Objašnjena je prisila tipa JavaScript

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[Uredi 5.2.2018.] : Ovaj je post sada dostupan na ruskom jeziku. Pljeska Serju Bulavyku za njegov trud.

Prisila tipa je postupak pretvaranja vrijednosti iz jedne vrste u drugu (poput niza u broj, objekta u logičku vrijednost i tako dalje). Bilo koji tip, bio on primitivan ili objekt, valjani je subjekt za prisilu tipa. Da se prisjetimo, primitivi su: broj, niz, logička vrijednost, nula, nedefinirano + simbol (dodano u ES6).

Kao primjer tipa prisile u praksi, pogledajte JavaScript Tablica za usporedbu, što pokazuje kako su labave jednakosti ==operator ponaša za drugačiji ai bvrsta. Ova matrica izgleda zastrašujuće zbog implicitne prisile tipa koju ==operater čini i teško je pamtiti sve te kombinacije. I to ne morate činiti - samo naučite temeljne principe prisile.

Ovaj članak detaljno govori o tome kako prisila tipa djeluje u JavaScriptu i pružit će vam osnovno znanje, tako da možete biti sigurni u objašnjavanje na što računaju sljedeći izrazi. Na kraju članka pokazat ću odgovore i objasniti ih.

true + false 12 / "6" "number" + 15 + 3 15 + 3 + "number" [1] > null "foo" + + "bar" 'true' == true false == 'false' null == '' !!"false" == !!"true" [‘x’] == ‘x’ [] + null + 1 [1,2,3] == [1,2,3] {}+[]+{}+[1] !+[]+[]+![] new Date(0) - 0 new Date(0) + 0

Da, ovaj je popis prepun prilično glupih stvari koje možete učiniti kao programer. U 90% slučajeva uporabe bolje je izbjegavati implicitnu prisilu tipa. Smatrajte ovaj popis vježbom učenja kako biste testirali svoje znanje o tome kako djeluje prisila tipa. Ako vam je dosadno, još primjera možete pronaći na wtfjs.com.

Usput, ponekad se s takvim pitanjima možete suočiti na razgovoru za mjesto programera za JavaScript. Pa, nastavi čitati?

Implicitna vs eksplicitna prisila

Prisilna vrsta može biti eksplicitna i implicitna.

Kad programer izrazi namjeru pretvoriti između tipova pisanjem odgovarajućeg koda, na primjer Number(value), to se naziva eksplicitna prisila tipa (ili lijevanje tipa).

Budući da je JavaScript slabo otkucan jezik, vrijednosti se također mogu automatski pretvoriti između različitih tipova, a to se naziva implicitnom prisilom tipa . To se obično događa kada operatore primijenite na vrijednosti različitih vrsta, poput

1 == null, 2/’5', null + new Date(), Ili to može biti potaknuta okolnog konteksta, kao i sa if (value) {…}, gdje valueje prisiljen boolean.

Jedan od operatora koji ne pokreće implicitnu vrstu prisile je ===, koji se naziva operator stroge jednakosti. Operator labave jednakosti ==s druge strane vrši usporedbu i prisilu tipa ako je potrebno.

Implicitna prisila mač je dvostrukog ruba: to je izvrstan izvor frustracija i nedostataka, ali i koristan mehanizam koji nam omogućuje da napišemo manje koda bez gubitka čitljivosti.

Tri vrste pretvorbe

Prvo pravilo koje treba znati jest da postoje samo tri vrste pretvorbe u JavaScript-u:

  • nizati
  • na boolean
  • na broj

Drugo, logika pretvorbe za primitive i objekte djeluje drugačije, ali i primitivi i objekti mogu se pretvoriti samo na ta tri načina.

Krenimo prvo s primitivcima.

