I wrote quite a long post on that topic (https://blog.ditectrev.com/blog/software-development/web-development/frontend/javascript/slice-vs-substring-vs-substr-complete-javascript-string-methods-comparison), but below is a basic summary:
| Feature | `slice()` / `String.prototype.slice()` | `substring()` / `String.prototype.substring()` | `substr()` / `String.prototype.substr()` | | --- | --- | --- | --- | | **Syntax** | `slice(start, end)` | `substring(start, end)` | `substr(start, length)` | | **Second Parameter** | End index (exclusive) | End index (exclusive) | Length of substring | | **Negative Index** | Supported (counts from end) | Not supported (treated as 0) | Supported (start only) | | **Swaps Arguments** | No | Yes (if start \> end) | No | | **Start \> End** | Returns empty string | Swaps arguments | N/A (second param is length, not end index) | | **ECMAScript Version** | ES3+ (modern standard) | ES1 (legacy) | Not part of standard | | **Specification** | Part of official JavaScript spec | Part of official JavaScript spec | Not part of ECMAScript standard | | **Array API Consistency** | Consistent with `Array.slice()` | N/A | N/A | | **NaN Handling** | NaN treated as 0 | NaN treated as 0 | NaN treated as 0 | | **Type Coercion** | Same for all: coerces non-numbers to numbers (NaN/undefined/null → 0) | Same for all: coerces non-numbers to numbers (NaN/undefined/null → 0) | Same for all: coerces non-numbers to numbers (NaN/undefined/null → 0) | | **Status** | Recommended ✅ | Legacy ⚠️ | Deprecated ❌ |

I wrote quite a long post on that topic (https://blog.ditectrev.com/blog/software-development/web-development/frontend/javascript/slice-vs-substring-vs-substr-complete-javascript-string-methods-comparison), but below is a basic summary:
| Feature | `slice()` / `String.prototype.slice()` | `substring()` / `String.prototype.substring()` | `substr()` / `String.prototype.substr()` | | --- | --- | --- | --- | | **Syntax** | `slice(start, end)` | `substring(start, end)` | `substr(start, length)` | | **Second Parameter** | End index (exclusive) | End index (exclusive) | Length of substring | | **Negative Index** | Supported (counts from end) | Not supported (treated as 0) | Supported (start only) | | **Swaps Arguments** | No | Yes (if start \> end) | No | | **Start \> End** | Returns empty string | Swaps arguments | N/A (second param is length, not end index) | | **ECMAScript Version** | ES3+ (modern standard) | ES1 (legacy) | Not part of standard | | **Specification** | Part of official JavaScript spec | Part of official JavaScript spec | Not part of ECMAScript standard | | **Array API Consistency** | Consistent with `Array.slice()` | N/A | N/A | | **NaN Handling** | NaN treated as 0 | NaN treated as 0 | NaN treated as 0 | | **Type Coercion** | Same for all: coerces non-numbers to numbers (NaN/undefined/null → 0) | Same for all: coerces non-numbers to numbers (NaN/undefined/null → 0) | Same for all: coerces non-numbers to numbers (NaN/undefined/null → 0) | | **Status** | Recommended ✅ | Legacy ⚠️ | Deprecated ❌ |