js_arithmetic.js

// fastroza (TM)
// Comments about https://mobile.twitter.com/lukaseder/status/867080556470730752

var js_behavior = false
function add(_x, _y) { // (x) + (b)
	// Normalization to Number
	var x = +_x
	var y = +_y

	if(!js_behavior)
		return x+y

	// How JS interp works
	if(typeof _x == 'string' || typeof _y == 'string')
		return _x.toString().concat(_y.toString())
	return x+y
}

function sub(_x, _y) { // (x) - (b)
	// Normalization to Number
	var x = +_x
	var y = +_y

	if(!js_behavior)
		return x-y

	// How JS interp works
	return x-y
}

function unary_add(x) { // + (x)
	return Number(x)
}

function unary_sub(x) { // - (x)
	return unary_add(x)
}

function test(exp) {
	console.log(exp + "=" + eval(exp))
}

js_behavior = false
console.log("\njs_behavior = false\n")
test("sub('5', 3)") 			            // '5' - 3
test("add('5', 3)") 			            // '5' + 3
test("sub('5', '4')") 			          // '5' - '4'
test("add('5', unary_add('5'))") 	    // '5' + + '5'
test("add('foo', unary_add('foo'))")	// 'foo' + + 'foo'
var x = 3
test("sub(add('5', x), x)")		        // '5' + x - x, it builds AST from left to right, no priorities here
test("add(sub('5', x), x)")		        // '5' - x + x, it builds AST from left to right, no priorities here

js_behavior = true
console.log("\njs_behavior = true\n")
test("sub('5', 3)")                     // '5' - 3
test("add('5', 3)")                     // '5' + 3
test("sub('5', '4')")                   // '5' - '4'
test("add('5', unary_add('5'))")        // '5' + + '5'
test("add('foo', unary_add('foo'))")    // 'foo' + + 'foo'
var x = 3
test("sub(add('5', x), x)")		          // '5' + x - x, it builds AST from left to right, no priorities here
test("add(sub('5', x), x)")             // '5' - x + x, it builds AST from left to right, no priorities here