How can I declare optional function parameters in JavaScript? [duplicate]

With ES6: This is now part of the language:

function myFunc(a, b = 0) {
   // function body
}

Please keep in mind that ES6 checks the values against undefined and not against truthy-ness (so only real undefined values get the default value – falsy values like null will not default).

With ES5:

function myFunc(a,b) {
  b = b || 0;

  // b will be set either to b or to 0.
}

This works as long as all values you explicitly pass in are truthy.
Values that are not truthy as per MiniGod’s comment: null, undefined, 0, false, ''

It’s pretty common to see JavaScript libraries to do a bunch of checks on optional inputs before the function actually starts.