Jest Making Test Await Promise Resolve

Question

I have the following code I am working with this API https://developer.mozilla.org/en-US/docs/Web/API/Performance/measureUserAgentSpecificMemory. The class is stripped right back

export class Memory {
  private stopped = false
  private isUserAgentSpecificMemorySupported = true
  public memoryData: any = []
  constructor() {}

  public startMonitoring(): () => void {
    if (this.isUserAgentSpecificMemorySupported) {
      this.scheduleMeasurement()
    }

    return () => {
      this.stopped = true
    }
  }

  private async performMeasurement(): Promise<void> {
    const memory = await (window.performance as any).measureUserAgentSpecificMemory()
    const type = memory.breakdown.filter((e: any) => e.types.includes('JavaScript'))
    this.memoryData.push(type[0].bytes)
  }
}

Jest file.

import {Memory} from './memory'

type UserAgentSpecificMemoryBreakdown = {
  bytes: number
  types: Array<string>
}

type UserAgentSpecificMemory = {
  bytes: number
  breakdown: Array<UserAgentSpecificMemoryBreakdown>
}

type MockWindow = {
  crossOriginIsolated?: boolean
  performance: {
    measureUserAgentSpecificMemory?: () => Promise<UserAgentSpecificMemory>
  }
}

const data = {
  bytes: 1500,
  breakdown: [
    {
      bytes: 1000000,
      types: ['JavaScript'],
    },
    {
      bytes: 0,
      types: ['DOM'],
    },
  ],
}

describe('Test Memory Class', () => {
  let mockWindow: MockWindow
  let windowSpy: jest.SpyInstance

  beforeEach(() => {
    windowSpy = jest.spyOn(window, 'window', 'get')
    mockWindow = {
      ...window,
      performance: {
        measureUserAgentSpecificMemory: jest.fn(() => Promise.resolve(data)),
      },
    }
    windowSpy.mockImplementation(() => mockWindow)
  })

  afterEach(() => {
    windowSpy.mockRestore()
  })

  it('should measure User Agent Specific Memory', async () => {
    let memory = new Memory()

    memory.startMonitoring()
    expect(memory.memoryData).toEqual([1000000])
  })
})

I am not sure how to make the test file await for the value in the test?

Any help would be great.

Answer 1

window is an object and if it doesn’t contain window function, you can not spy on it.

For your production code, just mock measureUserAgentSpecificMemory function are enough:

import { Memory } from './memory'


describe('Memory', () => {
  const data = {
    bytes: 1500,
    breakdown: [
      {
        bytes: 1000000,
        types: ['JavaScript'],
      },
      {
        bytes: 0,
        types: ['DOM'],
      },
    ],
  };

  let memory: Memory;
  let measureUserAgentSpecificMemory: jest.Mock;

  beforeEach(() => {
    measureUserAgentSpecificMemory = jest.fn().mockResolvedValue(data);
    (window as any).performance = {
      measureUserAgentSpecificMemory,
    };

    memory = new Memory();
  });

  it('should measure User Agent Specific Memory', async () => {
    memory.startMonitoring();

    expect(memory.memoryData).toEqual([1000000]);
    expect(measureUserAgentSpecificMemory).toHaveBeenCalled();
  });
});