sinonStubbing.js

const sinon = require('sinon')

function Example() {}

Example.prototype.someFunc1 = function ()   { return 42;   }
Example.prototype.func      = function (fn) { return fn(); }

const example = new Example()
console.log(example.func(example.someFunc1))
// >> 42

sinon.stub(example, 'someFunc1').returns(69);

console.log(example.func(example.someFunc1))
// >> 69