Creating a Module and Using the Function
In this final step, we will convert our function into a proper JavaScript module that can be imported and used in other files. This is a common practice in real-world JavaScript development.
First, let us create a module file for our function. Create a new file called replaceLastModule.js in the replace-last directory:
/**
* Replaces the last occurrence of a pattern in a string.
*
* @param {string} str - The input string.
* @param {string|RegExp} pattern - The pattern to replace (string or RegExp).
* @param {string} replacement - The replacement string.
* @returns {string} - The string with the last occurrence replaced.
*/
function replaceLast(str, pattern, replacement) {
// Ensure str is a string
if (typeof str !== "string") {
return str;
}
// If str is empty or pattern is not provided, return original string
if (str === "" || pattern === undefined) {
return str;
}
// Ensure replacement is a string
if (replacement === undefined) {
replacement = "";
} else if (typeof replacement !== "string") {
replacement = String(replacement);
}
// Handle string patterns
if (typeof pattern === "string") {
const lastIndex = str.lastIndexOf(pattern);
if (lastIndex === -1) {
return str;
}
return (
str.slice(0, lastIndex) +
replacement +
str.slice(lastIndex + pattern.length)
);
}
// Handle regular expression patterns
if (pattern instanceof RegExp) {
// Create a global version of the regex to find all matches
const globalRegex = new RegExp(
pattern.source,
"g" + (pattern.ignoreCase ? "i" : "") + (pattern.multiline ? "m" : "")
);
// Find all matches
const matches = str.match(globalRegex);
// If no matches, return original string
if (!matches || matches.length === 0) {
return str;
}
// Get the last match
const lastMatch = matches[matches.length - 1];
// Find the position of the last match
const lastIndex = str.lastIndexOf(lastMatch);
// Rebuild the string with the replacement
return (
str.slice(0, lastIndex) +
replacement +
str.slice(lastIndex + lastMatch.length)
);
}
// If pattern is neither string nor RegExp, return original string
return str;
}
// Export the function
module.exports = replaceLast;
Now, let us create another file to use our module. Create a new file called app.js in the replace-last directory:
// Import the replaceLast function
const replaceLast = require("./replaceLastModule");
// Examples of using the replaceLast function
console.log(
"Example 1:",
replaceLast("Hello world world", "world", "JavaScript")
);
console.log("Example 2:", replaceLast("abcabcabc", "abc", "123"));
console.log("Example 3:", replaceLast("file.txt.backup.txt", ".txt", ".md"));
console.log("Example 4:", replaceLast("123 456 789", /\d+/, "numbers"));
console.log(
"Example 5:",
replaceLast("The fox jumped over the lazy dog", /[a-z]+/i, "cat")
);
// Practical examples
const filePath = "/path/to/my/file.txt";
console.log("File with new extension:", replaceLast(filePath, ".txt", ".md"));
const url = "https://example.com/products/category/item?color=red";
console.log("URL with updated parameter:", replaceLast(url, "red", "blue"));
const htmlTag = "<div class='container'><p>Text</p></div>";
console.log(
"HTML with replaced tag:",
replaceLast(htmlTag, /<\/?\w+>/g, "<span>")
);
Run the app to see how the module works:
node app.js
You should see the output with all the examples demonstrating how the replaceLast function can be used in various scenarios.
Congratulations. You have successfully created a useful JavaScript utility function and packaged it as a module that can be reused across your projects.
