Update: Was curious around the performance of different solutions and so I've updated a basic benchmark here: https://www.measurethat.net/Benchmarks/Show/12738/0/trimming-leadingtrailing-characters

Some interesting and unexpected results running under Chrome. https://www.measurethat.net/Benchmarks/ShowResult/182877

+-----------------------------------+-----------------------+
| Test name                         | Executions per second |
+-----------------------------------+-----------------------+
| Index Version (Jason Larke)       | 949979.7 Ops/sec      |
| Substring Version (Pho3niX83)     | 197548.9 Ops/sec      |
| Regex Version (leaf)              | 107357.2 Ops/sec      |
| Boolean Filter Version (mbaer3000)| 94162.3 Ops/sec       |
| Spread Version (Robin F.)         | 4242.8 Ops/sec        |
+-----------------------------------+-----------------------+

Please note; tests were carried out on only a single test string (with both leading and trailing characters that needed trimming). In addition, this benchmark only gives an indication of raw speed; other factors like memory usage are also important to consider.

If you're dealing with longer strings I believe this should outperform most of the other options by reducing the number of allocated strings to either zero or one:

function trim(str, ch) {
    var start = 0, 
        end = str.length;

    while(start < end && str[start] === ch)
        ++start;

    while(end > start && str[end - 1] === ch)
        --end;

    return (start > 0 || end < str.length) ? str.substring(start, end) : str;
}

// Usage:
trim('|hello|world|', '|'); // => 'hello|world'

Or if you want to trim from a set of multiple characters:

function trimAny(str, chars) {
    var start = 0, 
        end = str.length;

    while(start < end && chars.indexOf(str[start]) >= 0)
        ++start;

    while(end > start && chars.indexOf(str[end - 1]) >= 0)
        --end;

    return (start > 0 || end < str.length) ? str.substring(start, end) : str;
}

// Usage:
trimAny('|hello|world   ', [ '|', ' ' ]); // => 'hello|world'
// because '.indexOf' is used, you could also pass a string for the 2nd parameter:
trimAny('|hello| world  ', '| '); // => 'hello|world'

EDIT: For fun, trim words (rather than individual characters)

// Helper function to detect if a string contains another string
//     at a specific position. 
// Equivalent to using `str.indexOf(substr, pos) === pos` but *should* be more efficient on longer strings as it can exit early (needs benchmarks to back this up).
function hasSubstringAt(str, substr, pos) {
    var idx = 0, len = substr.length;

    for (var max = str.length; idx < len; ++idx) {
        if ((pos + idx) >= max || str[pos + idx] != substr[idx])
            break;
    }

    return idx === len;
}

function trimWord(str, word) {
    var start = 0,
        end = str.length,
        len = word.length;

    while (start < end && hasSubstringAt(str, word, start))
        start += word.length;

    while (end > start && hasSubstringAt(str, word, end - len))
        end -= word.length

    return (start > 0 || end < str.length) ? str.substring(start, end) : str;
}

// Usage:
trimWord('blahrealmessageblah', 'blah');