CODEWITHSUNDEEP
javascriptecmascript-6default-parametersdestructuring
Daniel Herr
Daniel Herr

Reputation: 20508

Destructuring Default Properties

I am using default parameters, with this function:

function defaultstest({ test = { done: false, test: true }, done = false } = {}) {
 console.log(test, done)
}

If I call it as defaultstest(), defaultstest({}), or defaultstest({ done: true }), the defaults are set correctly.

But when I add a property to the test object, the others are not set to their default.

defaultstest({ test: { done: true } }) >> { done: true } false

How can I ensure that the optional properties are set to their defaults?

Upvotes: 2

Views: 121

Answers (2)

Tate Thurston
Tate Thurston

Reputation: 4660

You can't supply nested default arguments. In your example, if you supply test, then it is not going to be assigned to the default object you supplied with property values test and done.

Default values are only supplied if no value or undefined is passed to the function: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Default_parameters.

You could use Object.assign (ES6) or an extend function to accomplish this:

function defaultstest(test = {}, done = false ) {
  test = Object.assign({done: false, test: true}, test); 
  console.log(test, done)
}

Otherwise you are going to need to flatten your arguments.

If you use babel to transpile this to ES5 code, it may become more clear:

function defaultstest() {
  var _ref = arguments.length <= 0 || arguments[0] === undefined ? {} : arguments[0];

  var _ref$test = _ref.test;
  var test = _ref$test === undefined ? { done: false, test: true } : _ref$test;
  var _ref$done = _ref.done;
  var done = _ref$done === undefined ? false : _ref$done;

  console.log(test, done);
}

Specifically, note the assignment to test and done: 

var test = _ref$test === undefined ? { done: false, test: true } : _ref$test;
var done = _ref$done === undefined ? false : _ref$done;

Upvotes: 1

Darshan
Darshan

Reputation: 1072

There is no option for nested default arguments in ES6/ES2015. check out the transpiled source code code below.

function defaultstest() {
  var _ref = arguments.length <= 0 || arguments[0] === undefined ? {} : arguments[0];

  var _ref$test = _ref.test;
  var test = _ref$test === undefined ? { done: false, test: true } : _ref$test;
  var _ref$done = _ref.done;
  var done = _ref$done === undefined ? false : _ref$done;

  console.log(test, done);
}

Upvotes: 0

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