String.
Returns the index of the first occurrence of the given text in a string, or -1 when it is not there.
- Returns
- The index of the first occurrence (counted from
0), or-1when the text was not found. - Changes the original
- No
Example
#const text = 'Learn HTML, then CSS, then JavaScript'
console.log(text.indexOf('HTML'))
console.log(text.indexOf('then'))
console.log(text.lastIndexOf('then'))
console.log(text.indexOf('Python'))Definition and usage
#The indexOf() method looks for a piece of text and returns the position of its first character. Indexes start at 0. When the text is not found in the string, the result is -1, so compare the result against -1 instead of treating it as a boolean.
The second argument sets the index where the search starts, which lets you find successive occurrences in a loop. The sibling method lastIndexOf() searches the text from the end and returns the position of the last occurrence.
When you only need a yes-or-no answer, includes() is more readable. indexOf() comes in handy when you need the position for something else, such as cutting out a piece with slice().
Syntax
#string.indexOf(searchString)
string.indexOf(searchString, position)
string.lastIndexOf(searchString)Parameters
#searchString
string
The text to search for. Letter case matters.position
number, optional
The index where the search starts. Defaults to0.
More examples
#const title = 'JavaScript'
if (title.indexOf('Java')) {
console.log('Found')
} else {
console.log('Not found')
}
console.log(title.indexOf('Java') !== -1)indexOf() returned 0, and zero is falsy, so the condition fell into the else branch. Always compare the result against -1, or use includes().
const log = 'ok, error, ok, ok, error'
let count = 0
let position = log.indexOf('error')
while (position !== -1) {
count++
position = log.indexOf('error', position + 1)
}
console.log(count)const email = 'mia@example.com'
const at = email.indexOf('@')
console.log(email.slice(0, at))
console.log(email.slice(at + 1))Browser support
#Widely available
Works in all modern browsers, phones included. You can use it without worry.
- Chrome
- Edge
- Firefox
- Safari
Good practices
#- Never check the result with
if (text.indexOf(x)). A match found right at the start gives0, which is falsy. Compare against-1instead. - The method is case-sensitive. To search without case sensitivity, lowercase both strings first.
Related entries
#- String.prototype.includes()Checks whether a string contains the given piece of text. Returns true or false, and is case-sensitive.
- String.prototype.slice()Returns a portion of a string from a start index to an end index (not included). Supports negative indexes.
- String.prototype.match()Matches a string against a regular expression. Returns an array with the match or all matches, or null.
- String.prototype.at()Returns the character at the given index. Negative indexes count from the end, so at(-1) is the last character.
- Array.prototype.indexOf()Returns the index of the first occurrence of a value, or -1.
- String.prototype.lastIndexOf()Returns the index of the last occurrence of the given text in a string, or -1 when it is not there.
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