We Keep Coding

JavaScript variable assignment with OR vs if check [duplicate]

This question already has answers here:
What does the construct x = x || y mean?
(12 answers)
Closed 6 years ago.
In JavaScript I recently realized you could use the OR || logical operator for assignment, and I want to know if it's considered bad practice.
In particular I have some functions that have optional array input, if the input is null or undefined I should just set it to an empty array [], if it has content it should take the content.
I found that using the assignment using the OR operator handles that perfectly in a single line, it's clean. However, it feels like the kind of thing that might be considered bad practice, or may have some horrible pitfalls I'm not considering.
Another approach is a simple if check, which is fairly safe in general.
I want to know if using the || approach seen below has any pitfalls I'm not considering, although it works in this scenario I would appreciate knowing if it works well to keep using this in the future, or to stop using it altogether.
https://jsbin.com/nozuxiwawa/1/edit?js,console
var myArray = ['Some', 'Strings', 'Whatever'];
// Just assign using OR
var pathOne = function(maybeAnArray) {
var array = maybeAnArray || [];
console.log(array);
}
// Assign using IF
var pathTwo = function(maybeAnArray) {
var array = [];
// Covers null and undefined
if (maybeAnArray != null) {
array = maybeAnArray;
}
console.log(array);
}
console.log('Path one:');
pathOne(myArray); // ['Some', 'Strings', 'Whatever']
pathOne(null); // []
console.log('\nPath two:');
pathTwo(myArray); // ['Some', 'Strings', 'Whatever']
pathTwo(null); // []

IMHO the use of the OR || for the purposes of assignment is perfectly valid and is good practice. We certainly use it in our projects and I've seen it used in lots of 3rd party projects that we use.
The thing you need to be aware of is how certain JavaScript objects can be coerced to be other values. So for example, if you're ORing values such as "", false or 0 then they are treated as false... this means that when you have the following:
function f(o) {
var x = o || -1;
return x;
}
Calling:
f(0)
...will return -1... but calling
f(1)
Will return 1 ... even though in both cases you passed a number - because 0 is treated as false -1 is assigned to x.
...that said, as long as you're aware of how the OR operator will treat the operands that you use with it - then it is good JavaScript practice to use it.

i prefer the first option, it's clear for my eyes, but when i need to share my code with others will think about to use second, will be more clear for any.
Now i'm using sonar, and prefer the second option too, will more easy to comprend for machine in inegration works.
Last idea is to use
if(maybeAnArray !== void(0))
Two reasons:
use cast and type conditionals
void(0) will works same for all browsers
Expect it helps yopu

When given the option, I prefer concise code (which must still be readable).
I would say || is common enough that it is considered good practice. Once one has seen it a few times it reads just fine.

In my opinion there are few reasons why you should rather use the second option:
First of all it's much more readable - new developers that are still learning can have problems with understanding notation like var myArray = someArrayArg || [];
If you are using some kind of code checkers like JSLint, they will return warnings and/or errors like Expected a conditional expression and instead saw an assignment. for the statement with var myArray = someArrayArg || [];
We already have something like var myArray = someArrayArg ? someArrayArg : []; that works pretty well

JavaScript: alert object name as a string

I'm trying to alert any JavaScript object as a string, in a function. This means if the parameter given to the function is window.document, the actual object, it should alert "window.document" (without quotes) as a literal string.
The following calls...
example(window);
example(window.document);
example(document.getElementById('something'));
...calling this function...
function example(o) {/* A little help here please? */}
...should output the following strings...
window
window.document
document.getElementById('something')
I've attempted to do this with combinations of toString() and eval() among some more miscellaneous shots in the dark without success.
No need insane backwards compatibility, newer ECMAScript / JavaScript features/functions are fine. Feel free to inquire for clarifications though the goal should be pretty straight forward.

This is not possible to do in a self contained script.
If using a preprocessor would be an option, then you could write one which converts example(whatever) into example('whatever'). Other than that I'm afraid you're out of luck.

