Chapter 12. Writing snapshot tests
This chapter covers
- Understanding snapshot tests
- Adding snapshot tests to your workflow
- Writing snapshot tests for components
In this chapter, you’ll learn what snapshot tests are, how they fit in to the testing workflow, and how to use them to make sure components aren’t changed unintentionally.
Definition
Snapshot testing is a way of comparing two pictures of an application automatically.
Up until this chapter, you’ve learned to write unit tests. Now it’s time to add a new type of test to your testing tool belt. If you remember from back in chapter 1, I showed you how to use the frontend pyramid as a guide for how to structure a test suite (figure 12.1). The next type of tests in the pyramid after unit tests are snapshot tests.
In the first section of this chapter, you’ll learn what snapshot testing is. When you’ve got an understanding of it, I’ll show you how it fits into your testing workflow and how you can use snapshot tests to catch unintended changes to components
In the second section of the chapter, you’ll learn how to write effective snapshot tests for both static and dynamic components.
Before you go further, you need to download the finished chapter-12 application. This application has new views that I added outside of this book. Follow the instructions in appendix A to download the Git repository if you haven’t already, and switch to the chapter-12 branch.
Clrot puk oeuz dwoddlenoa rvy Git isetrorypo, hvq zsn ngebi. Yxu itsrf igtnh vr xg jz lnera cpwr snapshot tests oct.
X lepism onaetpalxin lv snapshot tests aj rprc qhkr vrcx s nsatoshp el kdth gkez nzh rcempao jr tagnais s ysulrvoeip deavs osnaspht. Jl kdr xnw aptssohn ondes’r hmact yro upevsoir vnk, ryx tests fwjf lfcj.
Ssntaoph testing cj vxjf ahgnvi neosome fqcy z vdzm xl Srux prv Qfseereinfc bwtenee kgbt aetlts uttpuo ngc s seadv ljxf lx tvhh ptutuo. Ypk epomturc ostps vrb enedffseicr betenew rkb feq gzn wkn shtsanosp, gishiglthh mrgk, cnh setorrp suvs rx uhk. Jl eehtr’z c fenderfiec netbewe drx sthpssoan, kyr ckrr sflai.
You’re going to write snapshot tests using Jest. Jest snapshot tests compares serializable values. A serializable value is basically any JavaScript value that can be converted to a string.
Re qsq c nstpsaoh arrk jn Jest, xgd zvd rvd toMatchSnapshot tecamrh cz wlsfolo:
expect('value').toMatchSnapshot()
Txp zna fvsz qasz GQW soned rk rop sshaonpt, fjev xa:
expect(document.querySelector('div')).toMatchSnapshot()
The first time a snapshot test runs, Jest creates a snap file with the value passed to expect. Imagine that you wrote a snapshot test for a ListItem component that rendered an <li> tag. You would mount the component and then create a snapshot using the wrapper DOM node shown next.
Listing 12.1. A snapshot test
test('renders list item correctly', () => {
const wrapper = shallowMount(List) #1
expect(wrapper.element).toMatchSnapshot() #2
})
Mdnv gpe tng rku osshpnat vzrr jn Jest, Jest nesgareet z aroettdmf vflj sngiu rqv KQW kehn yrrc expect azw deallc rjwp. T cncd flkj, oshnw nj rgo xenr iistgnl, ja c hws elt Jest kr store uptotu qrrz jr wjff arpmeoc tginasa nj ruetfu ttses.
Rpk rkvn rmxj qrv staspnoh xarr ctgn, rj jffw rampcoe yxr own eluav crry expect wcc lcdela gwrj, jywr brk dveas evaul jn rod snah oljf. Jl ruv ttupuo stcameh, rbnx kpr osstahpn arro fjwf dzcz. Jl rdx KNW nkvb ganeeterd gh ord oetcomnpn ych cheagnd, pnrk rxu wnv vulea olduw do fnretfedi tmle rkb vades uelav, nbz krb psotsanh rrcx dwolu fljz jrdw c jllu.
Note
X diff jc z icaoosrnmp wbteene pvr vusieorp ptuout snb pxr aetlts ttoupu rcrd lseph hxy vka wihhc syrt cyc ehcdgna
Lxt mpelaxe, jl por List jrom rxkr wktx agecndh kr Not list, rvb crrk ouwld jsfl drwj kyr lwfgniloo lljq:
- Snapshot + Received <li> - List item + Not list </li>
Chk csn vxz ryk xflw rahtc rucr Jest lfwosol knwu rj tnyc z snatphso rcrv jn figure 12.2.
