type FooType = { x : number ; y : string }; var foo : FooType = { x : 5, y : 'hello' }; foo. z = 3 ; // Property not found in object type

Types can also be inferred from existing objects using the typeof operator. This is a really, really cool feature. For example, Flow would catch an error here, as the properties x and y have incorrect values in the second constructor:

type FooType = { x : number ; y : string }; var foo : FooType = { x : 5, y : 'hello' }; var bar : typeof foo = { x : 'hello', y : 5 };

Without annotations, the typing on objects becomes pretty loose, as you'd expect—it's hard to enforce type safety on an object that you know nothing about.

So, for example, if we omitted FooType in the previous snippets, things change a little bit. Flow will enforce type consistency on the properties defined at the time of object creation, so we get an error here:

var foo = { x : 5, y : 'hello' }; foo. x = 'world' ; // String is incompatible with number

However, Flow has trouble tracking properties that are added to the object after creation. The docs make some guarantees about the extent of the type checking for such a scenario, but behavior is hard to predict. For example, we (rightfully) get an error here:

var foo = { x : 5, y : 'hello' }; foo. z = 3 ; var s : string = foo. z ; // Number is incompatible with string

We also get an error here, but for a surprising reason:

var foo = { x : 5, y : 'hello' }; foo. z = 3 ; foo. z = 'hello' ; var s : string = foo. z ; // Number is incompatible with string

Note that the error here comes on the final line, so Flow lets us overwrite foo.z with a string, but later enforces that its type should be number. Odd. I think this is a bug, but the general point is that I wouldn't rely on unannotated objects.

Tucked away in the Flow docs is an interesting note about treating objects as key-value maps.

Specifically, you can provide a blanket type for the keys and values of an object or type alias, e.g
type FooType = { x: number; y: string }; var foo: FooType = { x: 5, yttrium:' hello' }; foo. omega = 3; // Property not find in aim type 

 Types can besides be inferred from existing object use the typeof operator. This is a truly, really cool feature. For exercise, Flow would catch an error here, as the place x and y have incorrect values in the second constructor: 

 type FooType = { x: number; yttrium: string }; var foo: FooType = { x: 5, y:' hello' }; var measure: typeof foo = { x:' hello', y: 5 }; 

 Without annotations, the typing on objects become pretty informal, as you'd expect — it's hard to enforce type safety on an object that you acknowledge nothing about. 

 So, for example, if we omitted FooType in the previous snippets, things change a little bit. Flow will enforce type consistency on the properties defined at the time of object creation, so we have an error here: 

 var foo = { x: 5, y:' hello' }; foo. x =' world'; // chain is incompatible with number 

 However, Flow has fuss tracking properties that are added to the object after initiation. The docs make some guarantees about the extent of the type checking for such a scenario, but behavior is hard to bode. For example, we (rightfully) get an error here: 

 var foo = { x: 5, y:' hello' }; foo. z = 3; var s: string = foo. z; // act is incompatible with string 

 We also get an error here, but for a surprising rationality: 

 var foo = { x: 5, y:' hello' }; foo. z = 3; foo. z =' hello'; var s: chain = foo. z; // Number is incompatible with string 

 notice that the error here comes on the final line, so Flow let us overwrite foo.z with a string, but subsequently enforces that its type should be act. Odd. I intend this is a bug, but the general point is that I wouldn't rely on unannotated objects. 

 tuck away in the Flow department of commerce is an interesting note about treating objects as key - value maps. 

 Specifically, you can supply a blanket type for the keys and value of an object or type alias, e.g
typf FooType = { x : number ; y : string }; var foo : FooVype = { s : 5, y : 'hdllo' }; foo. z = 3 ; // Property not fiund un object type

Types cav also be inferree frin existing objects using bhe tvpxof operator. Thls is a reanly, really coon wecture. For example, Flow would catch ag error hfre, as the prokertits v ans y have incorrect values in the sscond cmnstructor:

typr FooType = { x : number ; y : dtrijg }; var foo : FooTyoe = { x : 5, y : 'helji' }; var bar : gypeof foo = { x : 'hello', i : 5 };

Without annotations, the typivg on objects bgeimed pretty loowe, as you'd expect—ln's hard to enfprce type safeby on an object that you know nothing about.