The first problem is that objects don't have names.
The second problem is that from your examples, you're not really wanting to print the (nonexistent) name of an object, you want to print the expression that evaluated into a reference to an object. That's what you're trying to do in this example:
example(document.getElementById('something'));
For that to print document.getElementById('something'), JavaScript would have had to keep the actual text of that expression somewhere that it would make available to you. But it doesn't do that. It merely evaluates the parsed and compiled expression without reference to the original text of the expression.
If you were willing to quote the argument to example(), then of course it would be trivial:
example( "document.getElementById('something')" );
Obviously in this case you could either print the string directly, or eval() it to get the result of the expression.
OTOH, if you want to try a real hack, here's a trick you could use in some very limited circumstances:
function example( value ) {
var code = arguments.callee.caller.toString();
var match = code.match( /example\s*\(\s*(.*)\s*\)/ );
console.log( match && match[1] );
}
function test() {
var a = (1);
example( document.getElementById('body') );
var b = (2);
}
test();
This will print what you wanted:
document.getElementById('body')
(The assignments to a and b in the test() function are just there to verify that the regular expression isn't picking up too much code.)
But this will fail if there's more than one call to example() in the calling function, or if that call is split across more than one line. Also, arguments.callee.caller has been deprecated for some time but is still supported by most browsers as long as you're not in strict mode. I suppose this hack could be useful for some kind of debugging purposes though.

Don't know why you need this, but you can try walking the object tree recursively and compare its nodes with your argument:
function objectName(x) {
function search(x, context, path) {
if(x === context)
return path;
if(typeof context != "object" || seen.indexOf(context) >= 0)
return;
seen.push(context);
for(var p in context) {
var q = search(x, context[p], path + "." + p);
if(q)
return q;
}
}
var seen = [];
return search(x, window, "window");
}
Example:
console.log(objectName(document.body))
prints for me
window.document.activeElement

How i can simplify my if condition in jQuery

I'm looking for a way to do the following:
$("#a" || "#b").val() === ""
as opposed to:
$("#a").val() === "" || $("#b").val() === ""
Any ideas?
Thanks in advance!

For two elements, I believe your example is about as short as you can make it and its meaning is clear. However, if you wish to repeat such logic or evaluate more elements, you might be able to improve upon it by creating a simple function to evaluate if any items in a set match a condition.
Extending jQuery
$.fn.any = function (evaluator) {
var items = $(this);
for (var i = 0; i < items.length; i++) {
if (evaluator(items[i]) === true) {
return true;
}
}
return false;
};
Demo: http://jsfiddle.net/xE76y/1/
This is similar to the Any() method implemented in the .Net LINQ library* (and I'm sure is implemented in other libraries, especially those geared towards functional programming). In c#, you would call such a method:
enumerable.Any( o => o.Value == "" );
JavaScript's syntax (sadly) isn't as concise; you end up with something like:
array.any( function(o){ return o.value === ""; } );
So far, this hasn't saved you anything. However, if you want to iterate over a large number of elements, it becomes much more elegant.
// there could be zero inputs or 100 inputs; it doesn't matter
var result = $("input").any(function (o) {
return o.value === "";
});
Native Solution
Note that we aren't relying on jQuery in our any() method. You could also consider a native JavaScript solution such as the Array.some() method.
some() executes the callback function once for each element present in
the array until it finds one where callback returns a true value. If
such an element is found, some immediately returns true.
Demo: http://jsfiddle.net/xE76y/2/
var result = jQuery.makeArray($("input")).some(function (o) {
return o.value === "";
});
Since this is an array method, it only works on an array. This unfortunately means that document.getElementsByTagName("input").some(...) will not work since getElementsByTagName() returns a NodeList.
Of course, you could push whatever you wanted into an array and call some() on that array. The call to jQuery.makeArray() in the example is just for convenience.
Abstracting the Evaluation Functions
Demo: http://jsfiddle.net/xE76y/3/
Perhaps the evaluation functions (such as testing for an empty string) will be reused. These can be abstracted further.
// ideally, this should NOT be left in global scope
function isEmpty(input) {
return input.value === "";
}
// the check now fits nicely in one line.
if ($("input").any(isEmpty)) {
alert("At least one input is empty.");
}
The resulting method calls are quite clean: $("#a, #b").any(isEmpty) and $("input").any(isEmpty)
* Also worth noting that LINQ has been recreated for JavaScript.

