Kyle Simpson getify

Parens And Performance

Years ago, some smart folks that worked on JS engines realized that not all JS that's loaded into a page/app initially is needed right away. They implemented JIT to optimize this situation.

JIT means Just-In-Time, which means essentially that the engine can defer processing (parsing, compiling) certain parts of a JS program until a later time, for example when the function in question is actually needed. This deferral means the engine is freer to spend the important cycles right now on the code that's going to run right now. This is a really good thing for JS performance.

Some time later, some JS engine devs realized that they needed to get some hints from the code as to which functions would run right away, and which ones wouldn't. In technical speak, these hints are called heuristics.

So they realized that one very common pattern for knowing that a function was going to run right away is if the first character before the function keyword was a (, because that usually m

	// maxOnlyEven(): undefined
	// maxOnlyEven(1): undefined
	// maxOnlyEven(1,3): undefined
	// maxOnlyEven(1,4): 4
	// maxOnlyEven(2): 2
	// maxOnlyEven(undefined,2): 2
	// maxOnlyEven(2,4): 4

	function maxOnlyEven(num1,num2) {
	var A = num1 % 2 == 0;

	// examples...
	//
	// maxEven(): undefined
	// maxEven(1): undefined
	// maxEven(1,13): undefined
	// maxEven(-Infinity,Infinity): undefined
	// maxEven(0): 0
	// maxEven(-4,3,-6,0,5,2,1,9): 2

	function maxEven(num1,...nums) {

	function whateverYouCallIt(obj) {
	function F(){};
	F.prototype = obj;
	return F;
	}

	var o = {};

	class C extends whateverYouCallIt(o) {}

	// All of the following are ways that template literals are NOT `strictly better` than " or ' delimited strings
	// RE: https://ponyfoo.com/articles/template-literals-strictly-better-strings

	// valid syntax, but doesn't turn on strict mode
	`use strict`;

	// invalid syntax
	const x = { `hello world`: 42 };

	// invalid syntax

	function foo(x,y = function(){ return x; }) {
	console.log( "a:", x );
	var x = 1;
	console.log( "b:", y() );
	}

	foo(2);

	class P {
	foo() { console.log( "P.foo" ); }
	}

	class C extends P {
	foo() {
	super();
	}
	}

	function printIf( msg, predicate ) {
	if (predicate( msg )) {
	console.log( msg );
	}
	}

	function isShortEnough(str) {
	return str.length <= 5;
	}

	function add1(v) { return v + 1; }
	function isOdd(v) { return v % 2 == 1; }
	function sum(total,v) { return total + v; }

	var list = [2,5,8,11,14,17,20];

	list
	.map( add1 )
	.filter( isOdd )
	.reduce( sum );

	function add1(v) {
	return v + 1;
	}
	function mul2(v) {
	return v * 2;
	}
	function div3(v) {
	return v / 3;
	}