title: Pseudomandatory parameters in ES6 functions tip-number: 12 tip-username: Avraam Mavridis tip-username-profile: https://github.com/AvraamMavridis tip-tldr: In many programming languages the parameters of a function are by default mandatory and the developer has to explicitly define that a parameter is optional.

• /en/pseudomandatory-parameters-in-es6-functions/

In many programming languages the parameters of a function are by default mandatory and the developer has to explicitly define that a parameter is optional. In Javascript, every parameter is optional, but we can enforce this behavior without messing with the actual body of a function, taking advantage of [es6’s default values for parameters] (http://exploringjs.com/es6/ch_parameter-handling.html#sec_parameter-default-values) feature.

const _err = function (message) {    throw new Error(message);};const getSum = (a = _err('a is not defined'), b = _err('b is not defined')) => a + b;getSum(10); // throws Error, b is not definedgetSum(undefined, 10); // throws Error, a is not defined

_err is a function that immediately throws an Error. If no value is passed for one of the parameters, the default value is going to be used, _err will be called and an Error will be thrown. You can see more examples for the default parameters feature on Mozilla’s Developer Network