Try like this instead:
if ($('#a,#b').is(':empty'))
{
alert("Either a or b is Empty!");
}
Try my demo
Edit:
If it is an input type like a textbox then it would be a little bit bulky but will achieve the same effect:
if ($.inArray("",[ $("#a").val(), $("#b").val() ])>=0)
{
alert("Either a or b is Empty!");
}
See another Demo

If you want to avoid duplication of the empty string "", you could do this:
if ($.inArray([ $("#a").val(), $("#b").val() ], ""))
Or if you only want to select once with jQuery:
if ($.inArray($("#a, #b").map(function() { return this.value; }), ""))
But I wouldn't use either of these myself. They are arguably both less efficient, more contrived, and certainly less readable than the "easy" way!

I'm not an expert in javaScript, but have you cross checked with :
http://api.jquery.com/multiple-selector/
jQuery selector regular expressions
Also, one way would be using the .each function as in
jQuery Multiple ID selectors

javascript: Only return if not false

Scenario: I'm searching for a specific object in a deep object. I'm using a recursive function that goes through the children and asks them if I'm searching for them or if I'm searching for their children or grandchildren and so on. When found, the found obj will be returned, else false. Basically this:
obj.find = function (match_id) {
if (this.id == match_id) return this;
for (var i = 0; i < this.length; i++) {
var result = this[i].find(match_id);
if (result !== false) return result;
};
return false;
}​
i'm wondering, is there something simpler than this?:
var result = this[i].find(match_id);
if (result) return result;
It annoys me to store the result in a variable (on each level!), i just want to check if it's not false and return the result. I also considered the following, but dislike it even more for obvious reasons.
if (this[i].find(match_id)) return this[i].find(match_id);
Btw I'm also wondering, is this approach even "recursive"? it isn't really calling itself that much...
Thank you very much.
[edit]
There is another possibility by using another function check_find (which just returns only true if found) in the if statement. In some really complicated cases (e.g. where you don't just find the object, but also alter it) this might be the best approach. Or am I wrong? D:

Although the solution you have is probably "best" as far as search algorithms go, and I wouldn't necessarily suggest changing it (or I would change it to use a map instead of an algorithm), the question is interesting to me, especially relating to the functional properties of the JavaScript language, and I would like to provide some thoughts.
Method 1
The following should work without having to explicitly declare variables within a function, although they are used as function arguments instead. It's also quite succinct, although a little terse.
var map = Function.prototype.call.bind(Array.prototype.map);
obj.find = function find(match_id) {
return this.id == match_id ? this : map(this, function(u) {
return find.call(u, match_id);
}).filter(function(u) { return u; })[0];
};​
How it works:
We test to see if this.id == match_id, if so, return this.
We use map (via Array.prototype.map) to convert this to an array of "found items", which are found using the recursive call to the find method. (Supposedly, one of these recursive calls will return our answer. The ones which don't result in an answer will return undefined.)
We filter the "found items" array so that any undefined results in the array are removed.
We return the first item in the array, and call it quits.
If there is no first item in the array, undefined will be returned.
Method 2
Another attempt to solve this problem could look like this:
var concat = Function.prototype.call.bind(Array.prototype.concat),
map = Function.prototype.call.bind(Array.prototype.map);
obj.find = function find(match_id) {
return (function buildObjArray(o) {
return concat([ o ], map(o, buildObjArray));
})(this).filter(function(u) { return u.id == match_id })[0];
};
How it works:
buildObjArray builds a single, big, 1-dimensional array containing obj and all of obj's children.
Then we filter based on the criteria that an object in the array must have an id of match_id.
We return the first match.
Both Method 1 and Method 2, while interesting, have the performance disadvantage that they will continue to search even after they've found a matching id. They don't realize they have what they need until the end of the search, and this is not very efficient.
Method 3
It is certainly possible to improve the efficiency, and now I think this one really gets close to what you were interested in.
var forEach = Function.prototype.call.bind(Array.prototype.forEach);
obj.find = function(match_id) {
try {
(function find(obj) {
if(obj.id == match_id) throw this;
forEach(obj, find);
})(obj);
} catch(found) {
return found;
}
};​
How it works:
We wrap the whole find function in a try/catch block so that once an item is found, we can throw and stop execution.
We create an internal find function (IIFE) inside the try which we reference to make recursive calls.
If this.id == match_id, we throw this, stopping our search algorithm.
If it doesn't match, we recursively call find on each child.
If it did match, the throw is caught by our catch block, and the found object is returned.
Since this algorithm is able to stop execution once the object is found, it would be close in performance to yours, although it still has the overhead of the try/catch block (which on old browsers can be expensive) and forEach is slower than a typical for loop. Still these are very small performance losses.
Method 4
Finally, although this method does not fit the confines of your request, it is much, much better performance if possible in your application, and something to think about. We rely on a map of ids which maps to objects. It would look something like this:
// Declare a map object.
var map = { };
// ...
// Whenever you add a child to an object...
obj[0] = new MyObject();
// .. also store it in the map.
map[obj[0].id] = obj[0];
// ...
// Whenever you want to find the object with a specific id, refer to the map:
console.log(map[match_id]); // <- This is the "found" object.
This way, no find method is needed at all!
The performance gains in your application by using this method will be HUGE. Please seriously consider it, if at all possible.
However, be careful to remove the object from the map whenever you will no longer be referencing that object.
delete map[obj.id];
This is necessary to prevent memory leaks.

