locutus/esm/perl/core/substr.js

Locutus other languages' standard libraries to JavaScript for fun and educational purposes

export function substr(str, offset, length) {
    //      discuss at: https://locutus.io/perl/substr/
    // parity verified: Perl 5.40
    //     original by: Kevin van Zonneveld (https://kvz.io)
    //          note 1: Perl's offset is 0-based, negative counts from end
    //       example 1: substr('hello', 0, 3)
    //       returns 1: 'hel'
    //       example 2: substr('hello', 1)
    //       returns 2: 'ello'
    //       example 3: substr('hello', -2)
    //       returns 3: 'lo'
    str = String(str);
    const len = str.length;
    // Handle negative offset (count from end)
    let start = offset < 0 ? len + offset : offset;
    // Clamp start to valid range
    if (start < 0) {
        start = 0;
    }
    if (start > len) {
        return '';
    }
    // If length is undefined, return rest of string
    if (length === undefined) {
        return str.substring(start);
    }
    // Handle negative length (leave that many chars at end)
    let end;
    if (length < 0) {
        end = len + length;
        if (end < start) {
            return '';
        }
    }
    else {
        end = start + length;
    }
    return str.substring(start, end);
}