What Does ≪≪-Constant Do

What does ! do in Flutter/Dart?

The exclamation mark is a dangerous but powerful operator.

It tells the compiler to assume the value is not null.

When you do this, the type gets cast from a nullable to a non-nullable type, and can be used in places that accept only non-nullable values.

When you do this, you must entirely trust your code not to accidentally use a null value.

More information in this question/answer.

In your specific situation, we can be sure that the value is not null because of your if statement that checks this.

Because of this, I would imagine you can just use _selectedFile without any additional operator. When you do this, you force the compiler to prove the safety of your code, rather than you just trusting yourself. If the compiler is satisfied, the code will compile. I haven't used Dart, so I don't know how smart the compiler is, but hopefully it can handle this circumstance without requiring you to take a risk with the ! operator.

What does *= do?

The *= operator is called multiplication assignment operator and is shorthand for multiplying the operand to the left with the operand to the right and assigning the result to the operand to the left. In this case, it's the same as:

x = x * 2;

Here integer promotion first takes place and the result of x * 2 is indeed 260.

However, an unsigned char can usually only carry values between 0 and 255 (inclusive) so the result overflows (wraps around) when you try assigning values above 255 and 260 % 256 == 4.

What does := do?


In computer programming languages, the equals sign typically denotes either a boolean operator to test equality of values (e.g. as in Pascal or Eiffel), which is consistent with the symbol's usage in mathematics, or an assignment operator (e.g. as in C-like languages). Languages making the former choice often use a colon-equals (:=) or ≔ to denote their assignment operator. Languages making the latter choice often use a double equals sign (==) to denote their boolean equality operator.

Note: I found this by searching for colon equals operator

What does ?. do in Dart?

It's a null safe operator.

Use ?. when you want to call a method/getter on an object IF that
object is not null (otherwise, return null).


Call open() only if it's not null.

More info: https://medium.com/@thinkdigitalsoftware/null-aware-operators-in-dart-53ffb8ae80bb

What does ? ... : ... do?

This is a ternary operator:

The expression (expr1) ? (expr2) : (expr3) evaluates to expr2 if expr1 evaluates to TRUE, and expr3 if expr1 evaluates to FALSE.

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?


  • 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


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:  —  Colon: labels

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


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?

What does '!!~' do in Javascript?

indexOf will return the index 0-x if the element is found, -1 otherwise.

~ will change all the bits in the number, turning -1 to 0 (the binary represantation of -1 is 1111 1111 ...).

0 is a falsy value, all other numbers are truthy.

!! will convert a falsy value to false and a truthy value to true. It wouldn't be needed here, because every doesn't care whether it recieves a truthy value or true.

As others have mentioned, nowadays you could use includes. However, includes is newer to the JavaScript ecosystem than indexOf, so this solution would work in IE, the solution with include would not. You could still use includes in IE either by providing a polyfill or by transpiling your code.

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