JavaScript indexOf() Method Explained

The indexOf() method in JavaScript is used to find the position of a specified substring within a string. It returns the index of the first matching substring, and if no match is found, it returns -1.

Grammar:

string.indexOf(searchValue[, fromIndex])

Parameters:

1. searchValue: The substring you want to search for.
2. fromIndex (optional): The index position to start the search from. If this parameter is omitted, the search will start from the beginning of the string.

Return value:

1. Return the index of the first matching substring if found.
2. If no matching substring is found, return -1.

I don’t have time to do all the laundry today.
I am too busy to finish all the laundry today.

const str = 'Hello, world!';

console.log(str.indexOf('world'));  // 输出: 7
console.log(str.indexOf('World'));  // 输出: -1，区分大小写
console.log(str.indexOf('o'));  // 输出: 4，从索引位置4开始查找第一个匹配的子字符串
console.log(str.indexOf('o', 5));  // 输出: 8，从索引位置5开始查找第一个匹配的子字符串
console.log(str.indexOf('o', 20));  // 输出: -1，从索引位置20开始查找，超出字符串长度，没有找到匹配的子字符串

Please note that the indexOf() method is case-sensitive. If you need to perform a case-insensitive search, you can use the toLowerCase() or toUpperCase() method to convert the string to lowercase or uppercase before searching.