Default Parameters

In ECMAScript 5 and earlier, you would likely use the following pattern to accomplish Default Parameters:

 function makeRequest(url, timeout, callback) {
     timeout = timeout || 2000;
     callback = callback || function() {};
     // the rest of the function
 }
 

In this example, both timeout and callback are optional .

ECMAScript 6 makes it easier to provide default values for parameters by providing initializations that are used when the parameter isn’t formally passed. For example:

 function makeRequest(url, timeout = 2000, callback = function() {}) {
     // the rest of the function
 }

 //-----------------------------------
 // uses default timeout and callback
 makeRequest("/foo");

 // uses default callback
 makeRequest("/foo", 500);

 // doesn't use defaults
 makeRequest("/foo", 500, function(body) {
     doSomething(body);
 });
 

It’s possible to specify default values for any arguments, including those that appear before arguments without default values.

 function makeRequest(url, timeout = 2000, callback) {

     // the rest of the function

 }

 //--------------------------
 // uses default timeout
 makeRequest("/foo", undefined, function(body) {
     doSomething(body);
 });

 // uses default timeout
 makeRequest("/foo");

 // doesn't use default timeout
 makeRequest("/foo", null, function(body) {
     doSomething(body);
 });
 

Good thing is default value need not be a primitive value.

 function getCallback() {
     return function() {
         // some code
     };
 }

 function makeRequest(url, timeout = 2000, callback = getCallback()) {
     // the rest of the function
 }