Overview
This lecture explains how the JavaScript forEach method works, how to use callback functions with it, and demonstrates practical examples for iterating over arrays.
Using forEach Method
- The
forEach method executes a function on every element of an array.
- The callback function receives three parameters: current value (item), index, and the array itself.
- Usually, only the item and index are used in practice.
- You can define the callback as a separate function or as an inline arrow function.
Example: Logging Array Elements
- Given an array
[1, 2, 3, 4, 5], using forEach logs each element with its index.
- Example output: "a of 0 is 1", "a of 1 is 2", etc.
- The callback can be simplified to an inline arrow function for convenience.
Example: Summing Array Elements
- Initialize a sum variable to zero.
- Use
forEach with an arrow function to add each array element to the sum.
- Updated array values (e.g.,
[1,2,3,4,5,10,15,25]) are handled the same way for summing.
Example: Counting Frequency of Letters
- Create an array of letters (e.g.,
['a', 'b', 'a', 'b', 'c', 'd', 'a']).
- Use an object to keep count of how many times each letter appears.
- The callback checks if a property exists in the object and increments or initializes it.
- The result is an object with letter keys and their occurrence counts.
Key Terms & Definitions
forEach method — An array method that calls a function on each element.- Callback function — A function passed as a parameter to another function (here, used with
forEach).
- Arrow function — Concise syntax for writing functions in JavaScript.
Action Items / Next Steps
- Practice using
forEach to perform different operations on arrays.
- Experiment with defining callback functions both separately and as arrow functions inline.
- Try implementing frequency counting for other data types or structures.