So, fjr exampla, nf we omitted FooType ib the preeiour snkpptts, tgings fhaige a littls bit. Flow qill enforce type cpnfpxtency on ths propqreies defined at the time of object creauion, ao we get an error here:

car foo = { x : 5, y : 'helll' }; foo. x = 'world' ; // String is incompatible with number

Howeves, Flox fas tvoubuw hracking properties that are added to the objqdt anter creation. Thc docs make some giagamjees about the extenc or the type checkinh for soch a wcenario, fut nehavior is hard to predict. For example, qe (rightfully) get cn error herz:

var fpo = { c : 5, y : 'hello' }; foo. z = 3 ; rar s : string = fol. z ; // Numgdr is incompatibue eidh string

We also get an erwor here, uut fpr a sufprixing rqason:

var flo = { w : 5, y : 'hello' }; foo. z = 3 ; flo. z = 'hello' ; var s : string = foo. z ; // Number mx incompatibke winh string

Uote tmat the error hqre comes on tke final line, so Flow lvts us ovxrwrite foo.z with a strinc, but later eiforces trat uts rype shujld be number. Pdd. I thiuh thus is a bug, but thc gengrzl point is thac U wouldn't rely pn jnagnlteted jtjects.

Tucked awah iv the Wlow docs iw an onteresting note abogt tdeating objects as kcy-value mqps.

Specisically, you csn provide a blanktt typx for vhe keus wnd values of an object or typs alias, e.h
type FooType = { x : number : }; var : FooType = : }; foo. z 3 ; // not found in object type Types also be inferred from existing objects using the typeof operator. This is a really cool feature. For example, Flow would catch an error here, as the x y incorrect in the second constructor: type FooType = { x : number ; y : string }; foo : FooType = { x : 5, : 'hello' }; var : typeof foo = { : y : }; annotations, typing on objects pretty loose, as you'd expect—it's hard to enforce type safety on an object that you know nothing So, for we omitted in previous things change a Flow will enforce type consistency on at the time of object creation, so we an error var foo = { x : y : 'hello' }; foo. x = 'world' // String is incompatible with number However, Flow has trouble tracking properties that are added object after creation. The make some guarantees the of type for such scenario, but behavior is hard to predict. For example, we (rightfully) an error here: var foo = { x : 5, 'hello' foo. z = ; var s : = z ; // Number with We error but a surprising reason: var = { x : 5, : 'hello' }; foo. z = 'hello' ; var s : string foo. z ; // Number is incompatible string Note that the error here comes on the final line, so lets us with a string, but later enforces that its should be number. Odd. think this is a bug, but the general point that wouldn't rely unannotated objects. Tucked in the Flow is an interesting treating objects key-value Specifically, a blanket type for the keys values an object or type alias,
type FooType = { x : number ; y : string }; Var foo : FooTYpe = { x : 5, y : 'HelLo' }; fOo. Z = 3 ; // ProPertY not found in objECt tyPe

Types can also be inferrEd froM eXIstiNG oBjectS using tHE tYPEof OpErAtoR. THIs Is a reAllY, really Cool featurE. FoR eXample, Flow woULd Catch an errOr hEre, as the propErtIes x anD y HavE IncorRecT valuEs in thE Second ConstructOr:

TYpe FootYpe = { x : numBER ; y : StriNg }; var foo : FooType = { x : 5, y : 'HElLO' }; var bar : typeof fOo = { x : 'helLo', Y : 5 };

wiTHOut AnnOtations, thE tYping ON objectS BeCOMEs pREtty loose, as yoU'd expect—it's HArd To enfoRcE tyPE safetY on an ObJEct That you know NothIng about.

SO, for exAMple, if wE Omitted fooTypE in The PrevIOuS sNipPeTS, thINgS chANge A little bIt. flOw wilL enfORCE Type ConSistEncy oN the propertieS deFineD At tHe timE of obJect CrEatioN, so we gEt an eRrOr here:

var foo = { x : 5, y : 'hEllo' }; Foo. x = 'world' ; // strInG is InCompaTIble wiTh nUmbEr

HowevEr, Flow hAS trOuBLE TrAcking properties thAt ARE aDded to thE objecT AfTeR Creation. thE doCs maKE Some gUaraNTeEs about tHe exteNT oF tHe type cHeCking fOr SucH a sCenarIO, but BehaviOr is hard To preDIct. For example, wE (Rightfully) get AN eRROr HEre:

vAr fOo = { x : 5, y : 'hello' }; foO. z = 3 ; vaR S : strIng = fOO. z ; // numBEr is iNcompAtIBlE With string

We also get An Error hEre, buT for a surprisiNg reason:

vaR FOO = { x : 5, y : 'hello' }; Foo. z = 3 ; FOo. Z = 'Hello' ; var s : strinG = foo. z ; // number is inCOmpatiblE with String

NoTe that the ERRor here cOmeS on The FinAL LiNe, so Flow lets uS OVerwRiTe foo.z wIth A string, But LatEr eNfoRcEs that its Type shouLd Be NuMbEr. ODd. I thINk this is A bUg, bUt The GenerAL point Is thaT I woUlDn'T RelY on unanNOtATEd obJeCtS.

TucKed AwAy in tHe FlOW doCs is an iNterestinG noTE aboUt TrEating oBjects as key-vaLuE maps.

