Check whether user input contains a required token
Validate input
JavaScript · Function
includes
String.prototype.includes() performs a case-sensitive substring search and reports whether the requested text occurs in a string. It returns a boolean and accepts an optional starting position.
How it works
Call includes() on a string with the text to find, optionally providing the zero-based position where searching should start. It returns a boolean and compares characters case-sensitively; the starting position is converted and constrained to the string’s bounds.
Syntax
includes(searchString)
includes(searchString, position)Parameters
| Name | Type | Description |
|---|---|---|
searchString | string | The string to look for within the receiver. It must not be a regular expression; other values are converted to strings. |
position | number | The zero-based position at which to begin searching. It defaults to 0. |
Returns
A boolean: true if the search string occurs at or after the starting position; otherwise false.
Key points
- It checks whether the receiver contains the requested substring using case-sensitive matching.
- The result is a boolean:
truewhen the substring is found andfalseotherwise. - An optional position lets you skip earlier characters when starting the search.
- If no starting position is supplied, searching begins at index zero.
- An empty search string returns
true, including when the source string itself is empty. - Search arguments other than regular expressions are converted to strings before matching.
Common mistakes
- The search value cannot be a regular expression; passing one throws a TypeError.
- Omitting the search argument or explicitly passing
undefinedsearches for the literal string "undefined". - Different letter casing does not match; normalize both strings first when a case-insensitive check is needed.
Example: Checking whether a string contains text
const text = "To be, or not to be, that is the question.";
console.log(text.includes("question")); // true
console.log(text.includes("TO BE")); // false
console.log(text.includes("be", 10)); // false
console.log(text.includes("")); // trueThe method performs a case-sensitive substring check and can begin searching at the supplied position.
When to use it
Keep strings that contain a specific word or phrase
Filter text
Choose a branch based on substring presence
Guard conditional logic
Ignore a prefix by starting at a later position
Search from an offset
Interview questions
QWhat does String.prototype.includes() return?
String.prototype.includes() returns a boolean indicating whether a string contains the supplied substring. The comparison is case-sensitive, and an empty search string is considered present.
Follow-up: What happens when the search argument is a regular expression? It throws a TypeError rather than performing a pattern search.
QHow does the position argument affect the search?
The optional position argument specifies where the search begins and defaults to zero. It is converted to an integer-like value and constrained to the string’s valid range.
Follow-up: Does a position beyond the string length match an empty search string? Yes, because the empty string is found at the effective endpoint.
QHow are non-string search arguments handled?
The method coerces non-regexp search values to strings. If the argument is omitted or is undefined, the searched value becomes the string "undefined".
Follow-up: How can a case-insensitive search be implemented? Convert both the source and search strings to the same case before calling includes().
Related
String.prototype.endsWith()String.prototype.indexOf()String.prototype.match()String.prototype.startsWith()