Solution Review

Review with explanations. (3 min. read)

assoc

We learned that a pure function must follow 2 rules

  1. Same inputs => same outputs
  2. No side-effects

assoc currently mutates the input object, which is a side-effect.

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const assoc = (key, value, object) => {
  object[key] = value;
};

Cure it by cloning the input and merging the desired properties.

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const assoc = (key, value, object) => ({
  ...object,
  [key]: value
});

getNames

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const getNames = (users) => {
  console.log('getting names!');
  
  return users.map((user) => user.name);
};

This was sort of a trick question. getNames is pure if you remove console.log.

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const getNames = (users) => {  
  return users.map((user) => user.name);
};

You can now refactor it to a single-line statement, if you prefer.

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const getNames = (users) => users.map((user) => user.name);

append

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const append = (item, list) => {
  list.push(item);
  
  return list;
};

This is impure because it mutates the input list. Like assoc, you can return a clone with the desired output.

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const append = (item, list) => [...list, item];

sortAscending

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const sortAscending = (numbers) => numbers
  .sort((a, b) => a > b);

Once again we’re using an impure Array method, sort. Cloning the input purifies us right up.
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const sortAscending = (numbers) => [...numbers]
  .sort((a, b) => a > b);