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Output of JavaScript Programs

  • Difficulty Level : Hard
  • Last Updated : 01 Dec, 2020

1. Predict and Explain the Output of the below JavaScript program.

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function f(a, b, c) {
    m = ["1", "2", "3"];
    a = 3;
    b[0] = "X";
    c.first = false;
var x = 4;
var y = ["A", "B", "C"];
var z = { first: true };
f(x, y, z);
console.log(x, y, z);
function g(a) {
    a = { first: true };


4 ["X", "B", "C"] {first:false} {first:true} {first:false}

Explanation: When (x, y, z) is logged, x gives value 4 (as primitives are passed by value and hence its value does not change even after function f()). y is an array, hence an object, and so it is passed by reference and its index 0 gets changed to X. So y logs X, B, C. Inside function f(), c.first has been changed to false and since it is passed by reference, it logs first:false. In function g(), a new object is created with value as true and so it logs first:true. Finally, in the last line, z.first is still equal to false and hence it logs first:false.

2. Predict and Explain the Output of the below JavaScript program:


function foo1() {
   return {
      bar: "hello";
function foo2() {
      bar: "hello";

Output: {bar:”hello”} undefined

Explanation: In foo1(), bar object is returned as it should and hence it gives the output {bar:”hello”}. But in foo2(), newline after return is interpreted differently. It implicitly puts a semicolon after return and the corresponding set of lines are treated as a block of statements. So foo2() has the following return statement- return; which gives output as undefined.

3. Predict and Explain the Output of the below JavaScript program:


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

Output: 1432

Explanation: The setTimeout() function is called only after the parent function has executed fully and returned. So even though console.log(3) has timeout of 0 milliseconds, it is executed only after the parent function has returned after logging 1 and 4. Then 3 is logged. Finally after a timeout of 1000 milliseconds, 2 is logged.

4. Predict and Explain the Output of the below JavaScript code snippet:


for (var i = 0; i < 5; i++) {
   (function(x) {
      setTimeout(function() {
      },  1000 );

Output: 0 1 2 3 4

Explanation: With the help of an Immediately Invoked Function Expression (IIFE), its own scope will be created, and we can pass i to the function. Variable i will be a local variable and value of i in every loop will be preserved and finally printed after timeout of 1 second.

5. Predict and Explain the Output of the below JavaScript program:


var x= 5;
function check(){
   y = 10;
   var x =10;
function check2(){

Output: undefined 10

Explanation: var x has been defined and initialized inside check() after it is logged. Hoisting works only for variable declaration and not for initialization, so it returns undefined. In check(), y has been initialized to 10. Since var is not used, the variable has its scope until it encounters a variable by the given name or the global object. So when check2() is called, it logs 10 as the output.

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