How to get the javascript function parameter names/values dynamically ?
In this article, we are given any arbitrary JavaScript function and the task is to return the parameter names of the function.
Approach: 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.
Example: This example explains the above-explained approach.
Javascript
// JavaScript program to get the function// name/values dynamicallyfunction 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 functionslet 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' ]
