如何使用Jest和JavaScript测试一个副作用函数

Question

我用Jest做测试。

我想测试myFunction( myArray )是否没有副作用：

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

如何编写myArray不被myFunction更改的Jest测试

编辑：

我期待一个Jest测试脚本的形式的答案，我想对任何函数和变量，而不仅仅是myFunction( myArray )，使用副作用测试算法。

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

在严格模式中，可以在测试函数之前对数组进行冰冻，任何直接的突变都会引发异常：

注意，下面代码中的数组元素是原语，但是要防止嵌套对象(包括数组)的突变，还需要冻结这些对象。

​

<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的详尽回答。以下是一个快速的答案：

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 );
});