In my particular case:
callback instanceof Function
or
typeof callback == "function"
does it even matter, what's the difference?
Additional Resource:
JavaScript-Garden typeof vs instanceof
instanceofjavascripttypeof
In my particular case:
callback instanceof Function
or
typeof callback == "function"
does it even matter, what's the difference?
Additional Resource:
JavaScript-Garden typeof vs instanceof
JavaScript has a number formatter (part of the Internationalization API).
// Create our number formatter.
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
// These options are needed to round to whole numbers if that's what you want.
//minimumFractionDigits: 0, // (this suffices for whole numbers, but will print 2500.10 as $2,500.1)
//maximumFractionDigits: 0, // (causes 2500.99 to be printed as $2,501)
});
formatter.format(2500); /* $2,500.00 */
Use undefined
in place of the first argument ('en-US'
in the example) to use the system locale (the user locale in case the code is running in a browser). Further explanation of the locale code.
Here's a list of the currency codes.
A final note comparing this to the older .toLocaleString
. They both offer essentially the same functionality. However, toLocaleString in its older incarnations (pre-Intl) does not actually support locales: it uses the system locale. So when debugging old browsers, be sure that you're using the correct version (MDN suggests to check for the existence of Intl
). There isn't any need to worry about this at all if you don't care about old browsers or just use the shim.
Also, the performance of both is the same for a single item, but if you have a lot of numbers to format, using Intl.NumberFormat
is ~70 times faster. Therefore, it's usually best to use Intl.NumberFormat
and instantiate only once per page load. Anyway, here's the equivalent usage of toLocaleString
:
(2500).toLocaleString('en-US', {
style: 'currency',
currency: 'USD',
}); /* $2,500.00 */
en-US
out of the box. One solution is to install full-icu, see here for more informationJavaScript has lexical (also called static) scoping and closures. This means you can tell the scope of an identifier by looking at the source code.
The four scopes are:
Outside of the special cases of global and module scope, variables are declared using var
(function scope), let
(block scope), and const
(block scope). Most other forms of identifier declaration have block scope in strict mode.
Scope is the region of the codebase over which an identifier is valid.
A lexical environment is a mapping between identifier names and the values associated with them.
Scope is formed of a linked nesting of lexical environments, with each level in the nesting corresponding to a lexical environment of an ancestor execution context.
These linked lexical environments form a scope "chain". Identifier resolution is the process of searching along this chain for a matching identifier.
Identifier resolution only occurs in one direction: outwards. In this way, outer lexical environments cannot "see" into inner lexical environments.
There are three pertinent factors in deciding the scope of an identifier in JavaScript:
Some of the ways identifiers can be declared:
var
, let
and const
var
in non-strict mode)import
statementseval
Some of the locations identifiers can be declared:
Identifiers declared using var
have function scope, apart from when they are declared directly in the global context, in which case they are added as properties on the global object and have global scope. There are separate rules for their use in eval
functions.
Identifiers declared using let
and const
have block scope, apart from when they are declared directly in the global context, in which case they have global scope.
Note: let
, const
and var
are all hoisted. This means that their logical position of definition is the top of their enclosing scope (block or function). However, variables declared using let
and const
cannot be read or assigned to until control has passed the point of declaration in the source code. The interim period is known as the temporal dead zone.
function f() {
function g() {
console.log(x)
}
let x = 1
g()
}
f() // 1 because x is hoisted even though declared with `let`!
Function parameter names are scoped to the function body. Note that there is a slight complexity to this. Functions declared as default arguments close over the parameter list, and not the body of the function.
Function declarations have block scope in strict mode and function scope in non-strict mode. Note: non-strict mode is a complicated set of emergent rules based on the quirky historical implementations of different browsers.
Named function expressions are scoped to themselves (e.g., for the purpose of recursion).
In non-strict mode, implicitly defined properties on the global object have global scope, because the global object sits at the top of the scope chain. In strict mode, these are not permitted.
In eval
strings, variables declared using var
will be placed in the current scope, or, if eval
is used indirectly, as properties on the global object.
The following will throw a ReferenceError because the namesx
, y
, and z
have no meaning outside of the function f
.
function f() {
var x = 1
let y = 1
const z = 1
}
console.log(typeof x) // undefined (because var has function scope!)
console.log(typeof y) // undefined (because the body of the function is a block)
console.log(typeof z) // undefined (because the body of the function is a block)
The following will throw a ReferenceError for y
and z
, but not for x
, because the visibility of x
is not constrained by the block. Blocks that define the bodies of control structures like if
, for
, and while
, behave similarly.
