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Make a Sinon Spy Return a Value

Aug 13, 2024

By design, sinon spies do not replace the function being called. They just record information about how the function is called, like which parameters the function was called with. If you want to make a spy return a value, you should use a Sinon stub and callsFake() instead.

const sinon = require('sinon');
const assert = require('assert');

const obj = {
  spyMethod() {
    return 'original spyMethod';
  },
  stubMethod() {
    return 'original stubMethod';
  }
}

// Create a sinon spy to record information about a function call
sinon.spy(obj, 'spyMethod');
obj.spyMethod(); // 'original spyMethod'

// Create a sinon stub to replace a function and return a pre-determined value
// Sinon spies don't have a `callsFake()` method
sinon.stub(obj, 'stubMethod').callsFake(() => 'wrapped stubMethod');
obj.stubMethod(); // 'wrapped stubMethod'

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Make a Sinon Spy Return a Value

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