Prefix validation
Validate a command or protocol marker before processing input.
JavaScript · Function
startsWith
String.prototype.startsWith() checks whether a string begins with a specified string and returns a boolean. An optional position lets the check begin at a later index.
How it works
Call startsWith() on a string with the text to find and, optionally, an index where the check should begin. The method compares the characters from that position, converting non-regular-expression search values to strings, and returns a boolean; the optional position defaults to zero.
Syntax
startsWith(searchString)
startsWith(searchString, position)Parameters
| Name | Type | Description |
|---|---|---|
searchString | string | The characters to search for at the start of the string. Regular expressions are rejected; other values are converted to strings. |
position | number, optional | The index where the first character of searchString is expected. Defaults to 0. |
Returns
true if the specified characters are found at the beginning from the selected position, including for an empty search string; otherwise false.
Key points
- It tests whether a string has a specified prefix.
- The result is always a boolean.
- The optional position selects where the prefix comparison starts.
- Without position, checking begins at the first character.
- An empty search string produces true.
- Regular expressions are invalid search values for this method.
Common mistakes
- Passing a regular expression as searchString throws a TypeError.
- Omitting searchString or passing undefined searches for the literal string "undefined", which is usually unintended.
- Uppercase and lowercase characters are compared distinctly.
Example: Checking string prefixes
const value = "Saturday night plans";
console.log(value.startsWith("Sat"));
console.log(value.startsWith("night", 9));
console.log(value.startsWith("sat"));Checks a prefix at the beginning of the string or from a specified starting index; matching is case-sensitive.
When to use it
Filtering by prefix
Select records or routes whose identifiers share a known leading token.
Offset comparison
Check a string from a known offset without creating a substring.
Interview questions
QWhat does String.prototype.startsWith() do?
startsWith(searchString, position) returns true when the specified characters occur at the beginning of the string starting at position; otherwise it returns false. The position defaults to 0.
Follow-up: What happens with an empty search string? It returns true, including when the requested position is at or beyond the string's effective search range.
QHow does startsWith() handle searchString?
The method converts non-regex search values to strings, but rejects a regular expression with TypeError. Thus, startsWith(42) searches for "42", while startsWith(/42/) throws.
Follow-up: Why is a regular expression rejected? The method is a literal prefix comparison rather than a pattern-matching operation.
QHow does the position argument affect the comparison?
position identifies the index where the first character of searchString is expected. It is converted to an integer-like value and clamped between zero and the string length.
Follow-up: What is the result if the prefix would extend beyond the string's end? The method returns false unless the search string is empty.
Related
String.prototype.endsWith()String.prototype.includes()String.prototype.indexOf()String.prototype.slice()