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Let's create our map method

machy44 on June 22, 2018

If you want to take a closer look on how JS works and how we created our own filter function, you can see my earlier post: ...
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Adrian B.G. • • Edited

Very nice as an example, just remember is an example that doesn't fully replace the builtin function, so don't use it in production.

As a completion, here is the lodash map source code 

function map(array, iteratee) {
  let index = -1
  const length = array == null ? 0 : array.length
  const result = new Array(length)

  while (++index < length) {
    result[index] = iteratee(array[index], index, array)
  }
  return result
}

Some side notes about the differences between the custom maps:

  • it pre-allocates the result array (we already know its length) const result = new Array(length)
  • it uses const where the values don't change (like the result and length)
  • both implementation lacks the last 2 optional parameters from the builtin Map functionality
  • by not using a for loop and storing the length, is not required to fetch the this.length value for each iteration
  • it doesn't pollute the global space (lodash export the functions, and you use them as you need), and most important it doesn't "modify" all instances of arrays from the entire project, I mean for many reasons it is bad to do Array.prototype.ownMap
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machy44 •

Yes, it's only for educational purpose. Thank you for giving a good example with notes and made this post better. I will certainly take a deeper look in lodash now.

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Adrian B.G. •

Lodash is a good example of an open source library that adds a huge value in your project, especially in the pre-JS6 era.

As for its code, as with any big open-source project, it has the added value from many of its contributor expertise. Crowd anything is general better then what a small team can achieve on its own.

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machy44 •

Yap, You are right. Take a look in some popular library code and learn in that way. I will personally take that path in future. Thank you for advice.

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Israel García •

Very good example, can I use it to with my students?

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machy44 • • Edited

Of course you can. It would be a pleasure.