How to trim a string to N chars in Javascript?

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byNikita Barsukov·Feb 22, 2025

To trim a string to N characters in JavaScript, you can use slice(0, N). If the length exceeds N, consider appending "..." to indicate truncation:

const trimString = (s, N) => s.length > N ? s.slice(0, N) + "..." : s;

You can then call trimString(yourString, N) to obtain the desired trimmed output. The slice method uses two arguments: the start index (inclusive) and the end index (exclusive). Remember, JavaScript allows slice to handle negative indexes—pretty neat for any "back to front" scenarios.

Exploring trim methods

JavaScript provides several handy methods for trimming strings:

substring: The precise cutter

JavaScript substring method helps us extract a portion of a string. It's like a surgeon's scalpel, ready to excise precisely what we need:

let word = "Hello, World!";
word.substring(0, 5);  // Why hello there! 👋

slice: End trimming and negative indexing

The slice method is handy for working with end trimming or negative indexing. It slices off what we need like a culinary chef:

word.slice(-5);  // Pump up the "orld!" 🌍

Short string? No problem!

For those times when your string is shorter than the desired trimming length, here's a method to handle these cases:

const reliableTrim = (s, N) => s.length > N ? s.slice(0, N) : s;

Custom methods

You can create custom methods by extending String.prototype. This is akin to adding tools to Batman's utility belt—powerful but use with caution:

if (!String.prototype.trimToLength) {
  String.prototype.trimToLength = function(N) { 
    return this.length > N ? this.slice(0, N) + "..." : this;
  };
}

Trimming precautions and tricks

Param validation

Cover your bases by validating your parameters—leave no room for sneaky surprises:

const robustTrim = (s, N) => {
  if (typeof N !== 'number' || N < 0) {
    throw new Error("Invalid trim length. You gotta give me a real number, not a Pikachu!");
  }
  return s.length > N ? s.slice(0, N) + "..." : s;
};

Strings are immutable

A golden rule in JavaScript—strings are immutable. Any slice or substring operation doesn't modify the original string; it just gives you a fresh one, like new socks in the morning.

substr vs. substring

Confusing these two could be worse than mixing up your twins' names on picture day. Remember, substr is deprecated and should be avoided in favor of slice or substring.

Practical tips and advanced usages

Dealing with Unicode and emojis

Unicode or emoji in your string? Careful! You may want to consider grapheme boundaries to prevent half-moon situations 🌓. You'll need a library like Intl.Segmenter for this, though.