JoeNoPhoto/Add,Remove, and Increment,Avoiding Array Mutations with concat(), slice(), and ...spread.js

## Add,Remove, and Increment,Avoiding Array Mutations with concat(), slice(), and ...spread.js
const addCounter = (list) => {
  return [...list, 0];
};

const removeCounter = (list, index) => {\
  return [
    ...list.slice(0, index),
    ...list.slice(index + 1)
  ];
};

const incrementCounter = (list, index) => {
  return [
    ...list.slice(0, index),
    list[index] + 1,
    ...list.slice(index + 1)
  ];
};

const testAddCounter = () => {
  const listBefore = [];
  const listAfter = [0];

  deepFreeze(listBefore);

  expect(
    addCounter(listBefore)
  ).toEqual(listAfter);
}

const testRemoveCounter = () => {
  const listBefore = [0, 10, 20];
  const listAfter = [0, 20];

  deepFreeze(listBefore);

  expect(
    removeCounter(listBefore, 1)
  ).toEqual(listAfter);
};

const testIncrementCounter = () => {
  const listBefore = [0, 10, 20];
  const listAfter = [0, 11, 20];

  deepFreeze(listBefore);

  expect(
    removeCounter(listBefore, 1)
  ).toEqual(listAfter);
};

}

testAddCounter();
testRemoveCounter();
testIncrementCounter();
console.log('all tests passed');

## index.html
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	<!--
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	</head>
	<body>
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	</html>