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JavaScript: The Definitive Guide by David Flanagan

JavaScript: The Definitive Guide by David Flanagan

An expression is a phrase that can be evaluated to produce a value.

Statements are full sentences that end with a semicolon.

A function is a named and parameterized block of JavaScript code we define once and can them invoke over and over again.

Methods are functions within objects, properties of an object.

The “this” keyword refers to the object on which the method is defined.

Variables allow a value to be referred to by name.

An array is an ordered collection of numbered values.

A function is an object that has executable code associated with it.

Constructor: a function written to be used with the new operator to initialize a newly created object.

The JavaScript interpreter performs automatic garbage collection for memory management. When an object is no longer referred to, the interpreter reclaims the memory.

Strings are an array of characters, they are immutable.

JavaScript converts values liberally from one type to another.

Variables are untyped.

Variables declared outside of a function are global variables and can be accessed anywhere.

String methods return a new string, they do not modify the string they are invoked on.

Undefined is not initialized or nonexistent.

Null == undefined // true, null === undefined // false

We can copy or compare objects or arrays with for loops.

Triple equals (===) does not perform conversions when testing for equality.

JavaScript has function scope, not block scope. Let keyword is block scoped.

JavaScript objects are compared by reference, not by value.

Short circuit: when an expression with && or || stops evaluating based on the first candidate.

|| (or) is often used in function bodies to supply default values for parameters.

For (variable in object) loops through properties of an object.

You cannot rely on a specific order with for/in on object properties.

Scope chain: a list of objects that are searched, in order, to perform variable name resolution.

The key feature of JavaScript is “prototypal inheritance.” Each object has a prototype which it inherits properties from.

Higher-order function: a function that operates on functions, taking one or more functions as arguments and returning a new function.

Memoization: cache previously computed results.

Classes define objects that share certain properties.

Instances have their own state in properties and behavior in methods.

Classes can be thought of as types.

Enumerated type: type with a finite set of values.

Modules export a public API: functions, classes, properties, and methods.

For/each loop is like the for/in loop but it iterates over the values instead of the properties.

An iterator is an object that allows iteration over some collection of values and keeps track of the current position in the collection.

Next() method returns the next value from the collection.

We often don’t use iterator object directly, instead, we use iterable objects.

You can call the iterators next() method which returns an array with two values, the first is a property name, the second is the value. for (let [k, v] in Iterator({ a: 1, b: 2 })).

Yield is used in generators to return values from a function.

Generator: an object that represents the current state of a generator function.

Return value of a nonblocking method cannot return the result. We need to provide a function to be invoked when the results are ready or the operation is complete. You can pass a function as an argument

The window object is the main entry point to all client-side JavaScript features and APIs.

The document represents the content displayed in the window.

JavaScript is commonly wrapped within a window.onload event handler.

Web apps use XMLHttpRequest objects to make scripted HTTP requests to obtain new information from the server without a page reload.

Embed JavaScript in HTML in 4 ways: 1. inline between script tags 2. external file in src attribute of script tag 3. in HTML event handler attribute, such as on click or onmouseover 4. in a URL that uses JavaScript.

JavaScript code in a script is executed once, in order to be interactive, a JavaScript program must define event handlers.

If a script defines a new global variable or function, that variable or function will be visible to any JavaScript code that runs after the script does.

Defer async on a script tag is used to tell the browser the script won’t be generating content and can continue to render the document while downloading the script.

Defer is wait until the document is loaded and parsed.

Async is run the scripts as soon as possible but do not block document parsing while the script is being downloaded.

Events have a name (type, ex. ‘click’), and a target (the object on which it occurred).

JavaScript is single-threaded so you never need to worry about locks, deadlock, or race conditions.

Use setTimeout or setInterval to run subtasks in the background while you update a progress indicator that displays feedback for the user.

Web worker: background thread for performing computationally intensive tasks without freezing the user interface.

Document.readyState == ‘loading’ when creating the document object and parsing the HTML elements. Script tags without async or defer are executed synchronously and the parser pauses while the script downloads and runs.

When the document is completely parsed, document.readyState == ‘interactive’.

When all content is loaded and all async scripts have loaded and executed, the document.readyState == ‘complete’ and the browser fires a load event on the window object.

Browser sniffer: code that determines the vendor and vision of the current browser.

A JavaScript program can close windows it opened itself.

The document object represents the content of the window.

DOM is an API for representing and manipulating document content.

Query a document for elements with id, name, tag, class, and css selector.

A NodeList object behaves like a read-only array of Element Objects.

Nodes are connected in a doubly linked list.

In general, to convert between the document and viewport coordinates we must add or subtract the scroll offsets.

PageXOffset and pageYOffset help determine the scrollbar positions for the browser window.

Viewport size: window.innerWidth and window.innerHeight.

Window or element.scrollTop can be set to make the browser scroll.

Event types are all lowercase.

Within the body of an event handler, ‘this’ refers to the event target. The single argument is the event object.

Like all JavaScript functions, event handlers are lexically scoped: they are executed in the scope in which they are defined, not the scope from which they are invoked.

If an event handler returns false, it tells the browser not to perform the default action.

All data stored in cookies is transmitted to the server with every HTTP request

Local storage is permanent.

SessionStorage exists until the window or tab is closed.

Use a manifest file to list all urls that should be cached.

Browser online check: navigator.onLine.

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Parker Klein ✌️

Parker Klein ✌️

Creator of Twos ✌️ (www.TwosApp.com): simply remember *things* Former Googler, programmer, reader, and writer