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Get Function Parameter Names Dynamically in JavaScript

Last Updated : 03 Nov, 2025
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In JavaScript, you can extract the parameter names of a function dynamically using string manipulation. This is useful for debugging, code analysis, or building custom frameworks that inspect functions at runtime.

  • JavaScript toString() method converts a function into its string representation.
  • You can extract parameters by isolating text between ( and ).
  • Regular expressions help remove comments, function bodies, and extra spaces.
  • Works with both regular functions and arrow functions.

[Approach]: Using toString() and Regex

JavaScript contains a method called toString() which is used to represent a function code in its string representation. This method is used to get the parameter names/values. 

  • First, get the function's code to its string equivalent using toString() method.
  • Then remove all the unnecessary codes like comments, function body, white spaces, and ES6 arrow (if any).
  • Identify the first occurrence of '(', it will be just before the starting of parameters.
  • The last character of the string will be ')' which removes all comments, function body, white spaces, and ES6 arrow.
  • Also, the last character will be just after the end of the parameters.
JavaScript 
// JavaScript program to get the function
// name/values dynamically
function getParams(func) {

    // String representation of the function code
    let str = func.toString();

    // Remove comments of the form /* ... */
    // Removing comments of the form //
    // Remove body of the function { ... }
    // removing '=>' if func is arrow function 
    str = str.replace(/\/\*[\s\S]*?\*\//g, '')
        .replace(/\/\/(.)*/g, '')
        .replace(/{[\s\S]*}/, '')
        .replace(/=>/g, '')
        .trim();

    // Start parameter names after first '('
    let start = str.indexOf("(") + 1;

    // End parameter names is just before last ')'
    let end = str.length - 1;

    let result = str.substring(start, end).split(", ");

    let params = [];

    result.forEach(element => {

        // Removing any default value
        element = element.replace(/=[\s\S]*/g, '').trim();

        if (element.length > 0)
            params.push(element);
    });

    return params;
}

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

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

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

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));

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

Advantages

  • Helps inspect and document functions automatically.
  • Useful for debugging and testing dynamic code.
  • Works without executing the function.

V

Vinod Tahelyani
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