Emulator

Updated: 2022/8/26 by

2 min read

How to start a new instance of emulator

Flow Javascript Testing Framework exposes emulator singleton allowing you to run and stop emulator instance programmatically. There are two methods available on it.

`emulator.start(options)`

Starts emulator on random available port, unless overriden in options. Returns Promise.

Arguments

Name	Type	Optional	Description
`options`	EmulatorOptions	✅	an object containing options for starting the emulator

EmulatorOptions

Key	Type	Optional	Description
`logging`	boolean	✅	whether log messages from emulator shall be added to the output (default: false)
`flags`	string	✅	custom command-line flags to supply to the emulator (default: "")
`adminPort`	number	✅	override the port which the emulator will run the admin server on (default: auto)
`restPort`	number	✅	override the port which the emulator will run the REST server on (default: auto)
`grpcPort`	number	✅	override the port which the emulator will run the GRPC server on (default: auto)

Returns

Type	Description
Promise	Promise, which resolves to true if emulator started successfully

Usage

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import {emulator, init} from "@onflow/flow-js-testing"
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describe("test setup", () => {
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  // Instantiate emulator and path to Cadence files
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  beforeEach(async () => {
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    const basePath = path.resolve(__dirname, "../cadence")
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    await init(basePath)
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    // Start emulator instance on auto-selected available ports
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    await emulator.start()
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  })
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})

`emulator.stop()`

Stops emulator instance. Returns Promise.

Arguments

This method does not expect any arguments.

Usage

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import {emulator, init} from "@onflow/flow-js-testing"
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describe("test setup", () => {
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  // Instantiate emulator and path to Cadence files
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  beforeEach(async () => {
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    const basePath = path.resolve(__dirname, "../cadence")
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    await init(basePath)
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    await emulator.start()
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  })
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  // Stop emulator, so it could be restarted
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  afterEach(async () => {
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    await emulator.stop()
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  })
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})

`emulator.setLogging(newState)`

Set logging flag on emulator, allowing to temporally enable/disable logging.

Arguments

Name	Type	Description
`newState`	boolean	Enable/disable logging

Usage

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import {emulator, init} from "@onflow/flow-js-testing"
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describe("test setup", () => {
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  // Instantiate emulator and path to Cadence files
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  beforeEach(async () => {
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    const basePath = path.resolve(__dirname, "../cadence")
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    await init(basePath)
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    await emulator.start()
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  })
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  // Stop emulator, so it could be restarted
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  afterEach(async () => {
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    await emulator.stop()
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  })
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  test("basic test", async () => {
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    // Turn on logging from begining
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    emulator.setLogging(true)
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    // some asserts and interactions
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    // Turn off logging for later calls
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    emulator.setLogging(false)
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    // more asserts and interactions here
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  })
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})