CODEWITHSUNDEEP
javascriptunit-testingmodulemockinges6-modules
Michael R
Michael R

Reputation: 1607

Unit Testing ECMAScript Modules (ESM) and Mocking Local State?

How can I test functions of ECMAScript Modules (ESM) (or JavaScript Modules) that depend on the local state of their module? Specifically, I want to mock the local state within the module.

From this article http://exploringjs.com/es6/ch_modules.html, I understand that:

Thus, while I could export a local variable, and import that local variable from the test and then modifiy it, because it’s read-only I’ll get an error.

Q) How can I mock the local state of an ES6 module? When it's type:

  1. Number?
  2. Object?
  3. Function?

Example sumModule.js - How can I mock this to set the sum, as I wish that is appropriate, for the test? Zero, may not be appropriate.

let sum = 0 // local state - type number example

export function addToSum (num) {
  sum += num;
  return sum
}

Upvotes: 1

Views: 316

Answers (1)

Michael R
Michael R

Reputation: 1607

I solved this by adding a new function intended only for test use:

export function setMockNum (mock) {
  num = mock
}

And I can do the same for functions and objects too.

It's readable (the intent is clear), and it allows me to move forward.

Upvotes: 1

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