JavaScript Glossary - Lesson #22

JavaScript String .indexOf() Method

This method checks for the first appearance of a provided string argument within the calling string and returns the index. It returns -1 if the string argument can’t be found within the calling String.

""Hello and welcome to the glossary, the end"".indexOf(""the"")
// 21
// New String
let sentence = "find the index of the first occurrence of me in this sentence"

const keyWord = "me"

let keyWordIndex = sentence.indexOf(keyWord)

console.log(keyWordIndex) // 42

The indexOf() method:

  • Takes a string argument
  • Checks if the provided argument exists within the calling string object
  • Returns the index of the first appearance of the provided argument, -1 if it can’t be found.
const indexOfWord = string.indexOf(searchParam, startIndex)

searchParam This is the parameter to search for within the string - required

startIndex This is the index at which the search will begin. Search typically begins from the arr.length - 1 index, which is the last position in the string. If the value provided is less that 0, the string will not be searched.

This method returns a number which is the index of the first occurence of the searched string.

The indexOf() method is case sensitive

The .indexOf() method is commonly used to verify the first occurence and position of a word in a sentence.

// New sentence
const sentence = 'Friday is the last day';

// Keyword to be searched for
const day = 'Friday';

// Find index of keyword
const hasDay = sentence.indexOf(day);

// log output
console.log(hasDay); // 0 

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Chris Sev

Chris Sev

Chris Sev is the co-founder of Better Dev. Coding better every day. Previously he created which was acquired.


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