What Does "Options = Options || {}" Mean in JavaScript

What does options = options || {} mean in Javascript?

This is useful to setting default values to function arguments, e.g.:

function test (options) {
  options = options || {};
}

If you call test without arguments, options will be initialized with an empty object.

The Logical OR || operator will return its second operand if the first one is falsy.

Falsy values are: 0, null, undefined, the empty string (""), NaN, and of course false.

ES6 UPDATE: Now, we have real default parameter values in the language since ES6.

function test (options = {}) {
  //...
}

If you call the function with no arguments, or if it's called explicitly with the value undefined, the options argument will take the default value. Unlike the || operator example, other falsy values will not cause the use of the default value.

What does = mean in JavaScript?

It's a new feature that introduced in ES6 and is called arrow function. The left part denotes the input of a function and the right part the output of that function.

So in your case

s.split('')

splits the input on empty spaces and for each element of the resulted array you apply the following function:

(x,index) => x.toUpperCase()+Array(index+1).join(x.toLowerCase())

The left part is the random element, x of the array (s.split('')) and it's corresponding index. The second part applies a transformation to this input.

What does 'this.x1 = options.x1 || 0'; mean in Javascript?

It's a coalesce... which means it'll assign 0 if options.x1 is falsy... and options.x1 otherwise.

What does this symbol mean in JavaScript?

See the documentation on MDN about expressions and operators and statements.

Basic keywords and general expressions

`this` keyword:

How does the "this" keyword work?

`var x = function()` vs. `function x()` — Function declaration syntax

var functionName = function() {} vs function functionName() {}

`(function(){`…`})()` — IIFE (Immediately Invoked Function Expression)

What is the purpose?, How is it called?
Why does (function(){…})(); work but function(){…}(); doesn't?
(function(){…})(); vs (function(){…}());
shorter alternatives:
- !function(){…}(); - What does the exclamation mark do before the function?
- +function(){…}(); - JavaScript plus sign in front of function expression
- !function(){ }() vs (function(){ })(), ! vs leading semicolon
(function(window, undefined){…}(window));

`someFunction()()` — Functions which return other functions

Two sets of parentheses after function call

`=>` — Equal sign, greater than: arrow function expression syntax

What's the meaning of "=>" (an arrow formed from equals & greater than) in JavaScript?

`|>` — Pipe, greater than: Pipeline operator

What does the "|>" operator do in JavaScript?

`function`, `yield`, `yield` — Star after `function` or `yield`: generator functions

What is "function*" in JavaScript?
What's the yield keyword in JavaScript?
Delegated yield (yield star, yield *) in generator functions

`[]`, `Array()` — Square brackets: array notation

What’s the difference between "Array()" and "[]" while declaring a JavaScript array?
What is array literal notation in javascript and when should you use it?

If the square brackets appear on the left side of an assignment ([a] = ...), or inside a function's parameters, it's a destructuring assignment.

`{key: value}` — Curly brackets: object literal syntax (not to be confused with blocks)

What do curly braces in JavaScript mean?
Javascript object literal: what exactly is {a, b, c}?
What do square brackets around a property name in an object literal mean?
How does this object method definition work without the "function" keyword? (ES2015 Method definitions)

If the curly brackets appear on the left side of an assignment ({ a } = ...) or inside a function's parameters, it's a destructuring assignment.

`…`${`…`}`…` — Backticks, dollar sign with curly brackets: template literals

What does this `…${…}…` code from the node docs mean?
Usage of the backtick character (`) in JavaScript?
What is the purpose of template literals (backticks) following a function in ES6?

`/`…`/` — Slashes: regular expression literals

Meaning of javascript text between two slashes

`$` — Dollar sign in regex replace patterns: `$$`, `$&`, $`, `$'`, `$n`

JavaScript replace() method dollar signs

`()` — Parentheses: grouping operator

MDN: Grouping operator

Property-related expressions

`obj.prop`, `obj[prop]`, `obj["prop"]` — Square brackets or dot: property accessors

JavaScript property access: dot notation vs. brackets?

`?.`, `?.[]`, `?.()` — Question mark, dot: optional chaining operator

Question mark after parameter
Null-safe property access (and conditional assignment) in ES6/2015
Optional Chaining in JavaScript
Is there a null-coalescing (Elvis) operator or safe navigation operator in javascript?
Is there a "null coalescing" operator in JavaScript?

`::` — Double colon: bind operator

JavaScript double colon (bind operator)

`new` operator

What is the 'new' keyword in JavaScript?
What is "new.target"?

`...iter` — Three dots: spread syntax; rest parameters

(...rest) => {} — What is the meaning of “…args” (three dots) in a function definition?
fn(...args) — What is the meaning of “foo(…arg)” (three dots in a function call)?
[...iter] — javascript es6 array feature […data, 0] “spread operator”
{...props} — Javascript Property with three dots (…), What does the '…rest' stand for in this object destructuring?

