我可以使用 Jest 模拟带有特定参数的函数吗? [英] Can I mock functions with specific arguments using Jest?

问题描述

我想用 Jest 模拟一个函数，但前提是它是用特定参数调用的，例如:

I want to mock a function with Jest, but only if it is called with specific arguments, for example:

function sum(x, y) {
  return x + y;
}
    
// mock sum(1, 1) to return 4
sum(1, 1) // returns 4 (mocked)
sum(1, 2) // returns 3 (not mocked)

Ruby 的 RSpec 库中实现了类似的功能:

There is a similar feature implemented in Ruby's RSpec library:

class Math
  def self.sum(x, y)
    return x + y
  end
end

allow(Math).to receive(:sum).with(1, 1).and_return(4)
Math.sum(1, 1) # returns 4 (mocked)
Math.sum(1, 2) # returns 3 (not mocked)

我在测试中试图实现的是更好的解耦，假设我想测试一个依赖于 sum 的函数:

What I'm trying to achieve in my tests is a better decoupling, let's say I want to test a function that relies on sum:

function sum2(x) {
  return sum(x, 2);
}

// I don't want to depend on the sum implementation in my tests, 
// so I would like to mock sum(1, 2) to be "anything I want", 
// and so be able to test:

expect(sum2(1)).toBe("anything I want");

// If this test passes, I've the guarantee that sum2(x) is returning
// sum(x, 2), but I don't have to know what sum(x, 2) should return

我知道有一种方法可以通过执行以下操作来实现:

I know that there is a way to implement this by doing something like:

sum = jest.fn(function (x, y) {
  if (x === 1 && y === 2) {
    return "anything I want";
  } else {
    return sum(x, y);
  }
});

expect(sum2(1)).toBe("anything I want");

但是如果我们有一些糖函数来简化它会很好.

But it would be nice if we had some sugar function to simplify it.

这听起来合理吗?我们是否已经在 J​​est 中提供了此功能?

Does it sounds reasonable? Do we already have this feature in Jest?

感谢您的反馈.

推荐答案

我发现了我同事最近写的这个库:jest-when

I found this library that a colleague of mine wrote recently: jest-when

import { when } from 'jest-when';

const fn = jest.fn();
when(fn).calledWith(1).mockReturnValue('yay!');

const result = fn(1);
expect(result).toEqual('yay!');

这是图书馆:https://github.com/timkindberg/jest-when

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