Node TAP 18.7.0

tap Intercept Plugin

@tapjs/intercept

A default tap plugin for doing object/global property/method interception and observing. These are sometimes refered to as "spy" or "mock" methods (though the term "mock" is extremely overloaded, and in tap is usually used to refer to dependency injection mocking).

Usage#

import t from 'tap'

const functionThatLogs = (n: number) => {
  console.log('the number is', n)
}

t.test('some child test', t => {
  // track console.log calls
  const results = t.capture(console, 'log')
  functionThatLogs(10)
  functionThatLogs(5)
  // results() returns the list of what was called, and resets
  // the store.
  t.match(results(), [
    { args: ['the number is', 10], returned: undefined },
    { args: ['the number is', 5], returned: undefined },
  ])
  functionThatLogs(1)
  t.match(results(), [
    { args: ['the number is', 1], returned: undefined },
  ])
  // when the test ends, the original is restored
  t.end()
})

t.test('capture with an implementation', t => {
  const results = t.capture(console, 'log', () => {
    throw new Error('thrown from stub')
  })
  t.throws(
    () => {
      functionThatLogs(3)
    },
    { message: 'thrown from stub' }
  )
  t.match(results(), { args: ['the number is', 3], threw: true })
  t.end()
})

t.test('capture and still call the function', t => {
  // to do this, we just pass the original in as the third arg
  const results = t.capture(console, 'log', console.log)
  // actually logs to the console
  functionThatLogs(1)
  t.match(results(), [
    { args: ['the number is', 1], returned: undefined },
  ])
  t.end()
})

t.test('intercept a property set/get', t => {
  // the last arg is a propertyDescriptor, but configurable: true
  // forced on it even if not provided, so that we can restore
  // it at the end of the test.
  // If a value is provided, then we still actually set to a
  // setter/getter so that we can track accesses.
  const results = t.intercept(process, 'version', {
    value: '1.2.3',
  })
  t.equal(process.version, '1.2.3')
  process.version = '2.4.6'
  // we didn't make it writable, so this didn't do anything.
  t.equal(process.version, '1.2.3')
  t.match(results(), [
    { receiver: process, type: 'get', value: '1.2.3', success: true },
    {
      receiver: process,
      type: 'set',
      value: '2.4.6',
      success: false,
    },
    { receiver: process, type: 'get', value: '1.2.3', success: true },
  ])
})

API#