Increment and decrement

`++`, `--` — Double plus or minus: pre- / post-increment / -decrement operators

++someVariable vs someVariable++ in Javascript

Unary and binary (arithmetic, logical, bitwise) operators

`delete` operator

What is the purpose of the delete operator in Javascript?

`void` operator

What does `void 0` mean?

`+`, `-` — Plus and minus: addition or concatenation, and subtraction operators; unary sign operators

What does = +_ mean in JavaScript, Single plus operator in javascript
What's the significant use of unary plus and minus operators?
Why is [1,2] + [3,4] = "1,23,4" in JavaScript?
Why does JavaScript handle the plus and minus operators between strings and numbers differently?

`|`, `&`, `^`, `~` — Single pipe, ampersand, circumflex, tilde: bitwise OR, AND, XOR, & NOT operators

What do these JavaScript bitwise operators do?
How to: The ~ operator?
Is there a & logical operator in Javascript
What does the "|" (single pipe) do in JavaScript?
What does the operator |= do in JavaScript?
What does the ^ (caret) symbol do in JavaScript?
Using bitwise OR 0 to floor a number, How does x|0 floor the number in JavaScript?
Why does ~1 equal -2?
What does ~~ ("double tilde") do in Javascript?
How does !!~ (not not tilde/bang bang tilde) alter the result of a 'contains/included' Array method call? (also here and here)

`%` — Percent sign: remainder operator

What does % do in JavaScript?

`&&`, `||`, `!` — Double ampersand, double pipe, exclamation point: logical operators

Logical operators in JavaScript — how do you use them?
Logical operator || in javascript, 0 stands for Boolean false?
What does "var FOO = FOO || {}" (assign a variable or an empty object to that variable) mean in Javascript?, JavaScript OR (||) variable assignment explanation, What does the construct x = x || y mean?
Javascript AND operator within assignment
What is "x && foo()"? (also here and here)
What is the !! (not not) operator in JavaScript?
What is an exclamation point in JavaScript?

`??` — Double question mark: nullish-coalescing operator

How is the nullish coalescing operator (??) different from the logical OR operator (||) in ECMAScript?
Is there a null-coalescing (Elvis) operator or safe navigation operator in javascript?
Is there a "null coalescing" operator in JavaScript?

`**` — Double star: power operator (exponentiation)

x ** 2 is equivalent to Math.pow(x, 2)
Is the double asterisk ** a valid JavaScript operator?
MDN documentation

Equality operators

`==`, `===` — Equal signs: equality operators

Which equals operator (== vs ===) should be used in JavaScript comparisons?
How does JS type coercion work?
In Javascript, <int-value> == "<int-value>" evaluates to true. Why is it so?
[] == ![] evaluates to true
Why does "undefined equals false" return false?
Why does !new Boolean(false) equals false in JavaScript?
Javascript 0 == '0'. Explain this example
Why false == "false" is false?

`!=`, `!==` — Exclamation point and equal signs: inequality operators

!= vs. !==
What is the difference between != and !== operators in JavaScript?

Bit shift operators

`<<`, `>>`, `>>>` — Two or three angle brackets: bit shift operators

What do these JavaScript bitwise operators do?
Double more-than symbol in JavaScript
What is the JavaScript >>> operator and how do you use it?

Conditional operator

…`?`…`:`… — Question mark and colon: conditional (ternary) operator

Question mark and colon in JavaScript
Operator precedence with Javascript Ternary operator
How do you use the ? : (conditional) operator in JavaScript?

Assignment operators

`=` — Equal sign: assignment operator

What is the difference between the `=` and `==` operators and what is `===`? (Single, double, and triple equals)

This symbol is also used for default parameters or default values in a destructuring assignment:

what does (state = {}) => state means
What does ({"key": "value"} = {}) syntax mean inside a JavaScript function

`%=` — Percent equals: remainder assignment

Having Confusion with Modulo operator

`+=` — Plus equals: addition assignment operator

How does += (plus equal) work?

`&&=`, `||=`, `??=` — Double ampersand, pipe, or question mark, followed by equal sign: logical assignments

What purpose do &&=, ||= and ??= serve?
Replace a value if null or undefined in JavaScript
Set a variable if undefined
Ruby’s ||= (or equals) in JavaScript?
Original proposal
Specification

`<<=`, `>>=`, `>>>=`, `&=`, `^=`, `|=` — Double less than, double greater than, triple greater than, ampersand, caret, or pipe followed by equal sign: bitwise assignments

What do these JavaScript bitwise operators do?

Destructuring

of function parameters: Where can I get info on the object parameter syntax for JavaScript functions?
of arrays: Multiple assignment in javascript? What does [a,b,c] = [1, 2, 3]; mean?
of objects/imports: Javascript object bracket notation ({ Navigation } =) on left side of assign

Comma operator

`,` — Comma operator (not to be confused with the comma used in variable declarations)

What does a comma do in JavaScript expressions?
Comma operator returns first value instead of second in argument list?
When is the comma operator useful?

