[Unit Testing] Unit Test a Function that Invokes a Callback with a Sinon Spy

Unit testing functions that invoke callbacks can require a lot of setup code. Using sinon.spy to create a fake callback function can simplify your unit tests while still allowing you to observe the output of the function you're testing.

const fs = require("fs");
const assert = require("assert");
const sinon = require("sinon");
const jQuery = require("jQuery");

function getTempFiles(callback) {
  const contents = fs.readdirSync("/tmp");
  return callback(contents);
}

describe("getTempFiles", () => {
  it("should call the provided callback", () => {
    const spy = sinon.spy();
    getTempFiles(spy);
    assert.equal(spy.callCount, 1);
    assert.ok(spy.getCall(0).args[0] instanceof Array);
  });

  it("should call the function with correct args", () => {
    var object = { method: function() {} };
    var spy = sinon.spy(object, "method");
    object.method(42);
    object.method(1);
    assert.ok(spy.withArgs(42).calledOnce);
    assert.ok(spy.withArgs(1).calledOnce);
  });

  it("should wrap a existing method", () => {
    sinon.spy(jQuery, "ajax");
    jQuery.getJSON("/some/resource");
    assert.ok(jQuery.ajax.calledOnce);
  });
});

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