Module @tapjs/intercept - v1.2.0


A default tap plugin for doing object/global property/method interception and observing.


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')
// 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 },
t.match(results(), [
{ args: ['the number is', 1], returned: undefined },
// when the test ends, the original is restored

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

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
t.match(results(), [
{ args: ['the number is', 1], returned: undefined },

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 },


  • Type ResultsFunction A function that returns data about the intercepted calls, and resets the tracking array. results.restore() will restore the method to its original state.

  • t.capture(obj, method, implementation = () => {}): CaptureResultFunction

    Replaces obj[method] with the supplied implementation.

    The results() method will return an array of objects with a receiver property indicating the this-context of the method call, an args array, an at CallSiteLike object, and either threw: true or returned: <value>.

    If t.teardown() is available (ie, if the @tapjs/after plugin is not disabled) then it will be automatically restored on test teardown. Otherwise, results.restore() must be called to restore the original method.

    Returned method also has a calls array which contains the results.

  • t.intercept(obj, property, desc?: PropertyDescriptor, strictMode: boolean = true): InterceptResultsFunction

    Similar to t.capture(), but can be used to track get/set operations for any arbitrary property. The results function returns a list of objects with:

    • receiver the object where the set/get is happening
    • type 'get' for get operations, 'set' for set operations
    • value The value that was returned by a get, or set in a set.
    • at call site where the get/set occurred.
    • threw whether or not the call threw.
    • success whether or not the call was sucessful.
  • t.captureFn(original: (...a:any[]) => any): WrappedFunction

    Similar to t.capture(), but just wraps the function and returns it.

    Returned function has a calls array property containing the results.




Type Aliases


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