Control flow

`{`…`}` — Curly brackets: blocks (not to be confused with object literal syntax)

JavaScript curly braces with no function or json

Declarations

`var`, `let`, `const` — Declaring variables

What's the difference between using "let" and "var"?
Are there constants in JavaScript?
What is the temporal dead zone?
var a, b; — Comma used in variable declarations (not to be confused with the comma operator): JavaScript variable definition: Commas vs. Semicolons

Label

`label:` — Colon: labels

What does the JavaScript syntax foo: mean?
What does ':' (colon) do in JavaScript?

Other

`123n` — `n` after integer: BigInt

What does character 'n' after numeric literal mean in JavaScript?

`#` — Hash (number sign): Private methods or private fields

What does the # symbol do in JavaScript?

`_` — Underscore: separator in numeric literals

Javascript numeric separators?
Is there a Javascript equivalent to the Ruby syntax using underscores (e.g. 10_000 = 10000) to make larger integers human readable?

In Javascript, what is an options object?

An options object is an object passed into a method (usually a method that builds a jQuery widget, or similar) which provides configuration info.

An options object is usually declared using object literal notation:

var options = {
 width: '325px',
 height: '100px'
};

The options that are valid depend on the method or widget that you are calling. There is nothing 'special' about an options object that makes it different from any other javascript object. The object literal syntax above gives the same result as:

var options = new Object();
options.width = '325px';
options.height = '100px';

Example:

$( ".selector" ).datepicker({ disabled: true });
//create a jQuery datepicker widget on the HTML elements matched by ".selector",
//using the option: disabled=true

Multiple arguments vs. options object

Like many of the others, I often prefer passing an options object to a function instead of passing a long list of parameters, but it really depends on the exact context.

I use code readability as the litmus test.

For instance, if I have this function call:

checkStringLength(inputStr, 10);

I think that code is quite readable the way it is and passing individual parameters is just fine.

On the other hand, there are functions with calls like this:

initiateTransferProtocol("http", false, 150, 90, null, true, 18);

Completely unreadable unless you do some research. On the other hand, this code reads well:

initiateTransferProtocol({
  "protocol": "http",
  "sync":      false,
  "delayBetweenRetries": 150,
  "randomVarianceBetweenRetries": 90,
  "retryCallback": null,
  "log": true,
  "maxRetries": 18
 });

It is more of an art than a science, but if I had to name rules of thumb:

Use an options parameter if:

You have more than four parameters
Any of the parameters are optional
You've ever had to look up the function to figure out what parameters it takes
If someone ever tries to strangle you while screaming "ARRRRRG!"

What does ({ key : value } = {}) syntax mean inside a JavaScript function

What does the syntax in the first example mean?

the used syntax is a combination of default parameters syntax and destructuring assignment syntax

What is the significance of the '= {}' part?

using the default parameters syntax: the '= {}' part set {} (an empty object) as the default value for the single parameter taken by getMovies

let's break it down:

here, we set 'flying' as default value for the power parameter

function giveSuperPower(power='flying'){
    console.log(`you have got: "${power}"`);
}

giveSuperPower('infinite energy');
// you have got: "infinite energy"
giveSuperPower();
// you have got: "flying"

what if the parameter was an object?

function giveSuperPower(power){
    console.log(`you have got: "${power.name||'flying'}"`);
    console.log(`it last for ${power.duration||'∞'}min`);
}
giveSuperPower({name: 'strength', duration: 5 });
// you have got: "strength"
// it last for 5min

looks good.. right?
lets try

giveSuperPower();
// Uncaught ReferenceError: power is not defined

okay that's a problem.

lets set a default parameter value (an empty object)

function giveSuperPower(power={}){
    console.log(`you have got: "${power.name||'flying'}"`);
    console.log(`it last for ${power.duration||'∞'}min`);
}

giveSuperPower();
//you have got: "flying" 
//it last for ∞min

let's destructure the power power parameter

function giveSuperPower({name, length} = {}){
    console.log(`you have got: "${name||'flying'}"`);
    console.log(`it last for ${duration||'∞'}min`);
}

let's set default parameter values for name and length

function giveSuperPower({name = 'flying', length = '∞'} = {}){
    console.log(`you have got: "${name}"`);
    console.log(`it last for ${duration}min`);
}

that's it :)

Typescript Type 'string' is not assignable to type

Update

As mentioned in @Simon_Weaver's answer, since TypeScript version 3.4 it's possible to assert it to const:

let fruit = "Banana" as const;

Old answer

You'll need to cast it:

export type Fruit = "Orange" | "Apple" | "Banana";
let myString: string = "Banana";

let myFruit: Fruit = myString as Fruit;

Also notice that when using string literals you need to use only one |