Trim After First.md

Use Case

Pseudo code:

output = "some@kind@of@weird@input".split('@')[0];

A couple possible responses you might have to seeing this kind of code a lot in the codebase you're working on:

1. "Well this is a code-smell! How can I refactor the code to not put us in this situation all the time? (And who the heck thought to do @-separated strings??)"
2. "Well that's not the most readable way to express that operation... and it creates an additional allocation! Surely there a better way??"

I can't help you with #1, unfortunately... but here's one possible way to answer #2:

JavaScript

/**
 * Returns the input string after removing the first instance of the separator and everything after it.
 * 
 * If the separator is not found, the original string is returned.
 */
String.prototype.trimAfterFirst = function(separator) {
    const separatorIndex = this.indexOf(separator);

    if(separatorIndex < 0) return this;

    return this.substring(0, separatorIndex);
};

Example Usage

const value = "this|that|the other".trimAfterFirst("|");

console.log(value); // "this"

C#

Two versions: one that takes a string separator (and a StringComparison), and another that just takes a char separator.

public static class TrimAfterFirstExtension
{
    /// <summary>
    /// Returns the input string after removing the first instance of the separator and everything after it.
    /// 
    /// If the separator is not found, the original string is returned.
    /// </summary>
    public static string TrimAfterFirst(this string input, string separator, StringComparison comparisonType = StringComparison.CurrentCulture)
    {
        var separatorIndex = input.IndexOf(separator, comparisonType);

        if(separatorIndex < 0) return input;

        return input[..separatorIndex];
    }
    
    /// <summary>
    /// Returns the input string after removing the first instance of the separator and everything after it.
    /// 
    /// If the separator is not found, the original string is returned.
    /// </summary>
    public static string TrimAfterFirst(this string input, char separator)
    {
        var separatorIndex = input.IndexOf(separator);

        if(separatorIndex < 0) return input;

        return input[..separatorIndex];
    }
}

Example Usage

var value = "this$that$the other".TrimAfterFirst('$');

Console.WriteLine(value); // "this"

Final Thoughts

What do you think about the name "trim after first X"? The name feels a little ambiguous to me - like, does that mean the first "X" found is included, or not?

• Maybe there's a better name? "take until first X", or something??
• Maybe just do the refactor to not put yourself in this position in the first place???