No there is no other clear way, storing the result in a variable isn't that much trouble, actually this is what variables are used for.
Yes, that approach is recursive:
you have the base case if (this.id==match_id) return this
you have the recursive step which call itself obj.find(match_id) { ... var result = this[i].find(match_id); }

I don't see any reason, why storing the variable would be bad. It's not a copy, but a reference, so it's efficient. Plus the temporary variable is the only way, that I can see right now (I may be wrong, though).
With that in mind, I don't think, that a method check_find would make very much sense (it's most probably basically the same implementation), so if you really need this check_find method, I'd implement it as
return this.find(match_id) !== false;
Whether the method is recursive is hard to say.
Basically, I'd say yes, as the implementations of 'find' are all the same for every object, so it's pretty much the same as
function find(obj, match_id) {
if (obj.id == match_id) return obj;
for (var i = 0; i < obj.length; ++i) {
var result = find(obj[i], match_id);
if (result !== false) return result;
}
}
which is definitely recursive (the function calls itself).
However, if you'd do
onesingleobjectinmydeepobject.find = function(x) { return this; }
I'm not quite sure, if you still would call this recursive.

Calling javascript function with an objectstring in dot notation

Suppose I have the string:
var string = "function";
With
window[string];
I can call a function with the name of "function".
But, when I have:
var string2 = "function.method.weHaveTogoDeeper";
it should call
window["function"]["method"]["weHaveTogoDeeper"]
I can't do:
window[string2]
in this case. I dont know the number of "." in the string, so I need some kind of routine.

you can split the string across . by using the String.split method:
var string2 = "function.method.weHaveTogoDeeper";
var methods = string2.split(".");
In this examples, methods will be the array ["function","method","weHaveTogoDeeper"]. You should now be able to do a simple iteration over this array, calling each function on the result of the previous one.
Edit
The iteration I had in mind was something like this:
var result = window;
for(var i in methods) {
result = result[methods[i]];
}
In your example, result should now hold the same output as
window["function"]["method"]["weHaveTogoDeeper"]

function index(x,i) {return x[i]}
string2.split('.').reduce(index, window);
edit: Of course if you are calling functions from strings of their names, you are likely doing something inelegant which would be frowned upon, especially in a collaborative coding settings. The only use case I can think of that is sane is writing a testing framework, though there are probably a few more cases. So please use caution when following this answer; one should instead use arrays, or ideally direct references.

I wrote one a while back:
function RecursiveMapper(handlerName, stack) {
// check if empty string
if(!handlerName || handlerName === '' || (handlerName.replace(/\s/g,'') === '')) return null;
var buf = handlerName.split('.');
stack = stack || window;
return (buf.length === 1) ? stack[buf[0]] : this.RecursiveMapper(buf.slice(1).join('.'), stack[buf[0]]);
}
Call it like this: RecursiveMapper(window[string2]);
This one also checks if the function is defined in window scope first and returns the global one fi found.