String.indexOf() — run online

String

String.indexOf()

Returns the index of the first occurrence of a substring, or -1 if not found

→ numberPHP: strpos()ES1+

stringstring

Source string

searchValuestring

Substring to search for

fromIndexnumberoptional

Position to start searching from

(string).indexOf(searchValue, fromIndex)

RESULT

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Parameter Reference

Parameter	Type	Status	Description
string	string	required	Source string
searchValue	string	required	Substring to search for
fromIndex	number	optional	Position to start searching from

About

indexOf() returns the position of the first occurrence of a substring, or -1 if not found. The optional fromIndex specifies where to start. Case-sensitive. For a simple presence check, includes() is more readable and does not require comparing with -1.

Browser Support

Available since ES1 (1997). In ES7 (2016), includes() was added as a more readable alternative when position is not needed.

Tips & Gotchas

For a presence check, includes() is more readable than indexOf() !== -1
fromIndex can be negative in lastIndexOf()
to find all occurrences, use a loop or matchAll() with a RegExp.