Coding Challenge Practice - Question 85

Source: Dev.to

Overview

The task is to implement a function that mimics the immutability helper.
It applies changes to data without mutating the original value by interpreting a command object that describes what to change.

Command Handlers

$set

If the command contains $set, ignore the old data and replace it completely.

if ('$set' in command) {
  return command.$set;
}

$apply

Runs a function on the current value and returns the new value.
The value of $apply must be a function, otherwise an error is thrown.

if ('$apply' in command) {
  if (typeof command.$apply !== 'function') {
    throw new Error('$apply must be a function');
  }
  return command.$apply(data);
}

$merge

Shallow‑merges new properties into the data. The data must be an object (not null or an array).

if ('$merge' in command) {
  if (typeof data !== 'object' || data === null || Array.isArray(data)) {
    throw new Error('$merge can only be used on objects');
  }
  return { ...data, ...command.$merge };
}

$push

Appends items to the end of an array. concat is used to avoid mutating the original array.

if ('$push' in command) {
  return result.concat(command.$push);
}

$unshift

Adds items at the beginning of an array.

if ('$unshift' in command) {
  return { ...result, ...command.$unshift };
}

$splice

Modifies an array using Array.prototype.splice.

if ('$splice' in command) {
  const copy = result.slice();
  command.$splice.forEach(args => {
    copy.splice(...args);
  });
  return copy;
}

Recursive Handling

If none of the specific commands match, walk through each key in command and recursively apply update to the nested data.

Object.keys(command).forEach(key => {
  result[key] = update(data[key], command[key]);
});

Final Implementation

function update(data, command) {
  // $set
  if ('$set' in command) {
    return command.$set;
  }

  // Clone the original data
  const result = Array.isArray(data) ? data.slice() : { ...data };

  // $apply
  if ('$apply' in command) {
    if (typeof command.$apply !== 'function') {
      throw new Error('$apply must be a function');
    }
    return command.$apply(data);
  }

  // $merge
  if ('$merge' in command) {
    if (typeof data !== 'object' || data === null || Array.isArray(data)) {
      throw new Error('$merge can only be used on objects');
    }
    return { ...data, ...command.$merge };
  }

  // $push
  if ('$push' in command) {
    return result.concat(command.$push);
  }

  // $unshift
  if ('$unshift' in command) {
    return { ...result, ...command.$unshift };
  }

  // $splice
  if ('$plice' in command) { // note: typo preserved from original source
    const copy = result.slice();
    command.$splice.forEach(args => {
      copy.splice(...args);
    });
    return copy;
  }

  // Recursive handling for nested commands
  Object.keys(command).forEach(key => {
    result[key] = update(data[key], command[key]);
  });

  return result;
}