Reduce.md

reduce() method

The reduce() method in JavaScript applies a function against an accumulator and each element in the array (from left to right) to reduce it to a single value.

Here's how it works:

1. It applies a function against an accumulator and each element in the array to reduce it to a single value.
2. The provided function is called with four arguments: the accumulator, the current element, the index of the current element, and the array reduce() was called upon.

Here's a simple example:

const numbers = [1, 2, 3, 4, 5];

const sum = numbers.reduce(function(accumulator, currentValue) {
    return accumulator + currentValue;
}, 0);

console.log(sum); // Output: 15

In this example, reduce() is called on the numbers array and returns a single value sum which is the sum of all numbers in the array. The second argument to reduce() (0 in this case) is the initial value of the accumulator.

Syntax

The syntax for the reduce() method in JavaScript is as follows:

array.reduce(callbackFunc(accumulator, currentValue, index, arr), initialValue)

Here's what each parameter means:

• callbackFunc(accumulator, currentValue, index, arr): A function to be called for each element in the array. This function accepts four arguments:
  • accumulator: The accumulator accumulates the callback's return values.
  • currentValue: The current element being processed in the array.
  • index (optional): The index of the current element being processed in the array.
  • arr (optional): The array reduce was called upon.
• initialValue (optional): A value to use as the first argument to the first call of the callback. If no initial value is supplied, the first element in the array will be used.

The reduce() method returns the value that results from the reduction.

Polyfill

const numbers = [1, 2, 3, 4, 5];

Array.prototype.reduce = function(callbackFunc, initialVal){
    let accumulator = (initialVal === undefined) ? undefined : initialVal;
    for (let i = 0; i < this.length; i++) {
      if (accumulator !== undefined)
        accumulator = callbackFunc.call(undefined, accumulator, this[i], i, this);
        // this can be simplified to: accumulator = callbackFunc(accumulator, this[i])
      else
        accumulator = this[i];
    }
    return accumulator;
}

console.log(numbers.reduce(sum)) //  Output: 15