sinon - spy on toString method

Question

In my file, I have something like this:

if(somevar.toString().length == 2) ....

How can I spy on toString from my test file? I know how to spy on things like parseInt with:

let spy = sinon.spy(global, 'parseInt')

But that doesn't work with toString since it's called on the value, I tried spying on Object and Object.prototype, but that doesn't work either.

Spying on Object.prototype will only work if toString() method was called on an instance of an object, same for arrays (using Array.prototype instead of Object.prototype). — Reuven Chacha, Apr 16 '18 at 21:49
I tested spying on String.prototype, and I can check that toString has been called indeed, but how can I check the parameters it was called on? calledWith doesn't work in this case. — Mankind1023, May 18 '18 at 18:08

score 1 · Answer 1 · answered Nov 20 '19 at 06:02

You can't call sinon.spy or sinon.stub on a method of primitive value like:

sinon.spy(1, 'toString'). This is wrong.

You should call them on the Class.prototype of primitive value. Here is the unit test solution:

index.ts:

export function main(somevar: number) {
  if (somevar.toString(2).length == 2) {
    console.log("haha");
  }
}

index.spec.ts:

import { main } from "./";
import sinon from "sinon";
import { expect } from "chai";

describe("49866123", () => {
  afterEach(() => {
    sinon.restore();
  });
  it("should log 'haha'", () => {
    const a = 1;
    const logSpy = sinon.spy(console, "log");
    const toStringSpy = sinon.stub(Number.prototype, "toString").returns("aa");
    main(a);
    expect(toStringSpy.calledWith(2)).to.be.true;
    expect(logSpy.calledWith("haha")).to.be.true;
  });
  it("should do nothing", () => {
    const a = 1;
    const logSpy = sinon.spy(console, "log");
    const toStringSpy = sinon.stub(Number.prototype, "toString").returns("a");
    main(a);
    expect(toStringSpy.calledWith(2)).to.be.true;
    expect(logSpy.notCalled).to.be.true;
  });
});

Unit test result with 100% coverage:

  49866123
haha
    ✓ should log 'haha'
    ✓ should do nothing


  2 passing (28ms)

---------------|----------|----------|----------|----------|-------------------|
File           |  % Stmts | % Branch |  % Funcs |  % Lines | Uncovered Line #s |
---------------|----------|----------|----------|----------|-------------------|
All files      |      100 |      100 |      100 |      100 |                   |
 index.spec.ts |      100 |      100 |      100 |      100 |                   |
 index.ts      |      100 |      100 |      100 |      100 |                   |
---------------|----------|----------|----------|----------|-------------------|

Source code: https://github.com/mrdulin/mocha-chai-sinon-codelab/tree/master/src/stackoverflow/49866123

Thank you that is what I needed. I am using Jest and needed to mock substring so by doing as you suggested in jest like: jest.spyOn(String.prototype, 'substring').mockImplementation(() => 'Hello'); works!! — Bartzilla, Apr 06 '21 at 14:34

sinon - spy on toString method

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