Pretvorba niza

Da biste eksplicitno pretvorili vrijednosti u niz, primijenite String()funkciju. Implicitnu prisilu pokreće binarni +operator, kada je bilo koji operand niz:

String(123) // explicit 123 + '' // implicit

Sve se primitivne vrijednosti prirodno pretvaraju u nizove kao što ste mogli očekivati:

String(123) // '123' String(-12.3) // '-12.3' String(null) // 'null' String(undefined) // 'undefined' String(true) // 'true' String(false) // 'false'

Pretvorba simbola pomalo je nezgodna, jer se može pretvoriti samo eksplicitno, ali ne i implicitno. Pročitajte više o Symbolpravilima prisile.

String(Symbol('my symbol')) // 'Symbol(my symbol)' '' + Symbol('my symbol') // TypeError is thrown

Logička pretvorba

Da biste eksplicitno pretvorili vrijednost u logičku vrijednost, primijenite Boolean()funkciju.

Implicitna pretvorba događa se u logičkom kontekstu ili je pokreću logički operatori ( ||&&!).

Boolean(2) // explicit if (2) { ... } // implicit due to logical context !!2 // implicit due to logical operator 2 || 'hello' // implicit due to logical operator

Napomena : Logički operatori kao što su ||i &&interno vrše logičke pretvorbe, ali zapravo vraćaju vrijednost izvornih operanda, čak i ako nisu logički.

// returns number 123, instead of returning true // 'hello' and 123 are still coerced to boolean internally to calculate the expression let x = 'hello' && 123; // x === 123

Čim postoje samo 2 moguća rezultata logičke pretvorbe: trueili falseje jednostavno lakše zapamtiti popis lažnih vrijednosti.

Boolean('') // false Boolean(0) // false Boolean(-0) // false Boolean(NaN) // false Boolean(null) // false Boolean(undefined) // false Boolean(false) // false

Svaka vrijednost koja nije na popisu se pretvara u true, uključujući i objekt, funkcija, Array, Date, korisnički definirane vrste, i tako dalje. Simboli su istinite vrijednosti. Prazni objekt i nizovi također su istinite vrijednosti:

Boolean({}) // true Boolean([]) // true Boolean(Symbol()) // true !!Symbol() // true Boolean(function() {}) // true

Numerička pretvorba

Za eksplicitnu pretvorbu samo primijenite Number()funkciju, istu kao što ste to učinili s Boolean()i String().

Implicitna pretvorba je lukav, jer se pokreće u više slučajeva:

  • usporedba operatora ( >, <, <=, >=)
  • bitni operatori ( |&^~)
  • aritmetički operatori ( -+*/%). Imajte na umu da binarni sustav +ne pokreće numeričku pretvorbu kada je bilo koji operand niz.
  • unarni +operator
  • operator labave jednakosti ==(uklj. !=).

    Imajte na umu da ==ne pokreće numeričku pretvorbu kada su oba operanda nizovi.

Number('123') // explicit +'123' // implicit 123 != '456' // implicit 4 > '5' // implicit 5/null // implicit true | 0 // implicit

Evo kako se primitivne vrijednosti pretvaraju u brojeve:

Number(null) // 0 Number(undefined) // NaN Number(true) // 1 Number(false) // 0 Number(" 12 ") // 12 Number("-12.34") // -12.34 Number("\n") // 0 Number(" 12s ") // NaN Number(123) // 123

Kada pretvaramo string u broj, motor prvi ukrasi vodeći i prateći razmak, \n, \tlikove, vraća NaNako je obrubljen niz ne predstavlja valjani broj. Ako je niz prazan, vraća se 0.

null and undefined are handled differently: null becomes 0, whereas undefined becomes NaN.

Symbols cannot be converted to a number neither explicitly nor implicitly. Moreover, TypeError is thrown, instead of silently converting to NaN, like it happens for undefined. See more on Symbol conversion rules on MDN.

Number(Symbol('my symbol')) // TypeError is thrown +Symbol('123') // TypeError is thrown

There are two special rules to remember:

  1. When applying == to null or undefined, numeric conversion does not happen. null equals only to null or undefined, and does not equal to anything else.
null == 0 // false, null is not converted to 0 null == null // true undefined == undefined // true null == undefined // true

2. NaN does not equal to anything even itself:

if (value !== value) { console.log("we're dealing with NaN here") }

Type coercion for objects

So far, we’ve looked at type coercion for primitive values. That’s not very exciting.

