Implement a function, to return the number of parameters a function expects.

function foo() {}
function bar(a) {}
function baz(a, b) {}

functionLength(foo); // 0
functionLength(bar); // 1
functionLength(baz); // 2

Solution

All function instances have a length property which indicates the number of parameters expected by the function.

/**
 * @param {Function} fn
 * @return {number}
 */
export default function functionLength(fn) {
  return fn.length;
}

Notes

The length property excludes the rest parameter and only includes parameters before the first one with a default value.

function foo(a, b = 2) {}
foo.length; // 1

function bar(a = 1, b = 2) {}
bar.length; // 0

function baz(...args) {}
baz.length; // 0

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