如何使用Jest和JavaScript测试副作用的功能

Question

我正在用玩笑进行测试。
我想测试myFunction（myArray）没有副作用：

test("that there are no side-effects" ...)

如何编写开玩笑的测试myArray不会通过myFunction ？

编辑：

我期望以嘲笑测试脚本的形式得到答案，我想对任何功能和任何变量使用该副作用测试算法，而不仅仅是myFunction（myarray）。

文件：myFunction.js

export default myFunction;
function myFunction( array ) {
  //any code goes here
  //maybe it changes array
  //maybe it doesn't
  //maybe it returns a value
  //maybe it doesn't
}

文件：myFunction.test.js

import myFunction from "myFunction.js";
test("that there are no side-effects", 
  //the Jest test code goes here
  //where I pass myArray into myFunction
  myFunction( myArray )
  //myArray should not be changed by myFunction
  //what should the test code be?
);

Answer 1

在严格模式 href =“ https://developer.mozilla.org/en-us/docs/web/javascript/Reference/global_objects/objects/object/object/freeze/freeze” rel =“ nofollow noreferrer”任何直接突变都会引发一个例外：

请注意，下面代码中数组的元素是原始数字，但是防止嵌套对象的突变（包括数组）将要求这些对象也被冻结。

<script type="module">

function test (name, fn) {
  try {
    fn();
    console.log('✅', name);
  }
  catch (ex) {
    console.log('❌', name);
    console.log(String(ex));
  }
}

function doubleArrayValues (array) {
  for (let i = 0; i < array.length; i += 1) {
    array[i] *= 2;
  }
}

function myFunctionPure (array) {
  const copy = [...array];
  doubleArrayValues(copy);
  return copy;
}

function myFunctionImpure (array) {
  doubleArrayValues(array);
  return array;
}

test('myFunctionPure: has no side-effects', () => {
  const myArray = Object.freeze([1, 2, 3]);
  myFunctionPure(myArray);
});

test('myFunctionImpure: has no side-effects', () => {
  const myArray = Object.freeze([1, 2, 3]);
  myFunctionImpure(myArray);
});

</script>

Answer 2

我喜欢@jsejcksn的透彻答案。
这是一个快速的答案：

import myFunction from "myFunction.js";

test("that there are no side-effects", () => {
  const myArray = [1,2,3];
  const myArrayCopy = [...myArray];
  const result = myFunction( myArray );
  
  //If the function returns an array,
  //it should not be the same reference
  expect( result ).not.toBe( myArray );
  
  //The original array values should remain the same
  expect( myArray ).toEqual( myArrayCopy );
});