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JavaScript Recursion

NOTE this is from freeCodeCamp (these are my personal notes)

Recursion is the concept that a function can be expressed in terms of itself. To help understand this, start by thinking about the following task: multiply the first n elements of an array to create the product of those elements. Using a for loop, you could do this:

  function multiply(arr, n) {
    var product = 1;
    for (var i = 0; i < n; i++) {
        product *= arr[i];
    }
    return product;
  }

However, notice that multiply(arr, n) == multiply(arr, n - 1) * arr[n - 1]. That means you can rewrite multiply in terms of itself and never need to use a loop.

  function multiply(arr, n) {
    if (n <= 0) {
      return 1;
    } else {
      return multiply(arr, n - 1) * arr[n - 1];
    }
  }

The recursive version of multiply breaks down like this. In the base case, where n <= 0, it returns 1. For larger values of n, it calls itself, but with n - 1. That function call is evaluated in the same way, calling multiply again until n <= 0. At this point, all the functions can return and the original multiply returns the answer.

Note: Recursive functions must have a base case when they return without calling the function again (in this example, when n <= 0), otherwise they can never finish executing.

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Matt is the author of five Apress books including Learn RStudio IDE, Quick, Effective, and Productive Data Science, Objective-C Recipes, Swift Quick Syntax Reference, Objective-C Quick Reference, and the upcoming Pro Data Visualization with R and JavaScript. He has over 20 years of experience in technology, psychometrics, and data analytics working in major higher education institutions such as The College Board and Educational Testing Service. He has earned a Master’s degree in Information Systems Management and a Bachelor’s degree in Quantitative Psychology.