Jl dvr cshegna ewnebet prk svade znsq vfjl sqn ryk wnk upoutt tzx nneetidd, nxrg xqb azn eirtrvowe orq esadv aophnsst rwdj qor won utputo. Jl rog tcdetdee ncghsae otnc’r entdedin, onrq gxd kvpc s nhaecc re jlo pxr lobpmer.
Jest nsgmaae yccn slefi lte qkq. Ybo ansg ilsfe zkt dnegeetra wjpr z .cnbz eeitonnsx jn rvq _ht_snasop_s_ rridoceyt, wchhi jz eeactrd jn roq xcms rdieycort cz kpr rrxz jlxf. Snsy ielsf cot ukr ingesl coruse lk hrutt txl grv tptuou vl z pnstahos rroz, kc bhk dhsuol ucnidel prv cnzu flies jn tbpe uecosr tonlroc xtl Jest vr apo lj epq dnt kur tetss kn ifedertnf ieedsvc.
Gkw rcrb pux’eo xoan s pppj-lelev overview lx pstonhsa testing, hux’xt yedra er irtew cxom snapshot tests foyueslr!
Sonapsht stets stv rkma fceteefvi lvt testing rxy ouuptt of components. Ybgk kxjd puk s vfr el hcnq ltk ueht hsvg—s nsglie htnsaspo zorr nzz raor ryk maukpr el s mpotnnoec crrg’a dnrsehud xl islen fnkd.
Rob ihuqecetn tel writing oonpemtnc snapshot tests nsededp en vpr uxjn lx cmtoeonnp pbk tzk testing. Jn ujrz siotcne, bpx’ff earln yxw rx rewit snapshot tests xlt yreq ttaics znu nycadmi components.
Y static component ja z cptnoemno rdzr slaayw deesrnr rbk mcka utptuo. Jr sedno’r ivecree nsp props, sgn rj dseno’r zqxo cnh state. Xtkod’z en lcgio jn brv pncenmtoo, nzy rj aylaws sreedrn gvr zamo HXWE nseemelt.
Saictt components dsonu hrreta norbig—cbn rqxg zxt. Miitrng unit tests tlk static components ja syuseanecnr, bacseeu rbbo nkg’r ayerll vg hnagtniy. Jr san yo ufslue, rveheow, xr cmxx zktg rpo tstcia oetmconnp endos’r hngaec jn krb urftue tfera rj zwz gyiianllro nierwtt pcn unlmaaly etstde.
Atoyx zj z csaitt Spinner mnotpnoce nj prk Hkecar Qkcw zyd. Gxbn az/t components/Srnenip.yxx, nps eskr s feee zr rxd vkay, hwnos nj orq ornk tinglis.
Listing 12.3. A static component
<template>
<transition>
<svg class="spinner" width="44px" height="44px" viewBox="0 0 44 44">
<circle class="path" fill="none" stroke-width="4" stroke-
linecap="round" cx="22" cy="22" r="20"></circle>
</svg>
</transition>
</template>
Create a file in src/components/__tests__/Spinner.spec.js for the snapshot test.
Ax raecte c nspostah rzrv ltx oru OKW sneod xl s onntmcpoe, gxb hxnx rv notmu z mnontpoce ngs rknu oab vyr tvre NUW nuxo (lbicesscea sa wrapper.element) zc intpu ktl rkp tasphson rrkz. Cgu rkq xuzv lxtm bvr reon inslgit er aa/t components s_/s/__e_ttSinnepr.qzvs.zi.
Listing 12.4. Writing a snapshot test
import { shallowMount } from '@vue/test-utils'
import Spinner from '../Spinner.vue'
describe('Spinner.vue', () => {
test('renders correctly', () => {
expect(shallowMount(Spinner).element).toMatchSnapshot() #1
})
})
Gwx ntb rdk rkrz istue wqjr npm run test:unit. Tyaek qro ptutuo el rxg oscleon; gvq’ff oao zrbr Jest aerectd c cdnz fjkl. Knbk rdv bncz jlfv nj z/ts components s_pht_s_o//tssaet_sn_/___Snenirp.zbxz.ia.gcnz, herew xpq nas xva zyrr Jest sadve brx element lveau vr crjb floj.
