JavaScript interview questions and answers

1. What’s relationship between JavaScript and ECMAScript? - ECMAScript is yet another name for JavaScript (other names include LiveScript). The current JavaScript that you see supported in browsers is ECMAScript revision 3.
2. What are JavaScript types? - Number, String, Boolean, Function, Object, Null, Undefined.
3. How do you convert numbers between different bases in JavaScript? - Use the parseInt() function, that takes a string as the first parameter, and the base as a second parameter. So to convert hexadecimal 3F to decimal, use parseInt ("3F", 16);
4. What does isNaN function do? - Return true if the argument is not a number.
5. What is negative infinity? - It’s a number in JavaScript, derived by dividing negative number by zero.
6. What boolean operators does JavaScript support? - &&, || and !
7. What does "1"+2+4 evaluate to? - Since 1 is a string, everything is a string, so the result is 124.
8. How about 2+5+"8"? - Since 2 and 5 are integers, this is number arithmetic, since 8 is a string, it’s concatenation, so 78 is the result.
9. What looping structures are there in JavaScript? - for, while, do-while loops, but no foreach.
10. How do you create a new object in JavaScript? - var obj = new Object(); or var obj = {};
11. How do you assign object properties? - obj["age"] = 17 or obj.age = 17.
12. What’s a way to append a value to an array? - arr[arr.length] = value;
13. What is this keyword? - It refers to the current object.

Posted in: JavaScript