DevEquiv

    PHP substr() Equivalent in JavaScript

    JavaScript's String.prototype.slice() is the modern equivalent of PHP's substr() — note that JavaScript also has its own substr(), but it's deprecated and should be avoided in new code.

    Close equivalent

    PHP source
    substr()
    Closest JavaScript API
    String.prototype.slice()

    Main limitation: PHP's `substr($str, $start, $length)` third argument is a *length*; JavaScript's `.slice(start, end)` second argument is an *end index* — `substr($s, 2, 3)` becomes `s.slice(2, 5)`, not `s.slice(2, 3)`.

    How to use each function

    Before comparing the two, here's how substr() and String.prototype.slice() work on their own.

    PHP: substr()

    substr(string $string, int $offset, ?int $length = null): string

    Extracts a portion of a string starting at $offset (negative counts from the end) and continuing for $length characters (negative trims that many characters off the end instead). Never raises an error for out-of-range offsets — it returns an empty or truncated string instead.

    Returns: The extracted substring.

    PHP 
    substr("hello world", 6, 5); // "world"
substr("hello world", -5); // "world"

    JavaScript: String.prototype.slice()

    str.slice(start, end)

    Extracts a substring from start up to, but not including, end — an end position, not a length like PHP's substr() third argument. Negative indices count from the end of the string. JavaScript also has a same-named .substr(start, length), but it's deprecated — prefer .slice().

    Returns: The extracted substring.

    JavaScript 
    "hello world".slice(6, 11); // "world"
"hello world".slice(-5);    // "world"

    Quick mapping

    PHP substr()
    Closest JavaScript API String.prototype.slice()

    How it differs

    • PHP's substr($str, $start, $length) third argument is a *length*; JavaScript's .slice(start, end) second argument is an *end index* — substr($s, 2, 3) becomes s.slice(2, 5), not s.slice(2, 3).
    • JavaScript also ships a same-named String.prototype.substr(start, length) that *does* take a length like PHP, but it's marked deprecated (legacy) in the spec — prefer .slice() for new code even though .substr() looks like a more direct name match.
    • Both substr() and .slice() support negative start indices to count from the end of the string, with matching semantics.

    Examples

    Basic substring by length

    PHP 
    substr("hello world", 6, 5);
// "world"
    JavaScript 
    "hello world".slice(6, 11);
// "world"
    PHP's length (5) becomes JavaScript's end index (6 + 5 = 11).

    Negative start (from the end)

    PHP 
    substr("hello world", -5);
// "world"
    JavaScript 
    "hello world".slice(-5);
// "world"
    Negative start indices behave identically in both languages.

    Edge cases

    Edge case

    Don't reach for JavaScript's .substr() just because the name matches PHP's substr() — it's deprecated; use .slice() and adjust the second argument from a length to an end index.

    Edge case

    .slice() with a negative end argument means "stop that many characters from the end" — e.g. "hello".slice(1, -1) is "ell" — this has no direct PHP substr() parallel using a single call, since PHP's negative length works differently (counts back from the end for where to *stop*, similar in effect but derived differently).

    References

    • PHP Manual — substr — checked 2026-07-29
    • MDN Web Docs — String.prototype.slice() — checked 2026-07-29

    Last verified 2026-07-29.