JavaScript function with optional parameters [duplicate]

Answer 1

You can check if parameters are defined and hard code that into your functions.

e.g.

 var preDefined = function(param) { 
    if(param === undefined) { 
        param = preDefinedValue
     }
     /* Rest of code goes here */
 }

ETA:

ES6 was allows for default parameter values (was unaware of this when I posted the answer).

Link

Answer 2

One way is to check if the parameter value is undefined and if so then assign a value.

function plotChart(data, xlabel, ylabel, chart_type) {
  if (typeof chart_type === 'undefined') {
     chart_type = 'l';
  }
} 

Also EcmaScript 2016 (ES6) offers Default Parameters. Since some browser don't yet support this feature you can use a transpiler such as babel to convert the code to ES5.

To make it work like in your python example you would have to pass an object containing the values instead of individual parameters.

function plotChart(options) {
    var data = options.data;
    var xlabel = options.xlabel;
    var ylabel = options.ylabel;
    var chart_type = (typeof options.chart_type === 'undefined' ? 'l' : options.chart_type);
}

Example usage

plotChart({
  xlabel: 'my label',
  chart_type: 'pie'
});

Answer 3

There's a ton of answers for this already, but I havn't seen what I considered to be the simplest solution to this.

var myFunc = function(param1) {
    var someData = param1 || "defaultValue";
}

Answer 4

A good way to do this would be to use an object for all of the arguments. Something like:

function plotChart(options) {
  // Set defaults
  options.chart_type = options.chart_type || '1';

  // Check if each required option is set
  // Whatever is used by the data
}

Then when the function is called:

plotChart({
  data: 'some data',
  xlabel: 'some xlabel',
  ylabel: 'some ylabel',
  chart_type: '5' // This is optional
});