Essential JavaScript Interview Questions

1. Explain the result of output:

(functiontest(){console.log({}.constructor===arguments.constructor,[].constructor===arguments.constructor);})();

2. Explain the result of output:

(functiontest(arguments){console.log(arguments[0]);})(100);

(functiontest(){vararguments;console.log(arguments[0]);})(200);

(functiontest(){functionsum(){varsum=0,i;for(iinarguments){sum+=i;}returnsum;}console.log(sum(10,20,30,40,50));})();

3. Explain the result of output:

(functiontest(){console.log(function(){}instanceofObject,function(){}instanceofFunction,FunctioninstanceofObject,ObjectinstanceofFunction);})();

4. Explain the result of output:

(functiontest(){console.log(function(){}.apply.length);})();

5. Explain the result of output:

(functiontest(){console.log(Function===Object.constructor,Function===Number.constructor,Function===Function.constructor,Function===Window.constructor,Function===Function.prototype.constructor,Object===Object.prototype.constructor,Number===Number.prototype.constructor,Array===Array.prototype.constructor,Window===Window.prototype.constructor);})();

6. Explain the result of output:

(functiontest(){console.log(typeofObject.prototype,typeofString.prototype,typeofRegExp.prototype,typeofFunction.prototype);})();

(functiontest(){console.log(typeofundefined,typeoftypeofundefined,typeofnull,typeof1/null,typeof[],typeof{},typeofdocument);})();

(functiontest(){console.log(typeofInfinity,typeofNaN,typeof{null: null}.null,typeof{NaN: NaN}.NaN,typeof{Infinity: Infinity}.Infinity);})();

7. Explain the result of output:

(functiontest(){varfn=function(){returnthis*2;};console.log(fn.apply(undefined));console.log(fn.apply(null));console.log(fn.apply(1));})();

(functiontest(){'use strict';varfn=function(){returnthis*2;};console.log(fn.apply(undefined));console.log(fn.apply(null));console.log(fn.apply(1));})();

8. Explain the result of output:

(functiontest(){console.log(Object.prototype.toString.call([]),Object.prototype.toString.call({}),Object.prototype.toString.call(Window),(16).toString(16),(true).toString(16),(false).toString(16));})();

9. Explain the result of output:

(functiontest(){varsum=function(a,b){returna+b;};console.log(typeofsum.call.apply);console.log(sum.call.apply(null,[1,2]));})();

10. Explain the result of output:

(functiontest(){console.log(void(p=1/""),p);})();

11. Explain the result of output:

(functiontest(){(function(){a=1;vara=2;})();console.log(a);})();

(functiontest(){vara=1;functiontest(){if(!a){vara=10;}console.log(a);}test();console.log(a);})();

(functiontest(){(function(){vara=b=3;})();console.log(typeofa,typeofb);})();

12. Which a variant is preferable and why?

(functiontest(){console.log(error!==undefined&&error.x);})();

(functiontest(){console.log(typeoferror!=='undefined'&&error.x);})();

13. Explain the result of output:

(functiontest(){vara=[];console.log(a.length,[,].length,[,,].length);a.length=-1;console.log(a.length);})();

(functiontest(){vara=[1,2,3,4,5];a.length=null;console.log(a[4]);})();

(functiontest(){vara=[1,2,3,4,5];a.length=undefined;console.log(a[4]);})();

(functiontest(){vara=[[[1],2],3];console.log(a.length);})();

(functiontest(){console.log([1,2,[3,4]]+[[5,6],7,8]);})();

14. Explain the result of output:

(functiontest(){console.log(9<5<1,2<6<1,1<3<4);})();

15. Explain the result of output:

(functiontest(){functionfn(){return{value: "test"}}console.log(typeoffn());})();

16. Explain the result of output:

(functiontest(){functionsum(a,b){returna+b;}functionsum(c){returnc;}console.log(sum(3));console.log(sum(2,4));})();

17. Explain the result of output:

(functiontest(){a=1;window.b=2;this.c=3;vard=4;deletea;deleteb;deletec;deleted;console.log(typeofa,typeofb,typeofc,typeofd);})();

18. Explain the result of output:

(functiontest(){vara=1;setTimeout(function(){a=0;console.log('Hi!');},0);while(a){}console.log('Hello!');})();

19. Explain the result of output:

(functiontest(){console.log([]-[],[]+[],{}-{},{}+{});})();

