The attribute(or the nested object) is selected dynamically based on conditions. It can be one of the 4 possibilities as follows:
var tempData = o.title ? o["properties"] || o["items"]["properties"] : o[k]["properties"] || o[k]["items"]["properties"];
Then I get this new data, I want to replace the above selected with.
var newData = //some new Object
I want to replace whatever above selected with the new data. I could do the following (go through the condition again and set the new data):
if(o.title){
if (o["properties"]) {
o["properties"] = newData;
} else if (o["items"]["properties"]) {
o["items"]["properties"] = newData;
}
}else{
if (o[k]["properties"]) {
o[k]["properties"] = newData;
} else if (o[k]["items"]["properties"]) {
o[k]["items"]["properties"] = newData;
}
}
But it doesn't look good. What is the more sophisticated way of achieving this?
It is unclear if you are generically attempting to replace any properties property with the newData, or if you are wanting it to specifically be one of the ones you have specified in your code. I have assumed that you are only wanting to replace the ones you specifically have shown in your code.
Note: The following assumes that it is not possible for the value of the properties property to evaluate to false. If it is possible for it to have a value that evaluates to false, this will fail.
As a first pass, I would do something like:
var p;
if (o.title) {
p=o;
} else {
p=o[k];
}
if (p.properties) {
p.properties = newData;
} else if (p.items.properties) {
p.items.properties = newData;
}
However, that relies on:
o is not null or undefined.
o.title does not evaluate to false, if you are trying to test for the existence of o.title.
k is valid/defined.
p (i.e. o[k]) is not null or undefined (i.e. is an Object)
p.properties does not evaluate to false, if you are testing for existence
p.items is not null or undefined (i.e. is an Object)
p.items.properties does not evaluate to false, if you are testing for existence
A more robust implementation would be:
if (typeof o === 'object' && o !== null) {
var p;
if (o.hasOwnProperty('title')) {
p = o;
} else {
p = o[k];
}
if (typeof p === 'object' && p !== null) {
if (p.hasOwnProperty('properties')) {
p.properties = newData;
} else if (typeof p.items === 'object' && p.items !== null
&& p.items.hasOwnProperty('properties')) {
p.items.properties = newData;
}
}
}
This still relies on:
k is valid/defined.
Basically, it is OK to use shortcuts like if(o.title) to test for existence, if you know that
the possible values for o can not include ones which might make your code throw an error (e.g o is null or undefined), and
the possible values for o.title do not evaluate to false when the property actually exists (e.g. o.title is null, undefined (yes, the property can exist, but have the value undefined), false, 0, '', etc.).
If you are going to perform the replacements in other areas of your code, or if you are going to use property keys other than hard coded items, and properties, then you should create a function. Assuming you are only performing this replacement in this section of your code, using a variable to hold the object in which you are looking for properties is faster/more efficient than creating a function.
Ok, from what i can understand here, it's like you are trying to replace the "properties" with the new data, and you want this to be able to be done dynamically, or maybe i can say, you need to do this regardless the structure.
lets see, if your objective is anything that end up with "properties", lets do it like this:
function recReplace(current,target,replacement){
for (var i in current){
if (i == target){
current[i] = replacement;
}
else{
recReplace(current[i],target,replacement);
}
}
}
And in the end you call
recReplace(o,"properties",newData);
But this will replace whole "properties" key with newData in DFS way, you can do additional conditional if you want to replace it only the first occurence
This error comes up a lot in javascript development.
cannot read property join of undefined
Is there a best way of dealing with this issue?
Some of the techniques I've used are:
Initialisation
question.tags = question.tags || [];
console.log(question.tags.join(', ');
If statements
if(question.tags) {
console.log(question.tags.join(', ');
}
You can use if..else, Object.hasOwnProperty(), Array.isArray() to determine if question exists and object has property tags and question.tags is an array
if (typeof question === "object"
&& question.hasOwnProperty("tags")
&& Array.isArray(question.tags)) {
//do stuff
} else {
// do other stuff, e.g
// question = {};
// question.tags = [];
}
There is no specific and exact way to do it. If there is an instance of the Array or Object or String, it inhertits the prototypal functions. Like an instance of array has a splice(), String instance has a replace ().
Now when this instance is undefined, it throws JS error. Lets assume a is supposedly an array. You can put a dirty check either by a logical ||
(a || []).length;
or a if block or a ternary property
return a ? a.length || undefined;
or a type check
(Array.isArray(a) || []).length
question.tags ? question.tags : []
You can use === and !== operators in if condition like
if(object.property !== undefined)
{
///write your code here
}
This operator will match your value + type so its easy to identify if mentioned property is undefined or not..hope this will help:)
I have this function in an AngularJS service:
self.IsValid = function (entity) {
if (entity == undefined || entity == "undefined")
return false;
if (entity == null)
return false;
if (entity == "00000000-0000-0000-0000-000000000000")
return false;
return true;
}
A working sample here.
