Why does using "0" with the ternary operator return the first value?

Question

I was playing around with JSconsole and found something strange. The value of "0" is false

"0" == false
=> true

The value of false when used in ternary returns the second value

false ? 71 : 16
=> 16

However the value "0" which equals false when used in ternary returns the first value.

"0" ? 8 : 10
=> 8

However, if you use 0 as the value, it returns the second value

0 ? 4 : 5
=> 5

0 == "0"
=> true

I'm afraid this doesn't make sense to me.

`==` in Javascript doesn't make sense. `'' == 0`, `0 == '0'`, but `'' != '0'`. Don't assume that because `x == y`, `x` and `y` will behave remotely the same. — user2357112, Jun 05 '15 at 22:23
This is one of the more confusing features of Javascript. There are at least four (!) scenarios where a value can `== false` yet not be "falsey". I've been putting together a reference guide under the question [All falsey values in JavaScript](http://stackoverflow.com/questions/19839952/all-falsey-values-in-javascript-values-where-value-is-true). I think I've got all of the falsey-related quirks and gotchas documented there... if I've missed any, please comment! — user56reinstatemonica8, Jun 05 '15 at 23:30
@CodesInChaos, you are mistaken: `!!"0"` evaluates to `true`. Agreed about `==`, though. — Andreas Rossberg, Jun 06 '15 at 11:43

xdazz · Answer 1 · 2015-06-05T16:16:03.547

Non-empty string is considered as truth value in conditional statements, conditional expressions and conditional constructs.

But when you compare a string with a number with ==, some conversion will take place.

When comparing a number and a string, the string is converted to a number value. JavaScript attempts to convert the string numeric literal to a Number type value. First, a mathematical value is derived from the string numeric literal. Next, this value is rounded to nearest Number type value.

And == don't have the Transitive Property of Equality:

you can't say if a == b, b == c, then a == c.

An example will be:

"0" == false // true
false == "\n" //true

and guess the result of "0" == "\n"? Yes, the result is false.

Some mention of `===` would make the answer stronger. – mattsilver Jun 06 '15 at 00:29 — mattsilver, Jun 06 '15 at 00:29

score 17 · Accepted Answer · edited Jun 20 '20 at 09:12

17

"0" is a string of length>0 which is true. Try

0 ? 8 : 10

and see. It will return 10.

== does type conversion and hence when you do

"0" == false

it returns true. When you do

0 == "0" //true

It also returns true as again type conversion is taking place. Even though one is a number and the other one is a string it returns true. But if you use ===, no type conversion is done and 0 === "0" will return false.

A nice explanation of == & === is given here.

From the docs:

The equality operator(==) converts the operands if they are not of the same type, then applies strict comparison.

The identity operator(===) returns true if the operands are strictly equal with no type conversion.

edited Jun 20 '20 at 09:12

Community

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answered Jun 05 '15 at 16:04

Zee

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A non-zero length string. – Adam Jenkins Jun 05 '15 at 16:04
1

When I typed in `"0" == false` I got true – Richard Hamilton Jun 05 '15 at 16:05
4

@user4703663 `==` will internally do `type conversion`, so use `===`. – Zee Jun 05 '15 at 16:06
1

@user4703663 A very nice explanation of both is given [here](http://stackoverflow.com/a/359509/3894168). – Zee Jun 05 '15 at 16:07

score 6 · Answer 3 · answered Jun 05 '15 at 16:04

6

I'm afraid this is an example of why you should use === - plain old == performs type conversion. Try

"0"===false

answered Jun 05 '15 at 16:04

Evan Knowles

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But wait, `"0"===true` yields false either. This may not answer the question straightaway, I guess. – TaoPR Jun 05 '15 at 17:34
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@TaoP.R. If you want to convert a value to a boolean in a way that respects how it acts in a ternary operator, use `!!value`. As in `!!"0" === true`. – Random832 Jun 05 '15 at 21:27
@Random832 Would `Boolean()` be completely equivalent? (i.e. `Boolean(x) === !!x` for all values of `x`) - that's probably the more explicit/readable/obvious way. – Bob Jun 06 '15 at 07:37

score 1 · Answer 4 · answered Jun 05 '15 at 16:55

JavaScript leads to tons of WTFs.

Check out "Javascript WTF" on YouTube...

Essentially, you are requesting a conversion from string to boolean.

This is defined as "string is not empty".

Whereas you assumed that javascript does string -> int -> boolean if the string happens to contain a number.

It's sensible. But these automatic conversions lead to programming errors, which is why I prefer typesafe languages (with compile time type checking) for larger projects.

For fun, try these:

("0" * 1) ? 71 : 16
("0" + false) ? 71 : 16

Why does using "0" with the ternary operator return the first value?

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