20. Explain the result of output:

(functiontest(){varholder={value: 1},holder2=holder;holder.result=holder={value: 0};console.log(holder.result,holder2);})();

21. Explain the result of output:

(functiontest(){vartest={property: 'Value',getPropertyValue: function(){returnthis.property;}};vargetPropertyValue=test.getPropertyValue;console.log(getPropertyValue(),test.getPropertyValue());})();

(functiontest(){"use strict";varholder,fn;holder={holderFn: function(){console.log(this);}};fn=holder.holderFn;holder.holderFn();fn();})();

22. What is the maximum depth of the stack, starting with the "test" function?

vara=[1,2,3,4,5];(functiontest(){console.log((newError()).stack);varitem=a.pop();item&&setTimeout(arguments.callee,0);})();

vara=[1,2,3,4,5];(functiontest(){console.log((newError()).stack);varitem=a.pop();item&&arguments.callee();})();

23. Explain the result of output:

(functiontest(){functionfn(){arguments.callee.count=arguments.callee.count||0;returnarguments.callee.count++;}console.log(fn(),fn(),fn());})();

24. Explain the result of output:

(functiontest(){vara='5',b=2,c=a+++b;console.log(c);})();

25. Explain the result of output:

console.log(016*2,0x16*2);

console.log(Math.floor(999.99)===~~999.99,Math.floor(-999.99)===~~-999.99,~function(){}(),~~function(){}(),~~null,~~undefined,~~[],~~{},~~'Test');

console.log(1+"2"+"2",1++"2"+"2",1+-"1"+"2",+"1"+"1"+"2","2"*3,"6"/2,"A"-"B"+"2","A"-"B"+2);

console.log(0.1+0.2);console.log(0.1+0.2===0.3);console.log((0.1+0.2)+0.3===0.1+(0.2+0.3));

console.log(Number('Test!'),Number(''),Number('00010'),Number(true),Number(NaN),parseInt('2',2),parseInt('011',8),parseInt('011',10),parseInt('00C',16),parseInt([10,9,8,7,6,5,4,3,2,1]),1/-0,isNaN(1/-0),isFinite(1/-0),0/-0,isNaN(0/-0),isFinite(0/-0),1/0,isNaN(1/0),isFinite(1/0),0/0,isNaN(0/0),isFinite(0/0),NaN===NaN,Infinity===Infinity);

26. Explain the result of output:

console.log(true==[1]&&true==[2]);

console.log(newBoolean()==true,newBoolean("")==true,newBoolean("0")==true,newBoolean("1")==true,newBoolean("true")==true);

console.log(false=='0',false==='0',true=='1',true==='1',true=='2',true==='2');

27. Explain the result of output:

(function(){varfn=function(){console.log(typeofthis);};fn.call("Hello World!");})();(function(){"use strict";varfn=function(){console.log(typeofthis);};fn.call("Hello World!");})();

console.log((function(){return(newthis).stack;}).apply(Error));

28. Explain the result of output:

(functiontest(){vara={},b={value: 'test1'},c={value: 'test2'};a[b]='test3';a[c]='test4';console.log(a[b]);})();

29. Explain the result of output:

console.log(Date(),newDate,+Date(),+newDate,+newDate(),+newDate()===+newDate);

30. Explain the result of output:

console.log(typeofconfirm('Do you like JavaScript?'));

31. Eliminate non-existent state of promise.

1. fulfilled
2. awaiting
3. pending
4. refused
5. rejected
6. interrupted

32. Explain the result of output:

(functiontest(){console.log([1,2,3,4,5].map(function(n){returnn===1&&1||arguments.callee(n-1)*n;}));})();

(functiontest(){"use strict";console.log([1,2,3,4,5].map(function(n){returnn===1&&1||arguments.callee(n-1)*n;}));})();

33. Explain the result of output:

(functiontest(){vars1='test',s2=newString('test'),s3=String('test');console.log(s1==s2,s1===s2,s1==s3,s1===s3,s1.constructor===s2.constructor,s1.constructor===s3.constructor,typeofs1,typeofs2,typeofs3);console.log(s1.slice()==s1,s1.slice()==s2,s1.slice()==s3,s1.slice()===s1,s1.slice()===s2,s1.slice()===s3);s1[2]='w';console.log(s1);})();