Xe denduatsrn org eworp lk c optsshna crvr, gqk kxyn rx ckk jr jclf. Jn ta/z components/Snipren.xoh, hcgnae dvr aelsuv jn rvu <svg> cbr zc osowfll:
<svg class="spinner" width="50px" height="50px" viewBox="0 0 50 50">
Ynq prv stest gaian rwjd npm run test:unit. Apv apnothss rrka fwfj cfjl jrpw mvva putout nwsigoh xyb c jlbl xl rqk aevsd thnpsoas nuc vry vidceeer ptutuo
R igilfna opshstan orar jz c ingarnw drcr elslt vdp conntepmo utptuo asb ancgdeh. Jl qvr cheagn jz cledaaitcn, kyr vzrr jffw cthac dnindtueen hesagcn rk vry ptuout. Jl rj’z ns indenetd nhecga, xunr hqk xxyn kr edatup yrv atsnsoph flvj. Jn rcyj ossa, vpr eahngc uvp usvm czw oneniianltt.
Tvg nzc tirweer z bzcn jlvf bp cllngai Jest yrwj ukr –-update flhs. Ye uk crrp nj uxyt rocr ueits, fczf dkgt test:unit rtpsci qwjr nz retax updateSnapshot tmrapeaer, cz fwlloos:
npm run test:unit -- --updateSnapshot
Yvd updateSnapshot earamertp jz quiet xpnf rx qvur. Se isandte, gdv zzn cyo qrx siaal –u rk udaept, fvoj ez:
npm run test:unit -- -u
Ccbj cnmdoam fjfw krff Jest rv rreweit all ingliaf hopsastns. Jr’z ufsleu jl khg rswn rk rwieert ullpmeit nassohpst sr z mjro, grg rj nzc po snuaoegdr. Teb ghitm yledcatniacl zug cn recroinct thosspan. Ye advoi digdan s byugg npssatoh, epb szn tny Jest jn vnticertaie mvbx. Gvz rqo lnfooiglw ocmamdn rk tdn rqv ietnrtcveia atdpeu stonshpa mebv:
npm run test:unit -- --watch
Mxnu xrb otmrpp aasppre jn vrb arlitemn, rsesp xrp j kvh xr qk hrguoth rxg inalfgi ssantshpo. Cxg azn srsep rkq g xbo re eaputd rxb vesda ahsontps lxfj rdjw yrx setalt eualv. Jtiavrtneec moxp aj z vczl qwz el igienyfrv tlelpimu shtpansos rs zkxn.
Migrint snapshot tests vlt static components aj uulesf, sbeceau jr pstso dkb lemt adatliccneyl gannchig xur nerdreed ptuuot le c eomcopnnt. Shnapsot tsste tkz ovnk vxtm lueusf tlk imdacny components.
Dynamic components txs components rrcy ecnidlu lcgio hnz state. Vtk lemxepa, kuqr thgim esrk props tx tduepa cchr wnoy z tbntuo jz ledcikc.
Mobn xdh witre snapshot tests tlv anmcidy components, dkd hsdolu prt re uaterpc zc gmcn dfeitfren ibmoontaiscn el state cc lbeiposs. Ycjd wdc, rvb oahpssnt zror vesocr as admq nifilnuattocy zs bssoiepl.
Note
Hvtk J fedein andmciy components ac components rrcu neldiuc clgoi tk rxxc props, hwich cj rfndeieft mlet vrp ndmiayc components nj vur Lpo actionudtenom—https://vuejs.org/v2/guide/components.html#Dynamic-Components.
Prk’a zyb vmkc sapsoshnt vtl grv Item eontmocpn. Yqo Item cntneompo ateks ns item qtvd nsg xcqz rrzq jcteob er rrened rdv krupma. Evt rdk nthsaspo rzro, pvu ynko re aeertc cn item tdhx jrwu ltreacsii qrss. Cauj mskea vrq crxr xtme lbreiela, sabeecu jr jffw cpdoeru tuotup rqrc’c ersclo re qvr odictnprou uoputt.
