Value for optional parameter by name (Javascript)

You can use an object as the function's parameter.

function drawImageOnCanvas(canvas, options){
    var scale = options.scale;
}

And in call site:

drawImageOnCanvas(canvas, {scale: 2});

Also to handle optional parameters, you can check their existence using an if or || or ?:.

var color = /*default color*/;
if (options.color){
    color = options.color;
}

Or

var color = options.color || /*default color*/;

Or

var color = options.color ? options.color : /*default color*/;

Note: If options contains parameters having false, 0, etc values then the methods above are not suitable anymore. For example, assume we have a parameter called isActive, then passing {isActive: false} will lead to the /*default isActive*/. To address this problem, you can use .hasOwnProperty or in.

var options = {scale: 2, isActive: false};

console.log('isActive' in options); // true
console.log(options.hasOwnProperty('isActive')); // true
console.log(options.isActive); // false

Anytime you have a function with many optional parameters, a much better option is to use a single parameter for all the optional parameters as an object.

In ES6, this case be more easily accomplished using destructuring and default parameters.

function drawImageOnCanvas(canvas, {scale = 1} = {}) {
    ...
}

Then you can call the function like this:

drawImageOnCanvas(canvas, { scale: 2 });

function test({ x = 10 } = {}) {
  console.log(x);
}

test({ x: 100 }); // passing x
test({ y: 200 }); // not passing x
test();           // not passing anything still works

You create an array having .length equal to the number of expected parameters to function, set parameters passed to function as elements of an array, set index 10 of array to value for scale, use rest element to pass array of parameters to function, destructuring assignment within function to define named variables

function drawImageOnCanvas(canvas, texturePath, color, type, onCall
, modelPath, scene, pX, pY, pZ, scale, transform) {
      console.log(canvas, texturePath, color, type, scale);
}

var args = Array.of(document.createElement("canvas"), 1, 2, 3, ...Array(7));

args[10] = {scale:456};

drawImageOnCanvas(...args);