Javascript : Guess the Output

Question

x = 23;
const x;

Answer

Syntax Error: Missing initializer in const declaration Temporal Dead Zone: It is a behaviour that occurs with variables declared with let and const keywords. Below code throws same error.

Question

x = 23;
let x;

Answer

ReferenceError: Cannot access 'x' before initialization Temporal Dead Zone: It is a behaviour that occurs with variables declared with let and const keywords. Below code throws same error.

Question

let x;
const y;
var z;

console.log(x);
console.log(y);
console.log(z);

Answer

undefined // x

const y;
      ^
SyntaxError: Missing initializer in const declaration

undefined // z

Question

console.log(1);
setTimeout(function () {
  console.log(2);
}, 1000);
setTimeout(function () {
  console.log(3);
}, 0);
console.log(4);

Answer

1 4 3 2 Even though the second timeout function has a waiting time of zero seconds, the javascript engine always evaluates the setTimeout function using the Web API and therefore, the complete function executes before the setTimeout function can execute.

Question

function x() {
  setTimeout(function () {
    console.log(i);
  }, 1000);
  var i = 1;
}

x();

Answer

1 Prints 1 after 1 second. It does not give error because hoisting takes place and variable declaration appears to be on the top of code.

Question

function x() {
  setTimeout(function () {
    console.log(i);
  }, 1000);
  let i = 1;
}

x();

Answer

1 // after 1 second Prints 1 after 1 second. Even though let variables are not hoisted, due to async nature of JS, the complete function code runs before the setTimeout function.

Question

function x() {
  setTimeout(function () {
    console.log(i);
  }, 1000);
  i = 1;
  let i;
}

x();

Answer

ReferenceError: Cannot access 'i' before initialization Temporal Dead Zone

Question

(function () {
  setTimeout(() => console.log(1), 2000);
  console.log(2);
  setTimeout(() => console.log(3), 0);
  console.log(4);
})();

Answer

2 4 3 1 // After two seconds

Even though the second timeout function has a waiting time of zero seconds, the javascript engine always evaluates the setTimeout function using the Web API and therefore, the complete function executes before the setTimeout function can execute
The function does not have a function name and is not invoked because it is an Immedidately Invoked Function.

Question

function func1() {
  setTimeout(() => {
    console.log(x);
    console.log(y);
  }, 3000);
  var x = 12;
  let y = 20;
}

func1();

Answer

12 20 Even though let variables are not hoisted, due to async nature of JS, the complete function code runs before the setTimeout function. Therefore, it has access to both x and y.

Question

function func2() {
  for (var i = 0; i < 3; i++) {
    setTimeout(() => console.log(i), 2000);
  }
}

func2();

Answer

3 3 3

Since variable declared with var keyword does not have block scope.
Also, inside the for loop, the variable i is incremented first and then checked.

Question

function func2() {
  for (let i = 0; i < 3; i++) {
    setTimeout(() => console.log(i), 2000);
  }
}

func2();

Answer

// after 2 seconds 0 1 2

let keyword has block scope.

Question

let x = {},
  y = { name: "Ronny" },
  z = { name: "John" };

x[y] = { name: "Vivek" };
x[z] = { name: "Akki" };

console.log(x[y]);

Answer

{ name: 'Akki' }

Adding objects as properties of another object should be done carefully.
Writing x[y] = {name:”Vivek”} , is same as writing x[‘object Object’] = {name:”Vivek”} ,
While setting a property of an object, javascript coerces the parameter into a string.
Therefore, since y is an object, it will be converted to ‘object Object’.
Both x[y] and x[z] are referencing the same property.

Question

function runFunc() {
  console.log("1" + 1);
  console.log("A" - 1);
  console.log(2 + "-2" + "2");
  console.log("Hello" - "World" + 78);
  console.log("Hello" + "78");
}

runFunc();

Answer

11 NaN 2-22 NaN Hello78

Coercion takes place
'+' converts Number to String and outputs String
'-' converts String to Number and performs operation, outputs Number ( typeof(NaN) = Number )

Question

let a = 0;
let b = false;
console.log(a == b);
console.log(a === b);

Answer

true // Equality Coercion false

Question

var x = 23;

(function () {
  var x = 43;
  (function random() {
    x++;
    console.log(x);
    var x = 21;
  })();
})();

Answer

NaN

random() function has functional scope, since x is declared and hoisted in the functional scope.

