---
layout: post
title: JavaScript notes I
tags:
- javascript
## Make sure to change these
published: true
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---
Let's review some JavaScript features in a hurry!
## Iterables
Objects that can be used in `for..of` are called iterable.
Iterables must implement the method named `Symbol.iterator`. The result
of `obj[Symbol.iterator]` is called an iterator. An iterator must have
the method named `next()` that returns an object `{done: Boolean, value:
any}`, the value is the next value. The `Symbol.iterator` method is
called automatically by `for..of`, but we also can do it directly.
`Array.from(obj[, mapFn, thisArg])` makes a real Array of an iterable or
array-like obj, and we can then use array methods on it. The optional
arguments mapFn and thisArg allow us to apply a function to each item.
## Destructuring
Map an object or array to variables.
Object syntax:
let {prop : varName = default, ...} = object
Array syntax:
let [item1 = default, item2, ...rest] = array
The first item goes to `item1`; the second goes into `item2`, all the rest
makes the array rest.
## Map and Set
`Map`, is a collection of keyed values, it allows objects to be keys and
provides a `size` property.
`Set`, is a collection of unique values, does not allow to reorder elements.
The following allow garbage-collection:
There ara also `WeakMap` and `WeakSet` which provide garbage collection,
I need to research more about these. Maybe the next post.
