Adv Conc- Closures, private, IIFE, Shorthand

Question 1

Is this a partial function?
function makeLogger(identifier) {
return function(msg) {
console.log(identifier + ‘ ‘ + msg);
};
}

Answer 1

Here, console.log takes exactly one argument and the function returned by makeLogger also takes exactly one argument. Since there is no difference in the number of arguments, we don’t have partial function application.

Question 2

Is this a partial function?
function makeLogger(identifier) {
return function(msg) {
console.log(identifier, msg);
};
}

Question 3

What is a partial function

Answer 3

A function return by another that applies some of the function’s arguments when called, and applies the remaining arguments when you call the returned function.

function add(first, second) {
return first + second;
}

function makeAdder(firstNumber) {
return function(secondNumber) {
return add(firstNumber, secondNumber);
};
}

let addFive = makeAdder(5);
let addTen = makeAdder(10);

console.log(addFive(3)); // 8
console.log(addFive(55)); // 60
console.log(addTen(3)); // 13
console.log(addTen(55)); // 65

Question 4

What’s a closure?

Answer 4

You can think of closure as a function combined with any variables from its lexical scope that the function needs.

Closures are created when you define a function or method. The closure essentially closes over its environment – what’s in lexical scope. In effect, the function definition and all the identifiers in its lexical scope become a single entity called a closure.

When the function is invoked, it can access any variables it needs from that environment. That is, the function can use variables from the lexical scope where the function was defined. Even if those variables aren’t in the lexical scope where you invoke the function, it can still access them.

Question 5

What is in a closure?

Answer 5

The closure essentially closes over its environment – what’s in lexical scope. In effect, the FUNCTION DEFINITION and all the IDENTIFIERS in its lexical scope become a single entity called a closure.

Question 6

When is a closure created?

Answer 6

Closures are created when you define a function or method.

Question 7

What is the relationship between closures and scope?

Answer 7

the FUNCTION DEFINITION and all the IDENTIFIERS in its lexical scope become a single entity called a closure.

Question 8

What do we mean when we say that closures are defined lexically?

Answer 8

A closure includes the variables it needs from the scope where you defined the function. Those variables may not be in scope when you invoke the function, but they’re still available to the function.

let apple = function() {
let orange = 1;
return function() {
return orange;
}
}
let purple = apple();

console.log(purple()); // 1
console.log(orange); //error

Question 9

When do closures use the lexical scope for a function to determine what variables that function can access.

Answer 9

at that function’s definition point

Question 10

Explain why private data is desirable. (3 things)

Answer 10

excellent way to force other developers to use the intended interface. By keeping the collections private, we enforce access via the provided methods.

Maybe the code forces you to use add to add an item to the list, which ensures that we can log every addition.

prevent the user of an object from becoming dependent on the implementation. Other developers shouldn’t care that a todo-list object stores todos in an array, and your code shouldn’t encourage them to depend on it. Instead, your object should supply an API that lets other developers manipulate the todo-list regardless of its implementation. If you later change the implementation, your API should remain the same.

Question 11

Make private data

Answer 11

function makeCounter() {
var count = 0; // declare a new variable
return function() {
count += 1; // references count from the outer scope
console.log(count);
};
}

Question 12

What do parenthesis do?

Answer 12

tell JavaScript that we first want to evaluate that first as an expression

Question 13

What happens with this?
let x = function() {return 2}()
console.log(x);

Answer 13

2

Question 14

(function() {
console.log(‘hello’);
}());

Answer 14

hello

the parenthesis basically make it an expression.

Question 15

((first, second) => first * second)(5, 6);

Answer 15

// => 30

Question 16

Rewrite this to use IIFE to hide the private data:
function makeUniqueIdGenerator() {
let count = 0;
return function() {
count += 1;
return count;
}
};

let makeUniqueId = makeUniqueIdGenerator();
makeUniqueId(); // => 1
makeUniqueId(); // => 2
makeUniqueId(); // => 3

Answer 16

const makeUniqueId = (function() {
let count = 0;
return function() {
++count;
return count;
};
})();

makeUniqueId(); // => 1
makeUniqueId(); // => 2
makeUniqueId(); // => 3

Question 17

IIFE can help prevent not knowing what variables are already in use for large files
What’s another way?

