Get Function Parameter Names Dynamically in JavaScript

In JavaScript, you can extract the parameter names of a function dynamically by converting the function into a string and processing its contents.

• The toString() method converts a function into its string representation.
• Regular expressions can remove comments, function bodies, and default values.
• The parameter section can then be extracted and converted into an array.
• This approach works with regular functions and arrow functions.

Using toString() and Regular Expressions

The toString() method returns the source code representation of a function. We can process this string to extract the function's parameter names.

Working:

• Convert the function into a string using toString().
• Remove comments, the function body, and the arrow (=>) syntax.
• Locate the opening ( and closing ) surrounding the parameters.
• Extract the parameter section and split it using commas.
• Remove default values and empty parameters.
• Return the parameter names as an array.

function getParams(func) {

    // Convert the function to a string
    let str = func.toString();

    // Remove comments, function body and arrow syntax
    str = str.replace(/\/\*[\s\S]*?\*\//g, '')
        .replace(/\/\/(.)*/g, '')
        .replace(/{[\s\S]*}/, '')
        .replace(/=>/g, '')
        .trim();

    // Find the starting position of parameters
    let start = str.indexOf("(") + 1;

    // Find the ending position of parameters
    let end = str.length - 1;

    // Extract and split parameters
    let result = str.substring(start, end).split(",");

    let params = [];

    result.forEach(element => {

        // Remove default values
        element = element.replace(/=[\s\S]*/g, '').trim();

        // Add non-empty parameters
        if (element.length > 0) {
            params.push(element);
        }
    });

    return params;
}

// Test functions
let fun1 = function (a) { };

function fun2(a = 5 * 6 / 3, b) { }

let fun3 = (a, /*
        */
    b, // comment
    c) => { };

// Display parameter names
console.log(`List of parameters of ${fun1.name}:`,
    getParams(fun1));

console.log(`List of parameters of ${fun2.name}:`,
    getParams(fun2));

console.log(`List of parameters of ${fun3.name}:`,
    getParams(fun3));

List of parameters of fun1: [ 'a' ]
List of parameters of fun2: [ 'a', 'b' ]
List of parameters of fun3: [ 'a', 'b', 'c' ]

• func.toString() gets the function's source code as a string.
• replace() removes comments, the function body, and arrow-function syntax.
• substring() extracts the portion containing the parameters.
• split(",") separates individual parameters.
• replace() removes default values such as a = 10.
• The resulting parameter names are stored in the params array.