When it comes to objects and engine encounters expression like [1] + [2,3], first it needs to convert an object to a primitive value, which is then converted to the final type. And still there are only three types of conversion: numeric, string and boolean.

The simplest case is boolean conversion: any non-primitive value is always

coerced to true, no matter if an object or an array is empty or not.

Objects are converted to primitives via the internal [[ToPrimitive]] method, which is responsible for both numeric and string conversion.

Here is a pseudo implementation of [[ToPrimitive]] method:

[[ToPrimitive]] is passed with an input value and preferred type of conversion: Number or String. preferredType is optional.

Both numeric and string conversion make use of two methods of the input object: valueOf and toString . Both methods are declared on Object.prototype and thus available for any derived types, such as Date, Array, etc.

In general the algorithm is as follows:

  1. If input is already a primitive, do nothing and return it.

2. Call input.toString(), if the result is primitive, return it.

3. Call input.valueOf(), if the result is primitive, return it.

4. If neither input.toString() nor input.valueOf() yields primitive, throw TypeError.

Numeric conversion first calls valueOf (3) with a fallback to toString (2). String conversion does the opposite: toString (2) followed by valueOf (3).

Most built-in types do not have valueOf, or have valueOf returning this object itself, so it’s ignored because it’s not a primitive. That’s why numeric and string conversion might work the same — both end up calling toString().

Different operators can trigger either numeric or string conversion with a help of preferredType parameter. But there are two exceptions: loose equality == and binary + operators trigger default conversion modes (preferredType is not specified, or equals to default). In this case, most built-in types assume numeric conversion as a default, except Date that does string conversion.

Here is an example of Date conversion behavior:

You can override the default toString() and valueOf() methods to hook into object-to-primitive conversion logic.

Notice how obj + ‘’ returns ‘101’ as a string. + operator triggers a default conversion mode, and as said before Object assumes numeric conversion as a default, thus using the valueOf() method first instead of toString().

ES6 Symbol.toPrimitive method

In ES5 you can hook into object-to-primitive conversion logic by overriding toString and valueOf methods.

In ES6 you can go farther and completely replace internal[[ToPrimitive]] routine by implementing the[Symbol.toPrimtive] method on an object.

Examples

Armed with the theory, now let’s get back to our examples:

true + false // 1 12 / "6" // 2 "number" + 15 + 3 // 'number153' 15 + 3 + "number" // '18number' [1] > null // true "foo" + + "bar" // 'fooNaN' 'true' == true // false false == 'false' // false null == '' // false !!"false" == !!"true" // true ['x'] == 'x' // true [] + null + 1 // 'null1' [1,2,3] == [1,2,3] // false {}+[]+{}+[1] // '0[object Object]1' !+[]+[]+![] // 'truefalse' new Date(0) - 0 // 0 new Date(0) + 0 // 'Thu Jan 01 1970 02:00:00(EET)0'

Below you can find explanation for each the expression.

Binary + operator triggers numeric conversion for true and false

true + false ==> 1 + 0 ==> 1

Arithmetic division operator / triggers numeric conversion for string '6' :

12 / '6' ==> 12 / 6 ==>> 2

Operator + has left-to-right associativity, so expression "number" + 15 runs first. Since one operand is a string, + operator triggers string conversion for the number 15. On the second step expression "number15" + 3 is evaluated similarly.

“number” + 15 + 3 ==> "number15" + 3 ==> "number153"

Expression 15 + 3 is evaluated first. No need for coercion at all, since both operands are numbers. On the second step, expression 18 + 'number' is evaluated, and since one operand is a string, it triggers a string conversion.

15 + 3 + "number" ==> 18 + "number" ==> "18number"

Comparison operator &gt; triggers numeric conversion for [1] and null .