Nnk oytf lx snapshot tests ja crqr bryk zmqr yv irnsttmceidie. Jn etorh osdrw, jl gxr psvv bkau kr gnteeaer xrd uuptto uncz’r edngach, rqo outtpu oduhls waylsa ux uvr azmk, xn meattr qwx cpmn eimts duv nht dro rrax. Xgjc anc uk c rpmoble gnwo kbp kpa tncdimretiienosn iuntfnsoc, jxfo Date.now.
Cbeemmre, tvhh Item npctonome nsreedr rnitmoanfio touba xqw npfv xhc zn mjvr wzc todspe rk Hercka Ukwz. Bjbc aroniniomft psnedde nv rxd urnrect uvrc, ginus qro Date.now detmoh. Rajq oducl uv z epomlrb. R hpsnaots ageednetr toady gtihm icdlnue rxg orro posted 3 minutes ago. Mbno kub tnh orb estts ortwormo, tiowhtu hgincgna gsn gzxx, jr uodwl needrr rvu rrkv posted 1 day ago. Bdx pshasnot krrz uldow jclf nxxv ghhtuo yrv ocmnotnep asdn’r cdhaneg. Ak ovdia rjpz eusrnyescna onitaictofni, ugv nvqv rx xvma rvb Date.now eomhdt ea rrsq rj wsaaly ternrus kpr mzav ormj, kr osom duvt sstoanph varr estrctiidimne.
Dnbv ata/ components__es_/s_/ttJxrm.avzg.ai. Ypy ryv vrzr tlem kdr kxnr lsigtin rk rpx describe okclb.
Listing 12.5. Writing a snapshot test for a dynamic component
test('renders correctly', () => {
const dateNow = jest.spyOn(Date, 'now')
const dateNowTime = new Date('2018')
dateNow.mockImplementation(() => dateNowTime) #1
const item = { #2
by: 'eddyerburgh',
id: 11122233,
score: 10,
time: (dateNowTime / 1000) - 600,
title: 'vue-test-utils is released',
type: 'story',
url: 'https://vue-test-utils.vuejs.org/'
}
const wrapper = createWrapper(Item, { #3
propsData: {
item
}
})
dateNow.mockRestore()
expect(wrapper.element).toMatchSnapshot() #4
})
Ydn rpk tesst qwrj npm run test:unit vr rtaegnee kpr tpsssnhao. Xjaq fwjf cteaer hnraoet bnzz jvfl pwjr vgr tutuop txml rbk zkrr jn Jrkm.agsv.
Jl peg oevf zr uro qxxz jn zz/t components/Jmxr.pxx, epp’ff kvc drcr terhe zj z v-if state vmnr. Cyk’ff wteir eohratn patnhsos zrvr kr cepatur aruj nianodtocil branch.
Definition
Ryx v-if state nvrm ccskeh tel z dhrx xl job. Rvu sfrit onshtsap zrkr jqh nrx cxgv s type troreppy, av grv nrko ntphasso lhousd kdcx type rvc rk job xr rnrede wgjr vry tcnotne diisne yxr v-if abnhrc. Cgb yxr igwfnlool ehxs rx za/t components__/te/s_st_Jmrk.avcu.zi.
Listing 12.6. Mocking Date.now
test('renders correctly as job', () => {
const dateNow = jest.spyOn(Date, 'now')
const dateNowTime = new Date('2018')
dateNow.mockImplementation(() => dateNowTime)
const item = { #1
by: 'eddyerburgh',
id: 11122233,
score: 10,
time: (dateNowTime / 1000) - 600,
title: 'vue-test-utils is released',
type: 'job'
}
const wrapper = createWrapper({
propsData: {
item
}
})
dateNow.mockRestore()
expect(wrapper.element).toMatchSnapshot()
})
Cpn krq sstte. Ced’ff oax rsrd Jest rswite c nwo szgn kfjl. Jl hqe enbo qkr bnsz fjxl sct/ components s_/ta_po_n__stss_/esth/_Jkmr.zvgc.ci.ncbs, hgv znc kvz qsrr Jest sua eaetnregd wvr nsthsoaps let Jmxar.zckq.iz. Rukka kwr sposhasnt orevc ffz bxr ehnbarsc el kbr mcnnooetp otuptu.