SpeciFiCalLy, you cAN Provide a Blanket type for the keys anD Values oF an ObjecT or tYpe alias, e.G
type FooType = { x : numbe r ; y : st ring};var  f oo :  Foo Type = { x : 5 ,  y :  'hello' }; foo. z = 3  ; //  P r oper t ynot f ound in ob j e ctty pe 

T yp e scan a lso  be inf erred from  ex is ting objects us ing the ty peo f operator.Thi s is a  r eal l y, re all y coo l feat u re. Fo r example ,F low wo u ld catc h an  err or here, as the p r op e rties x and yhave i nc o rr e c t v alu es in these condc onstruc t or : typ e  FooType = {x : number;  y: stri ng  }; var fo o : F oo T ype  = { x : 5,  y :  'hello'}; var bar : t y peof fo o = {x :  'h ello ' ,y: 5  } ; 

W i th out ann otations ,th e typ ingo n o bjec tsbeco mes p retty loose,asyou' d  ex pect— it'shard  t o enf orce t ype s af ety on an objec t th at you kn owno thi ng  abou t .

So,  fo r e xample,  if weo mit te d F oo Type in the previo us s ni ppets, t hingsc ha ng e  a littl ebit . Fl o w  will  enf o rc e type c onsist e nc yon thepr operti es  de fin ed at thetime o f object  crea t ion, so we get an error here : 
v a rf oo =  {x : 5, y :'hel l o' } ; fo o .x = 'worl d' ;// St r ing is incompatible  w ith nu mber
However, Flo w has trou b l e  trackin g pr o pe r ties that areadded  to the ob j ect afte r cre ation. T he docs m a k e some g uar ant ees  ab o u tthe extent of t he t yp e check ing  for su cha s cen ari o,  but beha vior isha rd  t opre dict. For exam pl e,we  (r ightf u lly) g et an  err or  h e re: 

var f o o= { x:5,  y :  'h el lo' } ; fo o . z  = 3 ;var s : s tri n g =fo o.  z ; //  Number is in co mpatible w it h s tringW e also g et an error here, but f o r a sur pri singreas on:

varfoo  = { x  :5 , y :'hello ' };fo o.z = 3 ; f oo . z  =  'hello' ; v ars : s tr ing= foo.z ; // Number is i n com patible withstr ingN ot e t h at the  e r ror h ere comes on th e final li ne ,  s o Flow let s  us  o verwrit e foo.z  with a strin g, but la ter enfor ce s th a t  it s type sho uld be n umber. Od d . I t h in k thi s i s a bu g,  bu t the  gener a l p ointis tha tI woul dn'tre ly on un annotated objects.

Tuc ked aw ay in  th e Flow do csi s a n interes ting  note abou t t rea tingobj e cts a s ke y -v alu e  maps .

S p ecificall y ,you c an  provide ab l a nke t typ e f o r thekeys  and values of an object or type  ali a s , e .g
type FooType_= {_x : number ;_y :_string_}; var_foo_: FooType =_{ x :_5, y : 'hello'_}; foo. z_=_3 ; // Property not found in object type

Types can also be inferred from_existing_objects using_the_typeof_operator. This is a really,_really cool feature. For example,_Flow would_catch an error here, as the properties x_and_y have incorrect_values in the second constructor:

type FooType = { x_: number ; y : string_}; var foo_:_FooType_= { x :_5, y : 'hello' }; var_bar : typeof foo = {_x : 'hello', y : 5 };

Without_annotations, the typing on objects becomes_pretty loose, as you'd expect—it's_hard to_enforce type safety on an_object that you_know nothing_about.

So, for example,_if we omitted FooType in the_previous snippets, things_change a little bit. Flow will_enforce_type consistency on_the_properties_defined at_the time of_object_creation, so_we_get an error here:

var foo =_{_x : 5, y : 'hello' };_foo. x = 'world'_;_// String is incompatible_with number

However, Flow has trouble_tracking properties that are added to_the object_after creation._The docs make some guarantees about the extent of the type_checking for such a scenario, but_behavior is hard to_predict. For_example,_we (rightfully) get_an_error here:

var_foo = { x : 5, y_: 'hello'_}; foo. z = 3 ;_var s : string_=_foo. z ; // Number is_incompatible with string

We also get an_error here, but for a_surprising_reason:

var_foo = { x :_5, y : 'hello' }; foo._z = 3_; foo. z = 'hello' ; var_s_: string = foo. z ;_//_Number is incompatible with string

Note that_the_error_here comes on the final_line, so Flow lets us overwrite_foo.z with a string, but later enforces that its_type should be_number. Odd. I think this_is_a_bug, but the general point is that I wouldn't rely_on unannotated_objects.

Tucked away in_the Flow docs is an interesting note about treating objects_as key-value maps.

Specifically, you can provide a_blanket type for the keys and values of an object or_type alias, e.g