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function API: function.length (with ES6 features)

The value of the length property (an integer) indicates the "typical" number of arguments expected by the function

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);


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