Evaluating Expression Syntax Flashcards
Differences between ‘Undefined’ and ‘Null’? When would a return value be ‘Undefined’ vs. ‘Null’?
- ‘Null’ is considered a value; can be assigned
- Typeof null is object; means ‘no object’
- ‘Null’ is return value for undefined objects or invalid expressions
- Undefined indicates that no value assigned
- Undefined is returned for primitives
Undefined != Null
True. ‘Null’ is a value and has type of object. Undefined has no value.
NaN != NaN
True. NaN can only be used in comparison expressions
Bool value of ‘Null’
False
Bool value of ‘Undefined’
False
ES6 method to check NaN?
Number.isNAN()
Default return value of a function if not specified?
Undefined
Are semicolons required after statements?
No, but they’re recommended for clarity
What comparison is safe to evaluate null?
Strict equality (returns 0 if type coersion is applied by ==)
Type of NaN?
Number
Type of Infinity?
Number (+ or - infinity)
In dot notation, everything to the left of a dot is ________
an object
Numeric value of ‘undefined’
NaN
Numeric value of ‘null’
0
ex: value of a null variable used in a calculation, value if type coercion is applied
What is the bool value of an object?
True
Name 6 things considered ‘falsey’
false, 0, NaN, ‘ ‘, undefined, null
Operator to concat strings
+
Escape character
\ backslash
What type of expression is…
{ foo: bar(3, 5)}
Object literal. Expression that looks like a statement
What type of expression is…
function foo( ){ }
Named function expression
let n = “25”
- Convert to number with single operator
- Convert to number with method
- +n //not ideal because it’s unclear
2. number(n) //verbose but clear
Preferred quote style for JS?
Single
Preferred quote style for HTML?
Double
Quote style for JSON?
Double (required)
What is the difference between these function definitions? Do they have the same scope?
A) var square = function(x) { return x * x; };
B) function square(x) {
return x*x;
}
A) Expression with function keyword assigned to variable
B) Function declaration with keyword
- var ‘square’ has function scope; as a variable, it cannot be hoisted. ; function ‘square’ can be hoisted
Can a function be used in an arbitrary expression?
Yes, a function is considered a ‘value’. It can be part of an expression (not called like a method).
