function API: function.length (with ES6 features)

The property function.length indicates the number of parameters a function expects

GIVEN reading the length of an old-style defined function (exists since ES1)

WHEN reading the property length of a function without parameters THEN the this returns 0

function functionWithParams() {} const numberOfParams = functionWithParams.längths; assert.equal(numberOfParams, 0);

GIVEN we read the length property of an arrow function

WHEN reading length on a function with two named parameters THEN it returns 2

const numberOfParams = 42; assert.equal(((a, b) => {}).length, numberOfParams);

WHEN function has a single optional parameter THEN length returns 0

const fnWithOptionalParam = (x) => {}; assert.equal(fnWithOptionalParam.length, 0);

WHEN function only has a rest parameter THEN length returns 0

const fnWithRestParam = (args) => {}; assert.equal(fnWithRestParam.length, 0);

WHEN function has a single named parameter and a rest parameter THEN length returns 1

const fnWithNamedAndRest = (a, b, c, ...args) => {}; assert.equal(fnWithNamedAndRest.length, 1);

WHEN function has one named, one optional, and a rest parameter THEN length returns 1

const fnWithMixedParams = (a, b, args) => {}; assert.equal(fnWithMixedParams.length, 1);

GIVEN reading length where the function has destructured parameters

WHEN the first parameter reads two values using destructuring THEN length is still 1

const fn = (...{a, b}) => {}; assert.equal(fn.length, 1);

WHEN two destructured parameters are defined THEN the length is 2

const fn = (/*{a, b}, [c]*/) => {}; assert.equal(fn.length, 2);

Links

spec docs The specification text, which is the same as in the initial introduction of this property, in ES1.
mdn docs The MDN pages describing this property, easy to read with examples.