JS array learning: iterate through arrays, calculate the sum of elements, and concatenate values ​​into strings

In the previous article "JS Array Learning: How to Splice All Elements and Return a String", we learned about the method of converting an array into a string. Friends in need can learn about it. ~

This article will talk about the iterative operation of arrays, and introduce two methods of calculating the sum of elements and linking values ​​into strings.

Method 1. Use the forEach() method

The forEach() method is used to call each element of the array and pass the element to Callback.

Syntax:

array.forEach(funtion callbackfn(value, index, array), thisValue)

funtion callbackfn(value, index, array): Required parameters, specify the callback function, which can receive up to three parameters:

• value: The value of the array element.

• index: Numeric index of the array element.

• array: Array object containing the element.

thisValue: an omitted parameter, an object that can be referenced by this in the callback function. If thisArg is omitted, the value of this is undefined.

Let’s learn more about it through code examples:

Example 1: Output array elements

function f(value,index,array) {
    console.log("a[" + index + "] = " + value);
}
var a = ['a', 'b', 'c'];
a.forEach(f);

Example 2: Accumulating and summing array values

var a = [10, 11, 12], sum = 0;
function f(value) {
	sum += value;
}

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

##Example 3: Concatenating array values ​​into a string

var a = ['ab', 'cd', 'ef'], str = "";
function f(value) {
	// str=str + value;
	str=str.concat(value);
}

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

Method 2. Use the reduce() method

reduce() method can reduce all elements in the array Call the specified callback function. 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 a string

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

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

If you want to use a delimiter to separate each array element, you can Change the empty string between the two " " operators in the callback function to the specified delimiter:

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

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

Okay, that's it for now. If you need it, you can See:

javascript advanced tutorial

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