JavaScript: Apply a function against an accumulator and each key in the object

JavaScript fundamental (ES6 Syntax): Exercise-118 with Solution

Write a JavaScript program to apply a function against an accumulator and each key in the object (from left to right).

• Use Object.keys() to iterate over each key in the object.
• Use Array.prototype.reduce() to apply the specified function against the given accumulator.

Sample Solution:

JavaScript Code:

//#Source https://bit.ly/2neWfJ2

// Define the 'transform' function.
const transform = (obj, fn, acc) => 
  // Use Object.keys to iterate over the keys of the object.
  Object.keys(obj).reduce(
    // Use reduce to apply the transformation function to each key-value pair.
    (a, k) => fn(a, obj[k], k, obj), // Call the transformation function with accumulator, value, key, and object.
    acc // Set the initial value of the accumulator.
  );

// Example usage: Transform an object using a custom function.
console.log(transform(
  { a: 1, b: 2, c: 1 }, // Input object
  // Transformation function: Group keys by their corresponding values.
  (r, v, k) => {
    // Initialize the array for the current value if it doesn't exist in the accumulator.
    (r[v] || (r[v] = [])).push(k);
    return r; // Return the updated accumulator.
  },
  {} // Initial value of the accumulator.
));
// Output: { '1': ['a', 'c'], '2': ['b'] }

Output:

{"1":["a","c"],"2":["b"]}

Flowchart:

Live Demo:

See the Pen javascript-basic-exercise-118-1 by w3resource (@w3resource) on CodePen.

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