[1] > null ==> '1' > 0 ==> 1 > 0 ==> true

Unary + operator has higher precedence over binary + operator. So +'bar' expression evaluates first. Unary plus triggers numeric conversion for string 'bar'. Since the string does not represent a valid number, the result is NaN. On the second step, expression 'foo' + NaN is evaluated.

"foo" + + "bar" ==> "foo" + (+"bar") ==> "foo" + NaN ==> "fooNaN"

== operator triggers numeric conversion, string 'true' is converted to NaN, boolean true is converted to 1.

'true' == true ==> NaN == 1 ==> false false == 'false' ==> 0 == NaN ==> false

== usually triggers numeric conversion, but it’s not the case with null . null equals to null or undefined only, and does not equal to anything else.

null == '' ==> false

!! operator converts both 'true' and 'false' strings to boolean true, since they are non-empty strings. Then, == just checks equality of two boolean true's without any coercion.

!!"false" == !!"true" ==> true == true ==> true

== operator triggers a numeric conversion for an array. Array’s valueOf() method returns the array itself, and is ignored because it’s not a primitive. Array’s toString() converts ['x'] to just 'x' string.

['x'] == 'x' ==> 'x' == 'x' ==> true

+ operator triggers numeric conversion for []. Array’s valueOf() method is ignored, because it returns array itself, which is non-primitive. Array’s toString returns an empty string.

On the the second step expression '' + null + 1 is evaluated.

[] + null + 1 ==> '' + null + 1 ==> 'null' + 1 ==> 'null1'

Logical || and && operators coerce operands to boolean, but return original operands (not booleans). 0 is falsy, whereas '0' is truthy, because it’s a non-empty string. {} empty object is truthy as well.

0 || "0" && {} ==> (0 || "0") && {} ==> (false || true) && true // internally ==> "0" && {} ==> true && true // internally ==> {}

No coercion is needed because both operands have same type. Since == checks for object identity (and not for object equality) and the two arrays are two different instances, the result is false.

[1,2,3] == [1,2,3] ==> false

All operands are non-primitive values, so + starts with the leftmost triggering numeric conversion. Both Object’s and Array’svalueOf method returns the object itself, so it’s ignored. toString() is used as a fallback. The trick here is that first {} is not considered as an object literal, but rather as a block declaration statement, so it’s ignored. Evaluation starts with next +[] expression, which is converted to an empty string via toString() method and then to 0 .

{}+[]+{}+[1] ==> +[]+{}+[1] ==> 0 + {} + [1] ==> 0 + '[object Object]' + [1] ==> '0[object Object]' + [1] ==> '0[object Object]' + '1' ==> '0[object Object]1'

This one is better explained step by step according to operator precedence.

!+[]+[]+![] ==> (!+[]) + [] + (![]) ==> !0 + [] + false ==> true + [] + false ==> true + '' + false ==> 'truefalse'

- operator triggers numeric conversion for Date. Date.valueOf() returns number of milliseconds since Unix epoch.

new Date(0) - 0 ==> 0 - 0 ==> 0

+ operator triggers default conversion. Date assumes string conversion as a default one, so toString() method is used, rather than valueOf().

new Date(0) + 0 ==> 'Thu Jan 01 1970 02:00:00 GMT+0200 (EET)' + 0 ==> 'Thu Jan 01 1970 02:00:00 GMT+0200 (EET)0'

Resources

I really want to recommend the excellent book “Understanding ES6” written by Nicholas C. Zakas. It’s a great ES6 learning resource, not too high-level, and does not dig into internals too much.

And here is a good book on ES5 only - SpeakingJS written by Axel Rauschmayer.

(Russian) Современный учебник Javascript — //learn.javascript.ru/. Especially these two pages on type coercion.

JavaScript Comparison Table — //dorey.github.io/JavaScript-Equality-Table/

wtfjs — a little code blog about that language we love despite giving us so much to hate — //wtfjs.com/