Create a gist now

Instantly share code, notes, and snippets.

What would you like to do?
True, False and Equal in JS
/* false, 0, undefined, null, NaN, "" are false */
> Boolean(false)
false
> Boolean(0)
false
> Boolean("")
false
> Boolean(undefined)
false
> Boolean(null)
false
> Boolean(NaN)
false
/* Anything else is true:
*/
> Boolean(true)
true
> Boolean(1)
true
> Boolean(2)
true
> Boolean("Hello world!")
true
/* Including empty array, Python developers!!! */
> Boolean([])
true
/* Let's try equal: */
> false == false
true
> 0 == false
true
> "" == false
true
/* So far, so good, but... : */
> undefined == false
false
> null == false
false
> NaN == false
false
/* The same for truthy works well: */
> true == true
true
> 1 == true
true
/* But: */
> 2 == true
false
> "Hello world!" == true
false
> [] == true
false
/* Let's avoid type coerition with triple-equals: */
> false === false
true
> 0 === false
false
> "" === false
false
> undefined === false
false
> null === false
false
> NaN === false
false
/* Good. And for truthy: */
> true === true
true
> 1 === true
false
> 2 === true
false
> [] === true
false
> "Hello world!" === true
false
/* Tripple equals returns always false when types of operands does not match." */
/* Equal is 'generous' about the type: */
> true == '1'
true
> true == '1.0'
true
/* Triple equals is not: */
> true === '1'
false
> true === '1.0'
false
/* It only goes with numbers: */
> 2 == '2'
true
/* But: */
> true == 'true'
false
> Boolean(' ')
true
> Boolean(' \n\n')
true
/* But: */
> ' ' == 0
true
> ' \n' == 0
true
> ' \n' == false
true
/* o_O ?? WAT */
/* Thank god for === */
> ' ' === 0
false
> ' \n' === 0
false
> ' \n' === false
false
/* These are quite obvious: */
> undefined == undefined
true
> undefined === undefined
true
> null == null
true
> null === null
true
/* These are not: */
> null == undefined
true
> null === undefined
false
/* But to check if global variable is not defined you can't do the following: */
> foo === undefined
ReferenceError: foo is not defined
/* There is an exception because foo is not defined. That's why: */
> typeof foo === 'undefined'
true
/* Typeof returns string containing name of a type. And undefined has type undefined, whereas: */
> typeof null
"object"
> typeof undefined
"undefined"
/* And these are useful for making default result: */
> null || "abc"
"abc"
> undefined || "abc"
"abc"
> NaN || "abc"
"abc"
/* Watch out for NaN! */
> NaN + NaN
NaN
// NaN are not equal:
> NaN == NaN
false
> NaN === NaN
false
// Think about it! A dog and a cat are Not a Number, but they are not equall as well. It make sense.
// Use Number.isNaN() instead:
> Number.isNaN(NaN)
true
> Number.isNaN(0)
false
> NaN && NaN
NaN
> NaN || NaN
NaN
// But:
> NaN || true
true
> true || NaN
true
> false && NaN
false
> NaN && false
NaN
// It all make sense, because condition is not evaluated after it's result is certain.
// But now:
> NaN | NaN
0
> NaN & NaN
0
// o_O ?? //
[[["\t\n 987654321e-400"]]] == 0

this is because during type coercion of an object to a number, the object is toStringed. Arrays don't have any wrapping in their string representation, so any level of nesting of arrays containing a single item will turn into the same string. During string coercion to number, leading whitespace is ignored.

Nice summary! At some points JavaScript is just stupid but === makes things a lot better.

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment