Related
I read this question about the "comma operator" in expressions (,) and the MDN docs about it, but I can't think of a scenario where it is useful.
So, when is the comma operator useful?
The following is probably not very useful as you don't write it yourself, but a minifier can shrink code using the comma operator. For example:
if(x){foo();return bar()}else{return 1}
would become:
return x?(foo(),bar()):1
The ? : operator can be used now, since the comma operator (to a certain extent) allows for two statements to be written as one statement.
This is useful in that it allows for some neat compression (39 -> 24 bytes here).
I'd like to stress the fact that the comma in var a, b is not the comma operator because it doesn't exist within an expression. The comma has a special meaning in var statements. a, b in an expression would be referring to the two variables and evaluate to b, which is not the case for var a, b.
The comma operator allows you to put multiple expressions in a place where one expression is expected. The resulting value of multiple expressions separate by a comma will be the value of the last comma separated expression.
I don't personally use it very often because there aren't that many situations where more than one expression is expected and there isn't a less confusing way to write the code than using the comma operator. One interesting possibility is at the end of a for loop when you want more than one variable to be incremented:
// j is initialized to some other value
// as the for loop executes both i and j are incremented
// because the comma operator allows two statements to be put in place of one
for (var i = 0; i < items.len; i++, j++) {
// loop code here that operates on items[i]
// and sometimes uses j to access a different array
}
Here you see that i++, j++ can be put in a place where one expression is allowed. In this particular case, the multiple expressions are used for side affects so it does not matter that the compound expressions takes on the value of the last one, but there are other cases where that might actually matter.
The Comma Operator is frequently useful when writing functional code in Javascript.
Consider this code I wrote for a SPA a while back which had something like the following
const actions = _.chain(options)
.pairs() // 1
.filter(selectActions) // 2
.map(createActionPromise) // 3
.reduce((state, pair) => (state[pair[0]] = pair[1], state), {}) // 4
.value();
This was a fairly complex, but real-world scenario. Bear with me while I explain what is happening, and in the process make the case for the Comma Operator.
This uses Underscore's chaining to
Take apart all of the options passed to this function using pairs
which will turn { a: 1, b: 2} into [['a', 1], ['b', 2]]
This array of property pairs is filtered by which ones are deemed to be 'actions' in the system.
Then the second index in the array is replaced with a function that returns a promise representing that action (using map)
Finally the call to reduce will merge each "property array" (['a', 1]) back into a final object.
The end result is a transformed version of the options argument, which contains only the appropriate keys and whose values are consumable by the calling function.
Looking at just
.reduce((state, pair) => (state[pair[0]] = pair[1], state), {})
You can see the reduce function starts with an empty state object, state, and for each pair representing a key and value, the function returns the same state object after adding a property to the object corresponding to the key/value pair. Because of ECMAScript 2015's arrow function syntax, the function body is an expression, and as a result, the Comma Operator allows a concise and useful "iteratee" function.
Personally I have come across numerous cases while writing Javascript in a more functional style with ECMAScript 2015 + Arrow Functions. Having said that, before encountering arrow functions (such as at the time of the writing of the question), I'd never used the comma operator in any deliberate way.
Another use for the comma operator is to hide results you don't care about in the repl or console, purely as a convenience.
For example, if you evaluate myVariable = aWholeLotOfText in the repl or console, it will print all the data you just assigned. This might be pages and pages, and if you'd prefer not to see it, you can instead evaluate myVariable = aWholeLotOfText, 'done', and the repl/console will just print 'done'.
Oriel correctly points out†that customized toString() or get() functions might even make this useful.
Comma operator is not specific to JavaScript, it is available in other languages like C and C++. As a binary operator this is useful when the first operand, which is generally an expression, has desired side effect required by second operand. One example from wikipedia:
i = a += 2, a + b;
Obviously you can write two different lines of codes, but using comma is another option and sometimes more readable.
I'd disagree with Flanagan, and say, that comma is really useful and allows to write more readable and elegant code, especially when you know what you're doing:
Here's the greatly detailed article on comma usage:
Several examples from out from there for the proof of demonstration:
function renderCurve() {
for(var a = 1, b = 10; a*b; a++, b--) {
console.log(new Array(a*b).join('*'));
}
}
A fibonacci generator:
for (
var i=2, r=[0,1];
i<15;
r.push(r[i-1] + r[i-2]), i++
);
// 0,1,1,2,3,5,8,13,21,34,55,89,144,233,377
Find first parent element, analogue of jQuery .parent() function:
function firstAncestor(el, tagName) {
while(el = el.parentNode, el && (el.tagName != tagName.toUpperCase()));
return el;
}
//element in http://ecma262-5.com/ELS5_HTML.htm
var a = $('Section_15.1.1.2');
firstAncestor(a, 'div'); //<div class="page">
I haven't found practical use of it other than that but here is one scenario in which James Padolsey nicely uses this technique for IE detection in a while loop:
var ie = (function(){
var undef,
v = 3,
div = document.createElement('div'),
all = div.getElementsByTagName('i');
while ( // <-- notice no while body here
div.innerHTML = '<!--[if gt IE ' + (++v) + ']><i></i><![endif]-->',
all[0]
);
return v > 4 ? v : undef;
}());
These two lines must to execute :
div.innerHTML = '<!--[if gt IE ' + (++v) + ']><i></i><![endif]-->',
all[0]
And inside comma operator, both are evaluated though one could have made them separate statements somehow.
There is something "odd" that can be done in JavaScript calling a function indirectly by using the comma operator.
There is a long description here:
Indirect function call in JavaScript
By using this syntax:
(function() {
"use strict";
var global = (function () { return this || (1,eval)("this"); })();
console.log('Global === window should be true: ', global === window);
var not_global = (function () { return this })();
console.log('not_global === window should be false: ', not_global === window);
}());
You can get access to the global variable because eval works differently when called directly vs called indirectly.
I've found the comma operator most useful when writing helpers like this.
const stopPropagation = event => (event.stopPropagation(), event);
const preventDefault = event => (event.preventDefault(), event);
const both = compose(stopPropagation, preventDefault);
You could replace the comma with either an || or &&, but then you'd need to know what the function returns.
More important than that, the comma separator communicates intent -- the code doesn't care what the left-operand evaluates to, whereas the alternatives may have another reason for being there. This in turn makes it easier to understand and refactor. If the function return type ever changes, the code above would not be affected.
Naturally you can achieve the same thing in other ways, but not as succinctly. If || and && found a place in common usage, so too can the comma operator.
One typical case I end up using it is during optional argument parsing. I think it makes it both more readable and more concise so that the argument parsing doesn't dominate the function body.
/**
* #param {string} [str]
* #param {object} [obj]
* #param {Date} [date]
*/
function f(str, obj, date) {
// handle optional arguments
if (typeof str !== "string") date = obj, obj = str, str = "default";
if (obj instanceof Date) date = obj, obj = {};
if (!(date instanceof Date)) date = new Date();
// ...
}
Let's say you have an array:
arr = [];
When you push onto that array, you are rarely interested in push's return value, namely the new length of the array, but rather the array itself:
arr.push('foo') // ['foo'] seems more interesting than 1
Using the comma operator, we can push onto the array, specify the array as the last operand to comma, and then use the result -- the array itself -- for a subsequent array method call, a sort of chaining:
(arr.push('bar'), arr.push('baz'), arr).sort(); // [ 'bar', 'baz', 'foo' ]
It saves you from using return in nested conditionals and it's very handy especially with the ternary operator. Such as;
function insert(v){
return this.node > v ? this.left.size < this.right.size ? ( this.left.insert(v)
, this
)
: ( this.left.insert(this.node)
, this.node = this.right.popmin()
, this.insert(v)
, this
)
: this.left.size < this.right.size ? ( this.right.insert(this.node)
, this.node = this.left.popmax()
, this.insert(v)
, this
)
: ( this.right.insert(v)
, this
)
}
I just came across this today looking at the proposals for pipeline operator proposal and partial application...
(https://github.com/tc39/proposal-pipeline-operator
(https://github.com/tc39/proposal-partial-application#hack-style-pipelines)
Also, Hack-style pipelines are already feasible without introducing new syntax today:
let $; // Hack-style topic variable
let result = (
$= books,
$= filter($, _ => _.title = "..."),
$= map($, _ => _.author),
$);
The use of comma expressions here can kind of fake the pipeline operator that isn't in the language yet.
Eliminating the space between $= simulates the feeling of a proper pipe token, |>. Note that the "topic" variable, $, can be anything here and that it's just shorthand for repeatedly overwriting the variable. So something more akin to ...
// blocking inside an IIFE
let result = (() => {
let $;
$ = books;
$ = filter($, _ => _.title = "..."),
$ = map($, _ => _.author),
return $;
})()
The "comma" version successfully cuts out some of the noise, getting you closer to what the proposal would be:
let result = books
|> filter($, _ => _.title = "..."
|> map($, _ => _.author)
Here's another example using it to compose functions:
const double = (x) => 2 * x;
const add = (x, y) => x + y;
const boundScore = (min, max, score) => Math.max(min, Math.min(max, score));
const calculateScore = ($) => (
$= double($),
$= add($, 20),
$= boundScore(0, 100, $),
(console.log($), $)
)
const score = calculateScore(28)
The comma operator (,) evaluates each of its operands (from left to right) and returns the value of the last operand. This lets you create a compound expression in which multiple expressions are evaluated, with the compound expression's final value being the value of the rightmost of its member expressions. This is commonly used to provide multiple parameters to a for loop.
let x = 1;
x = (x++, x);
console.log(x);
// expected output: 2
x = (2, 3);
console.log(x);
// expected output: 3
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Comma_Operator
Another area where comma operator can be used is Code Obfuscation.
Let's say a developper writes some code like this:
var foo = 'bar';
Now, she decides to obfuscate the code. The tool used may changed the code like this:
var Z0b=(45,87)>(195,3)?'bar':(54,65)>(1,0)?'':'baz';// Z0b == 'bar'
Demo: http://jsfiddle.net/uvDuE/
Just learning to code JavaScript, trying to learn if statements but my code isn't working:
var car = 8;
if (car = 9) {
document.write("your code is not working")
}
This executes the write command and I have no idea why. I'm using the tab button for indents, is that not allowed?
= is called assignment operator in JavaScript, it assigns the value of the right hand side expression to the variable on the left hand side.
You have to use comparison operator instead of assignment operator like this
if (car === 9)
We have two comparison operators in JavaScript, == and ===. The difference between them is that,
== checks if the values are the same, but === checks if the type and the value is also the same.
Go through the wonderful answers, to know more about == and ===
This line assigns car to the value of 9 and check if it is truthy (which 9 is).
if (car=9)
I think you want to use a comparison operator, like this:
if(car == 9)
use this code
var car = 8;
if (car==9)
{
document.write("your code is not working")
}
you need to understand about operators '=' is an assignment operator whereas '==' is a comparision operator.
See Tutorial
If you want
to compare if car is equal to 9, then you have to use code
if(car === 9){
/*Your code goes here*/
}
I read this question about the "comma operator" in expressions (,) and the MDN docs about it, but I can't think of a scenario where it is useful.
So, when is the comma operator useful?
The following is probably not very useful as you don't write it yourself, but a minifier can shrink code using the comma operator. For example:
if(x){foo();return bar()}else{return 1}
would become:
return x?(foo(),bar()):1
The ? : operator can be used now, since the comma operator (to a certain extent) allows for two statements to be written as one statement.
This is useful in that it allows for some neat compression (39 -> 24 bytes here).
I'd like to stress the fact that the comma in var a, b is not the comma operator because it doesn't exist within an expression. The comma has a special meaning in var statements. a, b in an expression would be referring to the two variables and evaluate to b, which is not the case for var a, b.
The comma operator allows you to put multiple expressions in a place where one expression is expected. The resulting value of multiple expressions separate by a comma will be the value of the last comma separated expression.
I don't personally use it very often because there aren't that many situations where more than one expression is expected and there isn't a less confusing way to write the code than using the comma operator. One interesting possibility is at the end of a for loop when you want more than one variable to be incremented:
// j is initialized to some other value
// as the for loop executes both i and j are incremented
// because the comma operator allows two statements to be put in place of one
for (var i = 0; i < items.len; i++, j++) {
// loop code here that operates on items[i]
// and sometimes uses j to access a different array
}
Here you see that i++, j++ can be put in a place where one expression is allowed. In this particular case, the multiple expressions are used for side affects so it does not matter that the compound expressions takes on the value of the last one, but there are other cases where that might actually matter.
The Comma Operator is frequently useful when writing functional code in Javascript.
Consider this code I wrote for a SPA a while back which had something like the following
const actions = _.chain(options)
.pairs() // 1
.filter(selectActions) // 2
.map(createActionPromise) // 3
.reduce((state, pair) => (state[pair[0]] = pair[1], state), {}) // 4
.value();
This was a fairly complex, but real-world scenario. Bear with me while I explain what is happening, and in the process make the case for the Comma Operator.
This uses Underscore's chaining to
Take apart all of the options passed to this function using pairs
which will turn { a: 1, b: 2} into [['a', 1], ['b', 2]]
This array of property pairs is filtered by which ones are deemed to be 'actions' in the system.
Then the second index in the array is replaced with a function that returns a promise representing that action (using map)
Finally the call to reduce will merge each "property array" (['a', 1]) back into a final object.
The end result is a transformed version of the options argument, which contains only the appropriate keys and whose values are consumable by the calling function.
Looking at just
.reduce((state, pair) => (state[pair[0]] = pair[1], state), {})
You can see the reduce function starts with an empty state object, state, and for each pair representing a key and value, the function returns the same state object after adding a property to the object corresponding to the key/value pair. Because of ECMAScript 2015's arrow function syntax, the function body is an expression, and as a result, the Comma Operator allows a concise and useful "iteratee" function.
Personally I have come across numerous cases while writing Javascript in a more functional style with ECMAScript 2015 + Arrow Functions. Having said that, before encountering arrow functions (such as at the time of the writing of the question), I'd never used the comma operator in any deliberate way.
Another use for the comma operator is to hide results you don't care about in the repl or console, purely as a convenience.
For example, if you evaluate myVariable = aWholeLotOfText in the repl or console, it will print all the data you just assigned. This might be pages and pages, and if you'd prefer not to see it, you can instead evaluate myVariable = aWholeLotOfText, 'done', and the repl/console will just print 'done'.
Oriel correctly points out†that customized toString() or get() functions might even make this useful.
Comma operator is not specific to JavaScript, it is available in other languages like C and C++. As a binary operator this is useful when the first operand, which is generally an expression, has desired side effect required by second operand. One example from wikipedia:
i = a += 2, a + b;
Obviously you can write two different lines of codes, but using comma is another option and sometimes more readable.
I'd disagree with Flanagan, and say, that comma is really useful and allows to write more readable and elegant code, especially when you know what you're doing:
Here's the greatly detailed article on comma usage:
Several examples from out from there for the proof of demonstration:
function renderCurve() {
for(var a = 1, b = 10; a*b; a++, b--) {
console.log(new Array(a*b).join('*'));
}
}
A fibonacci generator:
for (
var i=2, r=[0,1];
i<15;
r.push(r[i-1] + r[i-2]), i++
);
// 0,1,1,2,3,5,8,13,21,34,55,89,144,233,377
Find first parent element, analogue of jQuery .parent() function:
function firstAncestor(el, tagName) {
while(el = el.parentNode, el && (el.tagName != tagName.toUpperCase()));
return el;
}
//element in http://ecma262-5.com/ELS5_HTML.htm
var a = $('Section_15.1.1.2');
firstAncestor(a, 'div'); //<div class="page">
I haven't found practical use of it other than that but here is one scenario in which James Padolsey nicely uses this technique for IE detection in a while loop:
var ie = (function(){
var undef,
v = 3,
div = document.createElement('div'),
all = div.getElementsByTagName('i');
while ( // <-- notice no while body here
div.innerHTML = '<!--[if gt IE ' + (++v) + ']><i></i><![endif]-->',
all[0]
);
return v > 4 ? v : undef;
}());
These two lines must to execute :
div.innerHTML = '<!--[if gt IE ' + (++v) + ']><i></i><![endif]-->',
all[0]
And inside comma operator, both are evaluated though one could have made them separate statements somehow.
There is something "odd" that can be done in JavaScript calling a function indirectly by using the comma operator.
There is a long description here:
Indirect function call in JavaScript
By using this syntax:
(function() {
"use strict";
var global = (function () { return this || (1,eval)("this"); })();
console.log('Global === window should be true: ', global === window);
var not_global = (function () { return this })();
console.log('not_global === window should be false: ', not_global === window);
}());
You can get access to the global variable because eval works differently when called directly vs called indirectly.
I've found the comma operator most useful when writing helpers like this.
const stopPropagation = event => (event.stopPropagation(), event);
const preventDefault = event => (event.preventDefault(), event);
const both = compose(stopPropagation, preventDefault);
You could replace the comma with either an || or &&, but then you'd need to know what the function returns.
More important than that, the comma separator communicates intent -- the code doesn't care what the left-operand evaluates to, whereas the alternatives may have another reason for being there. This in turn makes it easier to understand and refactor. If the function return type ever changes, the code above would not be affected.
Naturally you can achieve the same thing in other ways, but not as succinctly. If || and && found a place in common usage, so too can the comma operator.
One typical case I end up using it is during optional argument parsing. I think it makes it both more readable and more concise so that the argument parsing doesn't dominate the function body.
/**
* #param {string} [str]
* #param {object} [obj]
* #param {Date} [date]
*/
function f(str, obj, date) {
// handle optional arguments
if (typeof str !== "string") date = obj, obj = str, str = "default";
if (obj instanceof Date) date = obj, obj = {};
if (!(date instanceof Date)) date = new Date();
// ...
}
Let's say you have an array:
arr = [];
When you push onto that array, you are rarely interested in push's return value, namely the new length of the array, but rather the array itself:
arr.push('foo') // ['foo'] seems more interesting than 1
Using the comma operator, we can push onto the array, specify the array as the last operand to comma, and then use the result -- the array itself -- for a subsequent array method call, a sort of chaining:
(arr.push('bar'), arr.push('baz'), arr).sort(); // [ 'bar', 'baz', 'foo' ]
It saves you from using return in nested conditionals and it's very handy especially with the ternary operator. Such as;
function insert(v){
return this.node > v ? this.left.size < this.right.size ? ( this.left.insert(v)
, this
)
: ( this.left.insert(this.node)
, this.node = this.right.popmin()
, this.insert(v)
, this
)
: this.left.size < this.right.size ? ( this.right.insert(this.node)
, this.node = this.left.popmax()
, this.insert(v)
, this
)
: ( this.right.insert(v)
, this
)
}
I just came across this today looking at the proposals for pipeline operator proposal and partial application...
(https://github.com/tc39/proposal-pipeline-operator
(https://github.com/tc39/proposal-partial-application#hack-style-pipelines)
Also, Hack-style pipelines are already feasible without introducing new syntax today:
let $; // Hack-style topic variable
let result = (
$= books,
$= filter($, _ => _.title = "..."),
$= map($, _ => _.author),
$);
The use of comma expressions here can kind of fake the pipeline operator that isn't in the language yet.
Eliminating the space between $= simulates the feeling of a proper pipe token, |>. Note that the "topic" variable, $, can be anything here and that it's just shorthand for repeatedly overwriting the variable. So something more akin to ...
// blocking inside an IIFE
let result = (() => {
let $;
$ = books;
$ = filter($, _ => _.title = "..."),
$ = map($, _ => _.author),
return $;
})()
The "comma" version successfully cuts out some of the noise, getting you closer to what the proposal would be:
let result = books
|> filter($, _ => _.title = "..."
|> map($, _ => _.author)
Here's another example using it to compose functions:
const double = (x) => 2 * x;
const add = (x, y) => x + y;
const boundScore = (min, max, score) => Math.max(min, Math.min(max, score));
const calculateScore = ($) => (
$= double($),
$= add($, 20),
$= boundScore(0, 100, $),
(console.log($), $)
)
const score = calculateScore(28)
The comma operator (,) evaluates each of its operands (from left to right) and returns the value of the last operand. This lets you create a compound expression in which multiple expressions are evaluated, with the compound expression's final value being the value of the rightmost of its member expressions. This is commonly used to provide multiple parameters to a for loop.
let x = 1;
x = (x++, x);
console.log(x);
// expected output: 2
x = (2, 3);
console.log(x);
// expected output: 3
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Comma_Operator
Another area where comma operator can be used is Code Obfuscation.
Let's say a developper writes some code like this:
var foo = 'bar';
Now, she decides to obfuscate the code. The tool used may changed the code like this:
var Z0b=(45,87)>(195,3)?'bar':(54,65)>(1,0)?'':'baz';// Z0b == 'bar'
Demo: http://jsfiddle.net/uvDuE/
In Ruby, I can write a || b and the function will choose a if it exists, and if not, b.
How can I do this in JQuery without writing something grossly cumbersome like:
if (a){
a
} else {
b
}
Also, does JQuery have an equivalent to Ruby's a ||= b?
jQuery is just a JavaScript library and in JavaScript we have the same.
var c = a || b;
This is because
If the first object is truthy, that gets returned. Otherwise, the second object gets returned.
In JavaScript, a || b evaluates to the first truthy value (or the last value if both are falsy), just as in Ruby. (Remember, jQuery is just a library for JavaScript.)
However, JavaScript has many more falsy (non-truthy values) than Ruby does so care may need to be taken. See Truthy and Falsy: When All is Not Equal in JavaScript.
For instance, in JavaScript: "" || "foo" will result in "foo" although it would have evaluated to "" in Ruby.
Happy coding.
And yes, JavaScript supports x Q= y for all binary operators x = x Q y. An easy way to find out is to just Try It And See :)
var myVariable = myVariable || "Default";
I've seen this format used in JavaScript code, but can't find a good source for the meaning.
Edit for a follow-up:
Thanks for all the quick answers! I figured it was something like that. Now, for bonus points:
can you use
(var1 ? var2)
to do the same thing as
if (var1) {
var2
}
?
It's known as a ternary (because it has three operands) conditional (because it's an if/else/then) operator.
It is evaluated to a value, so you would usually use it to assign a value, such as:
var result = condition ? value1 : value2;
Which is equivalent to:
var result;
if (condition == true) {
result = value1;
} else {
result = value2;
}
An example:
var message = "Length is " + len + " " + (len==1 ? "foot" : "feet");
Note ?: is the full operator. It's not a ? and : operator, so ? by itself is meaningless in Javascript.
Its a conditional operator.
It is
if var1 then var2 else var3
Read more here
Conditional Operator
The conditional operator is the only
JavaScript operator that takes three
operands. This operator is frequently
used as a shortcut for the if
statement.
if(var1) {
var2;
else {
var3;
}
The expression var1 ? var2 : var3 returns the value of var2 if var1 is considered to have a value equivalent to true else it returns teh value of var3.
Note this is not quite the same as:-
if (var1)
varX = var2
else
varX = var3
Since the above construct can not itself appear as part of a larger expression.
In ternery expression, as ? : is known, one should avoid allowing the component expressions to have side effects other than perhaps the side-effects of ++ or -- operators. For example this isn't a good idea:-
varX = var1 ? doSomethingSignificant() : doSomethingElseSignificant();
In this case it would be better to use the if else construct. On the hand:-
varX = var1 ? calcSomething(var2) : someOtherCalc(var2);
this is acceptable assuming the called functions don't themselves modify the program state significantly.
Edit:
I think I need to re-enforce this point. Do not use the ternary operator as means to short cut on if statements. The two have different purposes. If your code is full of ? : that should be if else it will be difficult to read. We expect logical flow to appear in if statements. We expect ? : when there is a simple logical component to an expression. Note expressions do not modify things only the results of them when assigned should modify things.
As an addendum for the first question, you can alternatively use
var result = (condition) && var1 || var2;
and obtain the same result
For the second question, in C the following works too :
(condition) && someinstruction;
but that does not seem to work in javascript (at least with my version of firefox).
This seems to be sort of a ternary operation. Short form of an if else operation, so to say. check here for details...