{
var x = 1
let y = 1
const z = 1
}
console.log(x) // 1
console.log(typeof y) // undefined because `y` has block scope
console.log(typeof z) // undefined because `z` has block scope
In the following, x
is visible outside of the loop because var
has function scope:
for(var x = 0; x < 5; ++x) {}
console.log(x) // 5 (note this is outside the loop!)
...because of this behavior, you need to be careful about closing over variables declared using var
in loops. There is only one instance of variable x
declared here, and it sits logically outside of the loop.
The following prints 5
, five times, and then prints 5
a sixth time for the console.log
outside the loop:
for(var x = 0; x < 5; ++x) {
setTimeout(() => console.log(x)) // closes over the `x` which is logically positioned at the top of the enclosing scope, above the loop
}
console.log(x) // note: visible outside the loop
The following prints undefined
because x
is block-scoped. The callbacks are run one by one asynchronously. New behavior for let
variables means that each anonymous function closed over a different variable named x
(unlike it would have done with var
), and so integers 0
through 4
are printed.:
for(let x = 0; x < 5; ++x) {
setTimeout(() => console.log(x)) // `let` declarations are re-declared on a per-iteration basis, so the closures capture different variables
}
console.log(typeof x) // undefined
The following will NOT throw a ReferenceError
because the visibility of x
is not constrained by the block; it will, however, print undefined
because the variable has not been initialised (because of the if
statement).
if(false) {
var x = 1
}
console.log(x) // here, `x` has been declared, but not initialised
A variable declared at the top of a for
loop using let
is scoped to the body of the loop:
for(let x = 0; x < 10; ++x) {}
console.log(typeof x) // undefined, because `x` is block-scoped
The following will throw a ReferenceError
because the visibility of x
is constrained by the block:
if(false) {
let x = 1
}
console.log(typeof x) // undefined, because `x` is block-scoped
Variables declared using var
, let
or const
are all scoped to modules:
// module1.js
var x = 0
export function f() {}
//module2.js
import f from 'module1.js'
console.log(x) // throws ReferenceError
The following will declare a property on the global object because variables declared using var
within the global context are added as properties to the global object:
var x = 1
console.log(window.hasOwnProperty('x')) // true
let
and const
in the global context do not add properties to the global object, but still have global scope:
let x = 1
console.log(window.hasOwnProperty('x')) // false
Function parameters can be considered to be declared in the function body:
function f(x) {}
console.log(typeof x) // undefined, because `x` is scoped to the function
Catch block parameters are scoped to the catch-block body:
try {} catch(e) {}
console.log(typeof e) // undefined, because `e` is scoped to the catch block
Named function expressions are scoped only to the expression itself:
(function foo() { console.log(foo) })()
console.log(typeof foo) // undefined, because `foo` is scoped to its own expression
In non-strict mode, implicitly defined properties on the global object are globally scoped. In strict mode, you get an error.
x = 1 // implicitly defined property on the global object (no "var"!)
console.log(x) // 1
console.log(window.hasOwnProperty('x')) // true
In non-strict mode, function declarations have function scope. In strict mode, they have block scope.
'use strict'
{
function foo() {}
}
console.log(typeof foo) // undefined, because `foo` is block-scoped
Scope is defined as the lexical region of code over which an identifier is valid.
In JavaScript, every function-object has a hidden [[Environment]]
reference that is a reference to the lexical environment of the execution context (stack frame) within which it was created.
When you invoke a function, the hidden [[Call]]
method is called. This method creates a new execution context and establishes a link between the new execution context and the lexical environment of the function-object. It does this by copying the [[Environment]]
value on the function-object, into an outer reference field on the lexical environment of the new execution context.
Note that this link between the new execution context and the lexical environment of the function object is called a closure.
Thus, in JavaScript, scope is implemented via lexical environments linked together in a "chain" by outer references. This chain of lexical environments is called the scope chain, and identifier resolution occurs by searching up the chain for a matching identifier.
Find out more.
Best Answer
Use
instanceof
for custom types:Use
typeof
for simple built in types:Use
instanceof
for complex built in types:And the last one is a little bit tricky: