How to get function parameter names/values dynamically?

Question

Is there a way to get the function parameter names of a function dynamically?

Let’s say my function looks like this:

function doSomething(param1, par         


        

Answer 1

Since JavaScript is a scripting language, I feel that its introspection should support getting function parameter names. Punting on that functionality is a violation of first principles, so I decided to explore the issue further.

That led me to this question but no built-in solutions. Which led me to this answer which explains that arguments is only deprecated outside the function, so we can no longer use myFunction.arguments or we get:

TypeError: 'caller', 'callee', and 'arguments' properties may not be accessed on strict mode functions or the arguments objects for calls to them

Time to roll up our sleeves and get to work:

⭐ Retrieving function parameters requires a parser because complex expressions like 4*(5/3) can be used as default values. So Gaafar's answer or James Drew's answer are so far the best approaches.

I tried the babylon and esprima parsers but unfortunately they can't parse standalone anonymous functions, as pointed out in Mateusz Charytoniuk's answer. I figured out another workaround though by surrounding the code in parentheses, so as not to change the logic:

const ast = parser.parse("(\n" + func.toString() + "\n)")

The newlines prevent issues with // (single-line comments).

⭐ If a parser is not available, the next-best option is to use a tried-and-true technique like Angular.js's dependency injector regular expressions. I combined a functional version of Lambder's answer with humbletim's answer and added an optional ARROW boolean for controlling whether ES6 fat arrow functions are allowed by the regular expressions.

---

Here are two solutions I put together. Note that these have no logic to detect whether a function has valid syntax, they only extract the arguments. This is generally ok since we usually pass parsed functions to getArguments() so their syntax is already valid.

I will try to curate these solutions as best I can, but without effort from the JavaScript maintainers, this will remain an open problem.

Node.js version (not runnable until StackOverflow supports Node.js):

const parserName = 'babylon';
// const parserName = 'esprima';
const parser = require(parserName);

function getArguments(func) {
    const maybe = function (x) {
        return x || {}; // optionals support
    }

    try {
        const ast = parser.parse("(\n" + func.toString() + "\n)");
        const program = parserName == 'babylon' ? ast.program : ast;

        return program
            .body[0]
            .expression
            .params
            .map(function(node) {
                return node.name || maybe(node.left).name || '...' + maybe(node.argument).name;
            });
    } catch (e) {
        return []; // could also return null
    }
};

////////// TESTS //////////

function logArgs(func) {
	let object = {};

	object[func] = getArguments(func);

	console.log(object);
// 	console.log(/*JSON.stringify(*/getArguments(func)/*)*/);
}

console.log('');
console.log('////////// MISC //////////');

logArgs((a, b) => {});
logArgs((a, b = 1) => {});
logArgs((a, b, ...args) => {});
logArgs(function(a, b, ...args) {});
logArgs(function(a, b = 1, c = 4 * (5 / 3), d = 2) {});
logArgs(async function(a, b, ...args) {});
logArgs(function async(a, b, ...args) {});

console.log('');
console.log('////////// FUNCTIONS //////////');

logArgs(function(a, b, c) {});
logArgs(function() {});
logArgs(function named(a, b, c) {});
logArgs(function(a /* = 1 */, b /* = true */) {});
logArgs(function fprintf(handle, fmt /*, ...*/) {});
logArgs(function(a, b = 1, c) {});
logArgs(function(a = 4 * (5 / 3), b) {});
// logArgs(function (a, // single-line comment xjunk) {});
// logArgs(function (a /* fooled you {});
// logArgs(function (a /* function() yes */, \n /* no, */b)/* omg! */ {});
// logArgs(function ( A, b \n,c ,d \n ) \n {});
logArgs(function(a, b) {});
logArgs(function $args(func) {});
logArgs(null);
logArgs(function Object() {});

console.log('');
console.log('////////// STRINGS //////////');

logArgs('function (a,b,c) {}');
logArgs('function () {}');
logArgs('function named(a, b, c) {}');
logArgs('function (a /* = 1 */, b /* = true */) {}');
logArgs('function fprintf(handle, fmt /*, ...*/) {}');
logArgs('function( a, b = 1, c ) {}');
logArgs('function (a=4*(5/3), b) {}');
logArgs('function (a, // single-line comment xjunk) {}');
logArgs('function (a /* fooled you {}');
logArgs('function (a /* function() yes */, \n /* no, */b)/* omg! */ {}');
logArgs('function ( A, b \n,c ,d \n ) \n {}');
logArgs('function (a,b) {}');
logArgs('function $args(func) {}');
logArgs('null');
logArgs('function Object() {}');

Full working example:

https://repl.it/repls/SandybrownPhonyAngles

Browser version (note that it stops at the first complex default value):

function getArguments(func) {
    const ARROW = true;
    const FUNC_ARGS = ARROW ? /^(function)?\s*[^\(]*\(\s*([^\)]*)\)/m : /^(function)\s*[^\(]*\(\s*([^\)]*)\)/m;
    const FUNC_ARG_SPLIT = /,/;
    const FUNC_ARG = /^\s*(_?)(.+?)\1\s*$/;
    const STRIP_COMMENTS = /((\/\/.*$)|(\/\*[\s\S]*?\*\/))/mg;

    return ((func || '').toString().replace(STRIP_COMMENTS, '').match(FUNC_ARGS) || ['', '', ''])[2]
        .split(FUNC_ARG_SPLIT)
        .map(function(arg) {
            return arg.replace(FUNC_ARG, function(all, underscore, name) {
                return name.split('=')[0].trim();
            });
        })
        .filter(String);
}

////////// TESTS //////////

function logArgs(func) {
	let object = {};

	object[func] = getArguments(func);

	console.log(object);
// 	console.log(/*JSON.stringify(*/getArguments(func)/*)*/);
}

console.log('');
console.log('////////// MISC //////////');

logArgs((a, b) => {});
logArgs((a, b = 1) => {});
logArgs((a, b, ...args) => {});
logArgs(function(a, b, ...args) {});
logArgs(function(a, b = 1, c = 4 * (5 / 3), d = 2) {});
logArgs(async function(a, b, ...args) {});
logArgs(function async(a, b, ...args) {});

console.log('');
console.log('////////// FUNCTIONS //////////');

logArgs(function(a, b, c) {});
logArgs(function() {});
logArgs(function named(a, b, c) {});
logArgs(function(a /* = 1 */, b /* = true */) {});
logArgs(function fprintf(handle, fmt /*, ...*/) {});
logArgs(function(a, b = 1, c) {});
logArgs(function(a = 4 * (5 / 3), b) {});
// logArgs(function (a, // single-line comment xjunk) {});
// logArgs(function (a /* fooled you {});
// logArgs(function (a /* function() yes */, \n /* no, */b)/* omg! */ {});
// logArgs(function ( A, b \n,c ,d \n ) \n {});
logArgs(function(a, b) {});
logArgs(function $args(func) {});
logArgs(null);
logArgs(function Object() {});

console.log('');
console.log('////////// STRINGS //////////');

logArgs('function (a,b,c) {}');
logArgs('function () {}');
logArgs('function named(a, b, c) {}');
logArgs('function (a /* = 1 */, b /* = true */) {}');
logArgs('function fprintf(handle, fmt /*, ...*/) {}');
logArgs('function( a, b = 1, c ) {}');
logArgs('function (a=4*(5/3), b) {}');
logArgs('function (a, // single-line comment xjunk) {}');
logArgs('function (a /* fooled you {}');
logArgs('function (a /* function() yes */, \n /* no, */b)/* omg! */ {}');
logArgs('function ( A, b \n,c ,d \n ) \n {}');
logArgs('function (a,b) {}');
logArgs('function $args(func) {}');
logArgs('null');
logArgs('function Object() {}');

Full working example:

https://repl.it/repls/StupendousShowyOffices