Implement isEqual (deep equality comparison)

When writing a program, there are many cases, such as when comparing objects or arrays, where a simple equality comparison (===) is not enough. We need a way to help us compare the value of different objects.This kind of comparison is also called deep equality comparison. It is called deep because it will compare not only the values of the properties, but also the properties of the properties and so on. There are various libraries and utility functions available in JavaScript that implement a deepEqual function, such as lodash.isEqual.

In the following example, object and other are two different objects, so a direct comparison of object === other will be false. But since the values in the two objects are the same, comparing them via isEqual will return true.

const object = { a: 1, b: [2, 3] };
const other = { a: 1, b: [2, 3] };

_.isEqual(object, other);
// => true

object === other;
// => false

In your daily development work, you can easily import such kind of method through libraries such as lodash. But in an frontend interview, the interviewer will ask you how to implement it by yourself. If you don't know how to do it, let's take a look together!

Implement deep comparison

The following is the implementation of deep equality comparison. If you are not sure why it is done like this, scroll down and there will be a detailed explanation for the code.

function isEqual(value, other) {
  if (typeof value !== "object" && typeof other !== "object") {
    return Object.is(value, other);
  }

  if (value === null && other === null) {
    return true;
  }

  if (typeof value !== typeof other) {
    return false;
  }

  if (value === other) {
    return true;
  }

  if (Array.isArray(value) && Array.isArray(other)) {
    if (value.length !== other.length) {
      return false;
    }

    for (let i = 0; i < value.length; i++) {
      if (!isEqual(value[i], other[i])) {
        return false;
      }
    }

    return true;
  }

  if (Array.isArray(value) || Array.isArray(other)) {
    return false;
  }

  if (Object.keys(value).length !== Object.keys(other).length) {
    return false;
  }

  for (const [k, v] of Object.entries(value)) {
    if (!(k in other)) {
      return false;
    }

    if (!isEqual(v, other[k])) {
      return false;
    }
  }

  return true;
}

Here is the version with detailed explanation

function isEqual(value, other) {
  // First handle the case where both values are primitive types.
  if (typeof value !== "object" && typeof other !== "object") {
    // we can avoid the NaN comparison issue if we check the equality of two primitive types using Object.is
    return Object.is(value, other);
  }

  // Although null is a primitive type in JavaScript, due to some historical bugs,
  // the type of null is object, so we need to additionally handling for null.
  if (value === null && other === null) {
    return true;
  }

  // After handling the case where both are primitive types,
  // we can handle one value is primitive type but the other is not.
  // If one is a primitive type and the other is an object type, then return false
  if (typeof value !== typeof other) {
    return false;
  }

  // If the above conditions are passed, it means that both values are object types,
  // So we compare the two objects first, and return true if they are from the same reference
  if (value === other) {
    return true;
  }

  // Next, check the case where both objects are arrays
  if (Array.isArray(value) && Array.isArray(other)) {
    // If the two arrays have different lengths, return false
    if (value.length !== other.length) {
      return false;
    }
    // iterate over each value in the array, then recursively compare two values with isEqual
    for (let i = 0; i < value.length; i++) {
      if (!isEqual(value[i], other[i])) {
        return false;
      }
    }

    return true;
  }

  // if one is an array, but the other is not, return false
  // Since we pass the && condition above, it means both not both values are arrays
  // here you can use || to check if one of them is an array
  if (Array.isArray(value) || Array.isArray(other)) {
    return false;
  }

  // If the above conditions are not met, the remaining possibility is that both values ​​are objects.
  // First check that the two objects have the same number of keys,
  // if not the same number means that the two objects must be different
  if (Object.keys(value).length !== Object.keys(other).length) {
    return false;
  }

  // If two objects have the same number of keys, iterate over the first object through Object.entires
  for (const [k, v] of Object.entries(value)) {
    // If a key in the first object does not exist in the second object, it means the two are different
    if (!(k in other)) {
      return false;
    }

    // If the key-value pair in the first object is different from the second, it also means that the two objects are different
    // Remember, because the value may also be an object, so use isEqual to recursively check whether the two values ​​are the same
    if (!isEqual(v, other[k])) {
      return false;
    }
  }

  return true;
}