Operators, data types, some syntax

Question 1

How can you include an external javascript file?

Answer 1

/script src=’myfile.js’/

Question 2

What’s better, putting scripts just before the closing body tag or in the head with ‘defer’ attribute?

Answer 2

Sort of a wash, but old browsers don’t understand defer so before probably better

Question 3

ECMAScript has how many data types?

Answer 3

5 simple: undefined, null, number, boolean, string;

and 1 complex: object

Question 4

how do you find the data type (type) of a variable? E.g. what is the type of myVar?

Answer 4

typeof myVar; (no need for parens since typeof is an operator, not a function). Thus, alert(typeof myVar);

Question 5

when should you set a variable to ‘null’

Answer 5

When you expect it to contain a reference to an object. That way, you can explicitly check for the value null to see if the var has been filled with an object reference.

Question 6

Why is this a good/bad idea:

if (a + b == 0.3) { do something… }

Answer 6

Bad idea because of how EMCAScript rounding errors with floating numbers. 0.1 + 0.2 will equal 0.300000000003 not just 0.3.

Question 7

true/false: NaN == NaN

Answer 7

false.

Question 8

How can you determine if something is NaN?

Answer 8

use isNaN() function.

Question 9

How best to convert something to a number or cast something as a number?

Answer 9

Depends, but usually use parseInt() rather than Number() since Number() has some unexpected values. For example: Number(“”) will return zero rather than NaN. Number(“023blue”) will be NaN whereas parseInt(“023blue”) will be 23.

Question 10

what is a “unary” operator?

Answer 10

only operates on one value, like ++ or – as in ++var.

Question 11

var a = 10;
alert( a++ + 10) gives you what?

Answer 11

20 because the ++ happens after the operation.

Question 12

var a = 10;
alert(++a + 10) gives you what?

Answer 12

21 because the ++ happens before the operation.

Question 13

var a = false. What is a++?

Answer 13

numeric 1

Question 14

var a = “1”; What is +a?

Answer 14

numeric 1

Question 15

10 % 9 = ?

Answer 15

1

Question 16

How can you find which word comes first, alphabetically, between “Brick” and “alphabet”?

Answer 16

“Brick”.toLowerCase() < “alphabet”.toLowerCase();
// false, as expected. Or
“Brick”.localeCompare(“alphabet”);

Question 17

10 % 2 = ?

Answer 17

0

Question 18

10 + “5” = ?

Answer 18

105

Question 19

5 - “2” = ?

Answer 19

3 because “2” is converted to 2.

Question 20

“a” < “b” (true or false)?

Answer 20

true

Question 21

“B” < “a” (true or false)?

Answer 21

true, because uppercase characters come first.

Question 22

How is the identically equal operator different from the equal operator?

Answer 22

It does the same thing, but doesn’t convert operands before testing equality. E.g., “55” === 55 will be false. However (“5”-2) === 3 will be true.

Question 23

What’s the difference between a do/while statement and just a while statement?

Answer 23

Do/while will execute at least once no matter what. While may never execute:
var i = 10; do { i += 10; } while (i<5);
// this will execute once and i will become 20.

Question 24

What does the continue statement do to a loop?

Answer 24

It stops the action in the given iteration, but then continues going.

Question 25

In a for loop, what is the term for parts a, b, and c?

for (part a, part b, part c){ // do something }

Answer 25

part a is the initialization; part b is the expression; part c is the post loop expression.

Question 26

what does the break statement do to a loop?

Answer 26

It not only stops the action in a given iteration, it stops the entire loop.

Question 27

In a switch statement, how can you test if variable var equals 10?

Answer 27

switch (var) { case 10: // do something then break

Question 28

In a switch statement, how can you test if variable var is between 10 and 20?

Answer 28

declare var outside of statement. Then say: switch(true) { case var >10 && var < 20: // do something then break

Question 29

How could you find how many arguments have been passed to a function?

Answer 29

With the arguments object, used within the function:

arguments.length;

Question 30

var n = null; what is typeof n?

Answer 30

object. Null is a primitive datatype, but it is also an object.

Question 31

why does typeof null === “object” return true?

Answer 31

because of how js was made in the beginning, it returns this “bogus” value (according to MDN).

Question 32

var x, y, z;
x = ( y = 1, z = 4 );
What happens here?

Answer 32

Comma operators go from left to right and return value on right.
x will = 4; y = 1; z = 4;.

Question 33

var x, y, z;
x = y = 1, z = 4;
x = ( y = 1, z = 4 );
What’s the difference?

Answer 33

Comma operators have the least precedence of all operators, so top sets x and y both to 1. z is 4.
Bottom sets x to 4 because parens require everything inside get evaluated first.