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True, False and Equal in JS
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| /* false, 0, undefined, null, NaN, "" are false */ | |
| > Boolean(false) | |
| false | |
| > Boolean(0) | |
| false | |
| > Boolean("") | |
| false | |
| > Boolean(undefined) | |
| false | |
| > Boolean(null) | |
| false | |
| > Boolean(NaN) | |
| false |
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| /* Anything else is true: | |
| */ | |
| > Boolean(true) | |
| true | |
| > Boolean(1) | |
| true | |
| > Boolean(2) | |
| true | |
| > Boolean("Hello world!") | |
| true | |
| /* Including empty array, Python developers!!! */ | |
| > Boolean([]) | |
| true |
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| /* 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 | |
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| /* The same for truthy works well: */ | |
| > true == true | |
| true | |
| > 1 == true | |
| true | |
| /* But: */ | |
| > 2 == true | |
| false | |
| > "Hello world!" == true | |
| false | |
| > [] == true | |
| false |
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| /* 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." */ |
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| /* 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 | |
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| > 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 |
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| /* 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" |
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| /* And these are useful for making default result: */ | |
| > null || "abc" | |
| "abc" | |
| > undefined || "abc" | |
| "abc" | |
| > NaN || "abc" | |
| "abc" |
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| /* 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 ?? // |
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Nice summary! At some points JavaScript is just stupid but
===makes things a lot better.