Javascript: "truthy" values for non-empty strings

Question

It's said that non-empty string in Javascript is considered "truthy". It explains why the code:

if ("0") { 
    console.log("OK")
}

prints "OK".

However, why does the code:

true == "0"

returns false?

The "0" is coerced to a number in the second case. true == "1" for example returns true — m0meni
– m0meni, Commented Jul 27, 2017 at 17:44
Possible duplicate of why does if("string") evaluate "string" as true but if ("string"==true) does not? — Vineesh
– Vineesh, Commented Jul 27, 2017 at 17:45
Any string, other than an empty string, 0, false will be coerced to true. — Scott Marcus
– Scott Marcus, Commented Jul 27, 2017 at 17:47
dorey.github.io/JavaScript-Equality-Table developer.mozilla.org/en-US/docs/Web/JavaScript/… — JGFMK
– JGFMK, Commented Jul 27, 2017 at 17:57

Anurag Singh Bisht · Accepted Answer · 2017-07-27 18:06:10Z

3

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. If both operands are objects, then JavaScript compares internal references which are equal when operands refer to the same object in memory.

(From Comparison Operators in Mozilla Developer Network)

So, while comparing true == '0', it first converts both into numbers.

Number(true) == Number('0') which evaluates to 1 == 0.

Hence the answer is false.

edited Jul 27, 2017 at 18:06

answered Jul 27, 2017 at 17:59

Anurag Singh Bisht

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6 Comments

Anurag Singh Bisht Over a year ago

I don't think, that's correct. But, Number(true) evaluates to 1, so if you compare any other value, true == '2' it evaluates to 1 == 2 which is false.

CrazySynthax Over a year ago

what about null and undefined ?

Anurag Singh Bisht Over a year ago

null and undefined both are considered as false values in javascript. So if you are talking about true == null or true == undefined, you can look in either way true == false or 1 == 0 ( Number(null) = 0 ), both will result in false value.

CrazySynthax Over a year ago

What happens if we have an object and a string? How do we convert an object to a string?

Anurag Singh Bisht Over a year ago

In Javascript,` Objects ( {} ) ` are ` true ` values. And, any string other than ```(empty string)` is also true value. To check, you can use Boolean({}) and Boolean(string). To keep it simple, if any of the values in == is Number, the values are converted into Number(), and for others, it is converted into Boolean().

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