JavaScript: functions, arrays and objects

The three ways of writing a function, default parameters, array methods (map, filter, reduce, slice, splice…) and objects, copied by reference.

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Arrow function: what is it?
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A short way of writing a function with an arrow, such as x => x * 2, where the expression's value is returned automatically when there are no braces.
With braces, return is required: x => { x * 2 } returns nothing (undefined). It is the most common slip.
What do a and c hold at the end?

```javascript
const a = [1, 2, 3];
const b = a;
b.push(4);
const c = a.map(x => x * 2).filter(x => x > 4);
`
- a = [1, 2, 3, 4] and c = [6, 8]
- a = [1, 2, 3] and c = [6, 8]
- a = [1, 2, 3, 4] and c = [6]
- a = [1, 2, 3] and c = [8]
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a = [1, 2, 3, 4] and c = [6, 8]
b = a does not copy: both names refer to the same array, so push on b changes a. Then map gives [2, 4, 6, 8] and filter keeps what exceeds 4: [6, 8].
An object property is read with … notation, such as client.nom, or with … notation, such as client['code-postal'], which is needed when the name is not a valid identifier.
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An object property is read with dot notation, such as client.nom, or with bracket notation, such as client['code-postal'], which is needed when the name is not a valid identifier.
Brackets also accept a variable: client[field]. A missing property gives undefined, with no error.
Default parameter: what is it?
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A value written in a function's declaration, used when the call gives no argument or gives undefined.
With function double(n = 2), double() and double(undefined) give 4, but double(null) gives 0: null is a value that was provided.
What does this program print?

```javascript
const items = [{ prix: 10 }, { prix: 5 }];
console.log(items.map(i => { i.prix }));
`
- [ 10, 5 ]
- [ undefined, undefined ]
- [ { prix: 10 }, { prix: 5 } ]
- undefined
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[ undefined, undefined ]
With braces, the arrow function has a function body: without return, it returns undefined on every turn. Write i => i.prix or i => { return i.prix }.

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