This document discusses various JavaScript concepts including semicolon insertion, function scope, string comparison, equality comparisons, and the differences between closure vs anonymous functions, functional vs imperative programming, prototypical vs class-based inheritance, and asynchronous vs multi-threaded code. It emphasizes that understanding core JavaScript concepts is important for developing robust JavaScript skills even if some things can work without fully understanding the language.

1. Javascriptwtfshow I learnedtostopworryingandlovethebotchthatisECMAScript

2. AboutmeRaphael PigullaBörseGo AGLead Developer JavaScriptpigulla@boerse-go.de@muddyb0y

3. WTF?functionfoo() {return { bar: true}}// whatis x?var x = bar();

4. Semicolon Insertionfunctionfoo() {return; { bar: true;};}// whatis x?var x = bar();

5. Semicolon Insertion{ 1 2 } 3{ 1 2 } 3;✖{ 1;2;} 3;{ 12 } 3✔i;++j;✔i++j

6. WTF?varfoo = 42;function bar() {foo = 13;}// foois 42bar();// foois?

7. WTF?varfoo = 42;function bar() {foo = 13;varfoo;}// foois 42bar();// foois?

8. DeclarationHoistingvarfoo = 42;function bar() {returnfoo;varfoo = 17;}// whatisthereturnvalueof bar() ?bar();

9. FunctionScopevar x = 42;functionfoo(n) { x = 13;while (--n > 0) {var x = 2 * n;}}

11. String Theory// firstattemptfunctionisString(v) {returntypeof v === ´string´;}isString(new String(´foo´)); // false!

12. String Theory// secondattemptfunctionisString(v) {return v instanceof String;}isString(´oh dear´); // false!

13. String Theory// thirdattemptfunctionisString(v) {returntypeof v === ´string´ || v instanceof String;}isString(popup.window.getSomeString());// mayreturnfalse

14. String Theory// fourthattemptfunctionisString(v) {returnObject.prototype.toString. ⏎call(v) === ´[object String]´;}// untilsomejokerchanges// Object.prototype.toString... :-/

15. // untilsomeoneoverwrites String// orRegExp.test-.-String Theory// fifthattemptfunctionisString(v) {returntypeofv ===´string´ ||!!v && /^function String\(\) {/⏎ .test(´´ + v.constructor);}

16. WTF?> [] == ![] <Array>[] == !<Array>[]<Array>[] == !ToBoolean(<Array>[]) <Array>[] == <Bool>false <Array>[] == ToNumber(<Bool>false) <Array>[] == <Number>0ToPrimitive(<Array>[]) == <Number>0<String>[].toString() ==<Number>0 <String>´´ == <Number>0

17. The Abstract EqualityComparisonAlgorithm

18. What‘sthebig deal?

19. Well, not foreverybody.for (franz=0;franz<items.length;franz++) { vare = items[franz];...

20. “It turns out thatif you haveabsolutelynoideawhatyou‘redoing in thelanguage, you can still generallymakethingswork.„Douglas Crockford

23. Whydoesit matter?JavaScript iseverywhereyou needfirstclass JavaScript developersweeding out thedudsisdifficultjQuery ≠ JavaScriptjQuery

24. The Checklist™Whatisthedifferencebetween...closure vs.anonymousfunctionfunctionalvs. imperative languagesprototypicalvs.class-basedinheritanceasynchronousvs. multi-threadedcode

25. Source: @markrendle on Twitter