compose-jb/tutorials/HTML/Using_Test_Utils/README.md

Using test-utils for unit testing

Dependencies

It's necessary to add compose.html.testUtils to jsTest dependencies:

sourceSets {
    val jsMain by getting {
        dependencies {
            implementation(compose.html.core)
            implementation(compose.runtime)
            //....
        }
    }
    val jsTest by getting {
        implementation(kotlin("test-js"))
        implementation(compose.html.testUtils)
        //...
    }
}

Example

// This is a function that we want to test
@Composable
fun TestButton(text: String, onButtonClick: () -> Unit) {
    Button(attrs = {
        onClick { onButtonClick() }
    }) {
        Text(text)
    }
}

Let's add a test to ensure that button has correct text, and it's onClick works properly.

import org.jetbrains.compose.web.testutils.ComposeWebExperimentalTestsApi
import org.jetbrains.compose.web.testutils.runTest
import androidx.compose.runtime.getValue
import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.setValue
import org.w3c.dom.HTMLButtonElement
import kotlin.test.Test
import kotlin.test.assertEquals

@OptIn(ComposeWebExperimentalTestsApi::class)
class TestsForButton {

    @Test
    fun testButton() = runTest {
        var counter by mutableStateOf(1)
        
        composition {
            TestButton(text = "$counter") {
                counter++
            }
        }

        assertEquals("<button>1</button>", root.innerHTML)

        (root.firstChild!! as HTMLButtonElement).click()
        waitForRecompositionComplete()
        assertEquals("<button>2</button>", root.innerHTML)

        counter = 10
        waitForRecompositionComplete()
        assertEquals("<button>10</button>", root.innerHTML)
    }
}

Let's break it down:

runTest { ... }

Provides the TestScope with useful functions to configure the test.

composition { ... }

Takes a @Composable block with a content that we want to test. It will automatically build and mount DOM into root element.

root

It's not supposed to be used for elements manipulation. It's mostly useful to make assertions on the html content (e.g. root.innerHtml)

nextChild() and currentChild()

Under the hood nextChild() iterates over root children, providing convenient access to them.

currentChild() doesn't move the iterator and returns the same element every time until nextChild() called.

waitForRecompositionComplete()

It suspends until recomposition completes. It's useful when state changes, and we want to test that content updates as well. waitForRecompositionComplete needs to be called after state change and before assertions.

waitForChanges(id: String)

It suspends until any change occur in the element with id. It's also useful to ensure that state changes make corresponding updates to the content.