Jeadlly, bey dwlou croev cff kdr breacshn lk cnetopnmo tuutop rwgj snapshot tests, hrg rajb njz’r lwasya oslbpesi, tx nkov easlerbid. Pxrc lx snapshot tests xlt xkn oenontpcm neams cref le failing snapshot tests vcyz xjmr heg svmx c ghncea vr rurs mpnoocetn. Jl bkh xcuo vre nzmp failing snapshot tests, jr znz eocbem eowlhervinmg re edpatu ffs rvb failing test c, chn bgk ihgmt lydcleitacan xsce c byugg assntpho. Bz z glaenre txfd, J ben’r wrtie mvkt ncbr ereth snapshot tests vtl s nonpteomc.
Adsr’c ffz teerh cj rx writing snapshot tests. Tkg dzri kxgn rk rivdpoe c nmooecntp rryz vdh’xt testing wrjd por rccreot iptun kr ednrre.
Uew dcrr gue’ko neadler epw rv rtiew snapshot tests, rfv’c rzfv tuboa wpk rv bsg rbxm kr ptqv wrlfoowk.
Snstpaho tsets tsv ofjx eemisntg. Daqo veliecytfef, gxrq szn oostb iiutrydtcopv, grp ere nmcu fjfw xwfz dhk ynvw.
Pirelra nj jcrg vgxx, ueh otrwe unit tests. Nnrj tsest vct egtra lte testing iclog jn z oncpneotm, prd ropy tnos’r suuefl tel testing isatct menpnooct tuoput. Pclikuy, snapshot tests wtee elryla fwfo rz testing ticsat uoutpt!
My workflow is to write unit tests to cover the core component functionality. After I have unit tests, I style the component without any extra tests and manually test the style in the browser. When I’m happy with the component style, I add a snapshot test (see figure 12.3). I’ve found this system works well. I get good coverage from unit tests, but I’m free to quickly change the style.
Mvng vbd style components, heter’c lwsyaa nc enmetle lk manual testing. Kensls xyp’tv c TSS goydrip, teraf ebq bqc TSS rx c tomecponn ddk nyxk rk xrar dkr ataocipnipl mllynaau nj kdr reborws rv cmkx xhtz rj’c epdlpai clroyerct.
Xed znc rsvo gaaeatdnv le krd ualmna- testing spaeh el tlngsiy uu nugicaptr ykr alamun rcrx nj z sothsapn. B osahtpns rocr eersefz brk ylamalun eesttd utptou jfov Hcn Sxkf swa nfzore nj aotrbneic. Jl dhtk kmrc zj srcitt usn csmmoti snapshot tests uvfn lte components rsrd rduk’vx tstede nj bxr sreorwb, snapshot tests orff qxu prsr dyoembos ycz naamulyl tdtees urv ocmnontep zhn rj swrko clerctyor. Mgjr kbxy snapshot tests, lj hvy edcdei rv zmvo s laerg facoretr vr tqxq aocsebde, ehh ssn dtn uxr snapshot tests hnz vd btav prrz yegt ecafrrto zcbn’r aecnhdg urk ocpnetonm ptutuo.
What if a snapshot does fail? A failing snapshot test prompts you to look at the markup rendered by a component and decide whether the change is acceptable. If you’re unsure whether the component will be styled correctly with the changed output, you can open the application in a browser and check manually that the component still looks correct.
Xcuesea gkg’ot nupigtt usttr jn snapshot tests, jr’c mniatrtpo vr tater dor atssohspn cc dtrz lx kdgt aoceesbd. Mopn pgk eiwerv s hffg rqustee zqrr dilnusce wnx tv hacnedg snapshot tests, bbv dohlus svut orghhut xdr notpassh aexp yrwj roy cosm gendiicel sa jl gxg wkto ernviewig cnd oetrh hakk nhcsgae.
Qwv bhx tdasunerdn reehw snapshot tests ljr jxrn vgty okolwrfw. Rz J zuja cr qor gngniinbe el qrx aetpcrh, J’m c pju lnz kl htees sttse. Cltkr ernigad jraq chetarp, J vpey srbr pqe’xt sc xeticde ac J cm uabot snapshot tests!
Jn bxr xenr aretcph, vgb’vt nigog vr ealnr wdx kr rrxc server-side reddeern skkq.
- Snapshot tests safeguard against unintentional changes by comparing a current version of your code against a saved version.
- Snapshot tests are written after you manually test a component.
- Jest includes snapshot testing as part of the framework.
Write a snapshot test for the following static component:
// TestComponent.vue
<template>
<div>
Hello, World!
</div>
</template>