Answer 17

blocks
{
}

Question 18

Do concise syntax for Property Initializers for an object

Answer 18

function xyzzy(foo, bar, qux) {
return {
foo,
bar,
qux,
};
}

Question 19

Do concise syntax for setting methods

Answer 19

let obj = {
foo() {
// do something
},

bar(arg1, arg2) {
// do something else with arg1 and arg2
},
}

Question 20

destructure an object’s properties into individual variables

Try omitting some names

Answer 20

let obj = {
foo: “foo”,
bar: “bar”,
qux: 42,
};

let { foo, bar, qux } = obj;
OR
let { foo } = obj;
let { bar, qux } = obj;

Question 21

destructure an object’s properties into individual variables and use different names

Answer 21

let { qux: newQux} = obj;

Question 22

destructure an object’s properties into individual function arguments

Answer 22

let obj = {
foo: 1,
bar: 2,
qux: 3,
};

function xyzzy({ foo: newFoo, bar, qux }) {
console.log(qux); // 3
console.log(bar); // 2
console.log(newFoo); // 1
}

xyzzy(obj);

Question 23

Destructure an object to reassign variables (as opposed to initialize them)

Answer 23

This does not work:
{ foo, bar, qux } = obj;
Nor does this
({ foo, bar, qux }) = obj;
So do this:
({ foo, bar, qux } = obj);

Question 24

destructure an array’s properties into individual variables

Answer 24

let foo = [1, 2, 3];
let [ first, second, third ] = foo;

Question 25

destructure an array’s properties into individual variables but skip some elements

Answer 25

let bar = [1, 2, 3, 4, 5, 6, 7];
let [ first, , , fourth, fifth, , seventh ] = bar;

Question 26

What’s good form for destructuring arrays?

Answer 26

The spaces inside the brackets aren’t required. However, it’s easier to read and more clearly distinguishes between destructuring and array literals.

let foo = [1, 2, 3];
let [ first, second, third ] = foo;

Question 27

Swap the values of variables using array destructuring

Answer 27

let one = 1;
let two = 2;

[ one, two ] = [two, one];

console.log(one); // 2
console.log(two); // 1

Question 28

What will this do?

let foo = [1, 2, 3, 4];
let [ bar, …qux ] = foo;
console.log(bar);
console.log(qux);

Answer 28

// 1
// [2, 3, 4]

Question 29

What does this do?
add3(…foo);

Answer 29

“spread” the elements of an array or object into separate items.

Question 30

let foo = [1, 2, 3];
let bar = […foo];
console.log(bar);
console.log(foo === bar);

Answer 30

// [1, 2, 3]
// false – bar is a new array

Question 31

let foo = [1, 2, 3];
let bar = [4, 5, 6];
let qux = […foo, …bar];
qux;

Answer 31

// => [1, 2, 3, 4, 5, 6]

Question 32

let foo = [1, 2, 3]
let bar = […foo, 4, 5, 6, …foo];
bar;

Answer 32

// => [1, 2, 3, 4, 5, 6, 1, 2, 3]

Question 33

let foo = { qux: 1, baz: 2 };
let bar = { …foo };
console.log(bar);
console.log(foo === bar);

Answer 33

// { qux: 1, baz: 2 }
// false – bar is a new object

Question 34

Spread syntax returns only enumeralbe properties of an object
t or f

Answer 34

True
Spread syntax with objects only returns the properties that Object.keys would return. That is, it only returns enumerable “own” properties. That means, in part, that it’s not the right choice when you need to duplicate objects that inherit from some other object. It also means that you lose the object prototype.

For now, all you need to know about enumerable properties is that the object prototype is not enumerable. Also, the length property of an array is not enumerable.

Question 35

What does this do?
let foo = [1, 2, 3, 4];
let [ bar, …otherStuff ] = foo;
console.log(bar);
console.log(otherStuff);

Answer 35

// 1
// [2, 3, 4]

Question 36

let foo = {bar: 1, qux: 2, baz: 3, xyz: 4};
let { bar, baz, …otherStuff } = foo;
console.log(bar);
console.log(baz);
console.log(otherStuff);

Answer 36

// 1
// 3
// {qux: 2, xyz: 4}

Question 37

let array1 = [1, 2, 3, 4]

let [one, …two, three] = array1;

console.log(two);

Answer 37

Syntax error

Question 38

function maxItem() {
let maximum = arguments[0];
[].forEach.call(arguments, value => {
if (value > maximum) {
maximum = value;
}
});

return maximum;
}

console.log(maxItem(2, 6, 10, 4, -3));

Answer 38

10

Question 39

rewrite

function maxItem() {
let maximum = arguments[0];
[].forEach.call(arguments, value => {
if (value > maximum) {
maximum = value;
}
});

return maximum;
}

console.log(maxItem(2, 6, 10, 4, -3));

using the rest syntax

Answer 39

function maxItem(first, …moreArgs) {
let maximum = first;
moreArgs.forEach(value => {
if (value > maximum) {
maximum = value;
}
});

return maximum;
}

console.log(maxItem(2, 6, 10, 4, -3));