String.prototype.substr(start, length?) returns a new string with length characters beginning at start. The second argument is a count, not an end index. Not the same as sub() (HTML wrapper) or substring() (two indexes).
Pass the required arguments shown below. Check return values and browser support before you rely on the result.
Call string.substr. Start with this line.
str.substr(start, length?)| Call | What it does |
|---|---|
str.substr(start, length?) | Primary signature. |
string.substr(...) | Call with the arguments your case needs. |
Built into JavaScript. Call substr(...) on a string (or String instance) — no import required.
str.substr(start, length?)| Parameter | Description |
|---|---|
| start | Required. Begin index; negatives count from the end |
| length | Optional. How many characters; omit → to the end; negative → "" |
| Return Value | Description |
|---|---|
| * | New string (original unchanged) |
substr()Extract by start index and optional length.
const str = "Mozilla";
console.log(str.substr(1, 2)); // "oz"
console.log(str.substr(2)); // "zilla""o"."o" and "z" → "oz".substr(2) copies from index 2 to the end → "zilla".str is still "Mozilla".Negative start counts from the end; negative length returns empty.
const str = "Mozilla";
console.log(str.substr(-3)); // "lla"
console.log(str.substr(-1, 1)); // "a"
console.log(str.substr(1, -1)); // ""
console.log(str.substr(-20, 2)); // "Mo"
console.log(str.substr(20, 2)); // ""-3 means three from the end → "lla".substr(1, -1) returns "" immediately.substr(-20, 2) still begins at the start → "Mo".substr(20, 2) has nothing to copy → "".slice()Same common cases with slice, plus the migration trap.
const str = "Mozilla";
// Common replacements
console.log(str.slice(1, 3)); // "oz" was: substr(1, 2)
console.log(str.slice(2)); // "zilla" was: substr(2)
console.log(str.slice(-3)); // "lla" was: substr(-3)
// Trap: negative length is NOT the same
const a = 1, l = -2;
const s = "01234";
console.log(s.substr(a, l)); // ""
console.log(s.slice(a, a + l)); // "123"substr(1, 2) matches slice(1, 1 + 2) → slice(1, 3).substr(i) becomes slice(i).substr(-n) becomes slice(-n).length makes substr and slice(a, a + l) disagree.String.prototype.substr() is still widely implemented for compatibility, but it is deprecated (ECMAScript Annex B). Prefer slice() for portable new code.
Available in major browsers and Node.js for legacy code. Not part of the main ECMAScript path — use slice() or substring() instead.
Bottom line: Recognize substr() when reading old code. For new extraction, use String.slice() with start/end indexes.
Extract between two indexes — and see how it differs from slice() and substr().
10 people found this page helpful