JavaScript Interview Questions
This page consists of JavaScript interview questions and answers.
Global variables are the variables that are available throughout the code.
If we create a variable without the var keyword then it becomes a global variable.
var
this
The this keyword is used to refer to the object from where it is called.
"use strict";
The "use strict"; in JavaScript is used to execute the code in strict mode.
So, for example in strict mode we can't use undeclared variables.
"use strict"; is a literal expression and not a statement and is ignored by older JavaScript versions.
setTimeout
The setTimeout function executes a function after a given delay.
In the following example we will get "Hello World" message in the browser console after 5000ms (milliseconds) delay.
setTimeout(function() { console.log('Hello World'); }, 5000)
setInterval
The setInterval function is used to execute a function repeatedly after a given delay. The execution stops only when we stop the timer using the clearInterval function.
clearInterval
In the following example we will get "Hello World" after every 3 seconds (3000ms) for 5 times.
var count = 0; var t = setInterval(function() { console.log('Hello World'); count++; if (count === 5) { clearInterval(t); console.log('End'); } }, 3000);
View state is specific to a page in a session. Whereas, Session state is specific to a user or browser session and is accessible from all pages.
Following are the loop structure available in JavaScript.
var str = 'Hello World'; for (var i = 0, len = str.length; i < len; i++) { console.log(str[i]); }
console.log(typeof typeof null);
The output is "string".
"string"
Explanation:
The typeof null will give us "object". This is in double quotes so, it is a string.
typeof null
"object"
Next, the typeof "object" will give us "string".
typeof "object"
Hence, the final answer is "string".
Void(0) is used to prevent a page from refreshing when used with hyperlinks.
Void(0)
In the following example we have a hyperlink which when clicked will not refresh the page or jump to the top of the page.
<a href="javascript:void(0);" ondblclick="alert('Hello World')">Double click me</a>