These days, I was preparing a small game for our team’s tech workshop. Thought it’d be a good opportunity of introducing the some fundamental and tricky stuffs around JavaScript. So I made 8 quiz to our team members. And hope they could solve them within 15 min. Eventually, it took all of them over 20 minutes to complete and most of them could solve 4-5 questions correctly.
You can take it as just a small test, each quiz has answer attached to the end of the code. Try to answer them first and then look at the answers. Good luck.
# What do these console.log print out?
# No. 1 – Doctor Pavlov has a dog
function Animal(){
this.type = "animal"
}
function Dog(){
this.name = "dog"
}
Dog.prototype = new Animal()
var PavlovPet = new Dog();
console.log(PavlovPet.__proto__ === Dog.prototype)
console.log(Dog.prototype.__proto__ === Animal.prototype)
Very basic stuff about the prototype chain.Answer for No. 1
// Output
true
true
# No. 2 – Be careful with the “sort”
var arr = [5, 22, 14, 9];
console.log(arr.sort());
Sorry, not [5, 9, 14, 22]. Each element is converted into string and then comparing the sequence of UTF-16 value. Check this out: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/sortAnswer for No. 2
// Output
[14, 22, 5, 9]
# No. 3 – Closure and event loop
for (var i = 0; i < 3; i++) {
const log = () => {
console.log(i)
}
setTimeout(log, 100)
}
It prints out 3 thrice since setTimout puts that Answer for No. 3
log function to the next macro task queue// Output
3
3
3
# No. 4 – There’s indentation
function createNewArray(item) {
return
[item]
}
console.log(createNewArray(0))
This returns Answer for No. 4
js•undefined. The break-line after the return is ignored.
Thus, the argument passed into the function is also ignored.undefined
# No. 5 – What’s inside the “numbers”
const length = 4
const numbers = []
for (var i = 0; i < length; i++);{
numbers.push(i + 1)
}
console.log(numbers)
This needs a pair of sharp eyes. See that semicolon between the parentheses and curly bracket?Answer for No. 5
// Output
[5]
# No. 6 – No length
const clothes = ['shirt', 'socks', 'jacket', 'pants', 'hat']
clothes.length = 0
console.log(clothes[3])
This is equivalant to removing all elements from the arrayAnswer for No. 6
// Output
undefined
# No. 7 – Variable went crazy
var a = 1
function output () {
console.log(a)
var a = 2
console.log(a)
}
console.log(a)
output()
console.log(a)
First output: using the global Second output: the first one in the Third output: print out after the declaration. Nothing special. Forth output: Answer for No. 7
js•var ajs•function output() using before declaration should be undefinedjs•var a never got changed by js•function output()// Output
1
undefined
2
1
# No. 8 – There’s an accidental declaration
function foo() {
let a = b = 0
a++
return a
}
foo()
console.log(typeof a)
console.log(typeof b)
Answer for No. 8
js•let a is a local variable. js•typeof a is checking undeclared variable.js•b is a global variable which value is assign in js•function foo().// Output
undefined
number
# In the end
Thanks so much for reading! Have you got them all correct?
