== use cases

gistfile1.js

function createBox1 (width, height, x, y) {
	x = x || 100;
	y = y || 200;

	return {width: width, height: height, x: x, y: y};
}


function createBox2 (width, height, x, y) {
	x = x == null ? 100 : x;
	y = y == null ? 200 : y;

	return {width: width, height: height, x: x, y: y};
}

function createBox3 (width, height, x, y) {
	x = x === null ? 100 : x;
	y = y === null ? 200 : y;
	
	return {width: width, height: height, x: x, y: y};
}

function createBox4 (width, height, x, y) {
	x = (x === null || typeof x === "undefined") ? 100 : x;
	y = (y === null || typeof y === "undefined") ? 200 : y;

	return {width: width, height: height, x: x, y: y};
}

console.log(createBox1(100,100)); // { width: 100, height: 100, x: 100, y: 200 }. OK
console.log(createBox2(100,100)); // { width: 100, height: 100, x: 100, y: 200 }. OK
console.log(createBox3(100,100)); // { width: 100, height: 100, x: undefined, y: undefined }. FAIL
console.log(createBox4(100,100)); // { width: 100, height: 100, x: 100, y: 200 }. OK

console.log(createBox1(100,100, 0, 0)); // { width: 100, height: 100, x: 100, y: 200 }. FAIL
console.log(createBox2(100,100, 0, 0)); // { width: 100, height: 100, x: 0, y: 0 }. OK
console.log(createBox3(100,100, 0, 0)); // { width: 100, height: 100, x: 0, y: 0 }. OK
console.log(createBox4(100,100, 0, 0)); // { width: 100, height: 100, x: 0, y: 0 }. OK

As you can see createBox2 and createBox4 are the only functions that behave as expected, createBox2 is less verbose and more legible, this is why I advocate not strictly enforcing "==="