sure if this is the correct order of conversion). To make the condition "exact" match(no conversion) use === inplace of ==

score 0 · Answer 9 · answered Sep 14 '12 at 09:51

0

Because of coercion. It is usually a better idea to use === for comparisons in JavaScript.

answered Sep 14 '12 at 09:51

skunkfrukt

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Why javascript treats 0 equal to empty string?

9 Answers9

To make a strict comparison:

Linked

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