What does the reduce() method in javascript do?

In JavaScript, the reduce() method is used to iterate (accumulate) array elements. It will call the specified callback function as an accumulator for all elements in the array. Each value in the array (from left to the right) begins to reduce and finally calculates to a value.

The operating environment of this tutorial: windows7 system, javascript version 1.8.5, Dell G3 computer.

In JavaScript, the reduce() method is used to iterate (accumulate) array elements. This method receives a function as an accumulator. Each value in the array (from left to right) begins to reduce, and finally Computes to a value.

The reduce() method can call the specified callback function for all elements in the array. The return value of this callback function is the cumulative result, and this return value is provided as a parameter the next time the callback function is called.

Syntax:

array.reduce(function callbackfn(previousValue, currentVaule, currentIndex, array), initialValue)

function callbackfn(previousValue, currentVaule, currentIndex, array): Required parameters, specify the callback function, at most Can receive 4 parameters:

• previousValue: the value obtained by calling the callback function last time. If initialValue is provided to the reduce() method, the previousValue is initialValue when the function is first called.

• currentValue: The value of the current element array.

• currentIndex: The numeric index of the current array element.

• array: Array object containing the element.

initialValue: Omissible parameter, initial value passed to the function.

Let’s learn more about it through code examples:

Example 1: Accumulate and sum array values

var a = [11, 12, 13], sum = 0;
function f(pre,curr) {
	sum=pre+curr;
	return sum;
}

a.reduce(f);
console.log(sum);

Output results:

36

Example 2: Concatenate array values ​​into strings

var a = [11, 12, 13], str = '';
function f(pre,curr) {
	str=pre+'-'+curr;
	return str;
}

a.reduce(f);
console.log(str);

[Recommended learning: javascript advanced tutorial]

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