Rewriting the random function;

function random() {
  var x; // x is hoisted
  x++; // x is not a number since it is not initialized yet
  console.log(x); // Outputs NaN
  x = 21; // Initialization of x
}

Question

let hero = {
  powerLevel: 99,
  getPower() {
    return this.powerLevel;
  },
};

console.log(hero.getPower());

let getPower = hero.getPower;

let hero2 = { powerLevel: 42 };
console.log(getPower());
console.log(getPower.apply(hero2));

Answer

99 undefined 42 In the line

let getPower = hero.getPower;

only getPower() function inside hero object is assigned. So, when the function is invoked, it is invoked referencing the global object:

window.getPower() = getPower();

Question

const a = function () {
  console.log("a");
  console.log(this);

  const b = {
    func1: function () {
      console.log("b");
      console.log(this);
    },
  };

  const c = {
    func2: () => {
      console.log("c");
      console.log(this);
    },
  };

  b.func1();
  c.func2();
};

a();

Answer

a global/window object b object "b" c global/window object

Since we are using arrow function inside func2, this keyword refers to the global object.

Question

const b = {
  name: "Vivek",
  f: function () {
    var self = this;
    console.log(this.name);
    (function () {
      console.log(this.name);
      console.log(self.name);
    })();
  },
};

b.f();

Answer

'Vivek' undefined "Vivek Only in the IIFE (Immediately Invoked Functions) inside the function f , the this keyword refers to the global/window object.

Each time bigFunc is called, an array of size 700 is being created, Modify the code so that we don't create the same array again and again.

function bigFunc(element) {
  let newArray = new Array(700).fill("♥");
  return newArray[element];
}

console.log(bigFunc(599)); // Array is created
console.log(bigFunc(670)); // Array is created again

Answer

code can be modified by using closures

function bigFunc() {
  let newArray = new Array(700).fill("♥");
  return (element) => newArray[element];
}

let getElement = bigFunc(); // Array is created only once
getElement(599);
getElement(670);

Question

(function (a) {
  return (function () {
    console.log(a);
    a = 23;
  })();
})(45);

Answer

45 Even though a is defined in the outer function, due to closure the inner functions have access to it.

The following code outputs 2 and 2 after waiting for one second. Modify the code to output 0 and 1 after one second.

function randomFunc() {
  for (var i = 0; i < 2; i++) {
    setTimeout(() => console.log(i), 1000);
  }
}

randomFunc();

Answer

Using let keyword :

function randomFunc() {
  for (let i = 0; i < 2; i++) {
    setTimeout(() => console.log(i), 1000);
  }
}

randomFunc();

Using closure :

function randomFunc() {
  for (var i = 0; i < 2; i++) {
    (function (i) {
      setTimeout(() => console.log(i), 1000);
    })(i);
  }
}

randomFunc();

We have to store the name of every item in an array. What will be your approach?

[
  {
    name: "Menu 1",
    link: "https://google.com",
    subitems: [
      {
        name: "Menu 2",
        link: "https://google.com",
      },
    ],
  },
  {
    name: "Menu 3",
    link: "https://google.com",
    subitems: [
      {
        name: "Menu 4",
        link: "https://google.com",
        subitems: [
          {
            name: "Menu 5",
            link: "https://google.com",
          },
          {
            name: "Menu 6",
            link: "https://google.com",
          },
        ],
      },
    ],
  },
];

Answer

Deep Copy
Using Recursion

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Previousjavascript Nextobject-oriented-programming

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hashtagEach time bigFunc is called, an array of size 700 is being created, Modify the code so that we don't create the same array again and again.

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hashtagThe following code outputs 2 and 2 after waiting for one second. Modify the code to output 0 and 1 after one second.

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Each time bigFunc is called, an array of size 700 is being created, Modify the code so that we don't create the same array again and again.

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The following code outputs 2 and 2 after waiting for one second. Modify the code to output 0 and 1 after one second.

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