How to set default parameter values in JavaScript

Dec 5, 2018

Learn how to set default parameter values in JavaScript functions, the ES6 way with param = value, and how to set defaults for destructured options objects.

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Default parameter values have been introduced in ES6 in 2015, and are widely implemented in modern browsers.

This is a doSomething function which accepts param1.

const doSomething = (param1) => {

}

We can add a default value for param1 if the function is invoked without specifying a parameter:

const doSomething = (param1 = 'test') => {

}

This works for more parameters as well, of course:

const doSomething = (param1 = 'test', param2 = 'test2') => {

}

What if you have an unique object with parameters values in it?

Once upon a time, if we had to pass an object of options to a function, in order to have default values of those options if one of them was not defined, you had to add a little bit of code inside the function:

const colorize = (options) => {
  if (!options) {
    options = {}
  }

  const color = ('color' in options) ? options.color : 'yellow'
  ...
}

With destructuring you can provide default values, which simplifies the code a lot:

const colorize = ({ color = 'yellow' }) => {
  ...
}

If no object is passed when calling our colorize function, similarly we can assign an empty object by default:

const spin = ({ color = 'yellow' } = {}) => {
  ...
}

Tagged: JavaScript · All topics

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