String.prototype.indexOf() - JavaScript | MDN

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coerced to strings

Substring to search for. All values are coerced to strings, so omitting it or passing undefined causes indexOf() to search for the string "undefined", which is rarely what you want.

indexOf() and case-sensitivity

The following example defines two string variables.

String.prototype.charAt()

String.prototype.lastIndexOf()

String.prototype.includes()

String.prototype.split()

Array.prototype.indexOf()

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This page was last modified on Jul 10, 2025 by MDN contributors.

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[Symbol.iterator]()

[Symbol.hasInstance]()

__defineGetter__()

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__lookupGetter__()

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