L1-Object Methods

Question 1

Object methods: keys, values, entries, assign, freeze

Question 2

Object methods: keys, values, entries, assign, freeze

Question 3

Assign entries from the second object argument to the end of the first object argument.
+
Mutates the first object argument
+
Returns the mutated first argument object.

SIMPLE LANGUAGE
- a method for combining and/or copying objects

Answer 3

Object.assign(myObj1, myObj2)

Question 4

What is logged to console from:
const target = { a: 1, b: 2 };
const source = { b: 4, c: 5 };
const returnedTarget = Object.assign(target, source);

console. log(target);
console. log(source);
console. log(returnedTarget);

Answer 4

// expected output: Object { a: 1, b: 4, c: 5 }
// expected output: Object  { b: 4, c: 5 }
// expected output: Object { a: 1, b: 4, c: 5 }

Question 5

What is logged to console from:
let myObject1 = {one: "1"};
let myObject2 = {2: "2"};
Object.assign(myObject1, myObject2)['one'] = 'seven';
console.log(myObject1);

Why

Answer 5

// expected output: { ‘2’: ‘2’, one: ‘seven’ }

Object.assign() returns the mutated first argument object.

Question 6

What is logged to console.

const target = { a: 1, b: 2 };
const source = { b: 4, c: 5 };
const returnedTarget = Object.assign(target, source);

console. log(target);
console. log(source);
console. log(returnedTarget);

Answer 6

// expected output: Object { a: 1, b: 4, c: 5 }

// expected output: Object { b: 4, c: 5 }

// expected output: Object { a: 1, b: 4, c: 5 }

Question 7

const target = { a: 1, b: 2 };
const source = { b: 4, c: 5, d: {e: 6}};
const returnedTarget = Object.assign(target, source);

What is returned from the following

target. b = 444;
console. log(target);
console. log(source);
console. log(returnedTarget);

source. b = 444;
console. log(target);
console. log(source);
console. log(returnedTarget);

source. d.e = 666;
console. log(target);
console. log(source);
console. log(returnedTarget);

Answer 7

Both target and returned target are changed. But not the source.

Only the source is changed.

The nested object of the source, target and returned target are all changed (it’s the same object being referenced)

Question 8

What happens when a source has the same key name as the target?

let user = { name: “John” };

Object.assign(user, { name: “Pete” });

alert(user.name);

Answer 8

The target key gets overridden

Question 9

Does Object.assign({}, oldObject) do a deep or shallow copy?

Answer 9

Shallow

Question 10

What does
Object.assign()
do?
and return?

Answer 10

Copies all enumerable own properties from one or more source objects to a target object
Object.assign(target, …sources)

Returns the target object argument.

Question 11

Convert a JSON file into an object

Answer 11

let apple = require(‘./test.json’);

let newApple = JSON.parse(JSON.stringify(apple));

console.log(newApple.apple);

Question 12

Stop things happening in array/object argument (shallow)

Answer 12

Object.freeze(myObjectorArray)

Only freezes 1 object at a time

Question 13

What does
Object.freeze()
do?
and return?

Answer 13

Freezes an object. A frozen object can no longer be changed; freezing an object prevents:

new properties from being added to it
existing properties from being removed
prevents changing the enumerability
configurability or writability of existing properties
and prevents the values of existing properties from being changed.
In addition, freezing an object also prevents its prototype from being changed. freeze() returns the same object that was passed in.
Values that are objects can still be modified, unless they are also frozen.

Returns the object.

Question 14

Which of these throw an error?

let obj = Object.freeze({ a: ‘foo’ });
let arr = Object.freeze([‘a’, ‘b’, ‘c’]);

obj[‘b’] = ‘bar’;

obj.c = ‘ccc’;

arr[3] = ‘x’;

arr.push(‘d’);

Answer 14

Trying to use a method to mutate a frozen object raises an exception. However, if we try to use assignment, the assignment fails silently. Another one of those JavaScript quirks!

let obj = Object.freeze({ a: ‘foo’ });
let arr = Object.freeze([‘a’, ‘b’, ‘c’]);

obj[‘b’] = ‘bar’;
obj; // => { a: ‘foo’ }

obj.c = ‘ccc’;
obj; // => { a: ‘foo’ }

arr[3] = ‘x’;
arr; // => [ ‘a’, ‘b’, ‘c’ ]

arr.push(‘d’); // => TypeError: Cannot add property 3, object is not extensible

Question 15

What does this do?
let target = [1, 2];
let source = [3, 4];

Object.assign(target, source);

console.log(target);

Answer 15

[3, 4]

Because the target and source have the same keynames!