Is there anything I can do to improve on this? Or maybe there's a better way altogether?
PS: Resharper says that the entity == null check is always false, but that doesn't feel right, I could have passed null.
You can use
self.IsValid = function(entity) {
return (entity !== "undefined" && entity !== "00000000-0000-0000-0000-000000000000") && !!entity;
};
entity !== "undefined" && entity !== "00000000-0000-0000-0000-000000000000": Checks if the value of entity variable is not 'undefined' and '00000000-0000-0000-0000-000000000000'. If the expression evaluates to false the result will be returned.
!! will cast the value to Boolean. See What is the !! (not not) operator in JavaScript?
Updated Plunker
If you can identify all of the invalid cases, then you can simply list them in an array and then check the entitity against them
var invalid = [undefined, "undefined", null, "0000-..."];
var isInArray = invalid.indexOf(entity) >= 0;
return isInArray;
This will be easier to maintain than a large compound boolean expression. You could even move this list out into a separate config file, to be shared and injected into your separate services.
If Not (oResponse.selectSingleNode("BigGroupType") Is Nothing) Then
End If
I need to convert this to javascript. Is that enough to check null?
This was my lead's answer, plz verify this,
if(typeof $(data).find("BigGroupType").text() != "undefined" && $(data).find("BigGroupType").text() != null) {
}
JavaScript has two values which mean "nothing", undefined and null. undefined has a much stronger "nothing" meaning than null because it is the default value of every variable. No variable can be null unless it is set to null, but variables are undefined by default.
var x;
console.log(x === undefined); // => true
var X = { foo: 'bar' };
console.log(X.baz); // => undefined
If you want to check to see if something is undefined, you should use === because == isn't good enough to distinguish it from null.
var x = null;
console.log(x == undefined); // => true
console.log(x === undefined); // => false
However, this can be useful because sometimes you want to know if something is undefined or null, so you can just do if (value == null) to test if it is either.
Finally, if you want to test whether a variable even exists in scope, you can use typeof. This can be helpful when testing for built-ins which may not exist in older browsers, such as JSON.
if (typeof JSON == 'undefined') {
// Either no variable named JSON exists, or it exists and
// its value is undefined.
}
You need to check for both null and undefined, this implicitly does so
if( oResponse.selectSingleNode("BigGroupType") != null ) {
}
It is the equivalent of:
var node = oResponse.selectSingleNode("BigGroupType");
if( node !== null &&
node !== void 0 ) {
}
void 0 being a bulletproof expression to get undefined
In JavaScript equvalent for Nothing is undefined
if(oResponse.selectSingleNode("BigGroupType") != undefined){
}
This logic:
If Not (oResponse.selectSingleNode("BigGroupType") Is Nothing)
Can be written like this in JavaScript:
if (typeof oResponse.selectSingleNode("BigGroupType") != 'undefined')
Nothing would equal undefined, but checking against undefined is not recommended for several reasons, it’s generally safer to use typeof.
However, if the selectSingleNode can return other falsy values such as null, it’s probably OK to just do a simple check if it is truthy:
if (oResponse.selectSingleNode("BigGroupType"))
JavaScript:-
(document.getElementById(“BigGroupType”) == undefined) // Returns true
JQuery:-
($(“#BigGroupType”).val() === “undefined”) // Returns true
Note in above examples undefined is a keyword in JavaScript, where as in JQuery it is just a string.
We are frequently using the following code pattern in our JavaScript code
if (typeof(some_variable) != 'undefined' && some_variable != null)
{
// Do something with some_variable
}
Is there a less verbose way of checking that has the same effect?
According to some forums and literature saying simply the following should have the same effect.
if (some_variable)
{
// Do something with some_variable
}
Unfortunately, Firebug evaluates such a statement as error on runtime when some_variable is undefined, whereas the first one is just fine for it. Is this only an (unwanted) behavior of Firebug or is there really some difference between those two ways?
I think the most efficient way to test for "value is null or undefined" is
if ( some_variable == null ){
// some_variable is either null or undefined
}
So these two lines are equivalent:
if ( typeof(some_variable) !== "undefined" && some_variable !== null ) {}
if ( some_variable != null ) {}
Note 1
As mentioned in the question, the short variant requires that some_variable has been declared, otherwise a ReferenceError will be thrown. However in many use cases you can assume that this is safe:
check for optional arguments:
function(foo){
if( foo == null ) {...}
check for properties on an existing object
if(my_obj.foo == null) {...}
On the other hand typeof can deal with undeclared global variables (simply returns undefined). Yet these cases should be reduced to a minimum for good reasons, as Alsciende explained.
Note 2
This - even shorter - variant is not equivalent:
if ( !some_variable ) {
// some_variable is either null, undefined, 0, NaN, false, or an empty string
}
so
if ( some_variable ) {
// we don't get here if some_variable is null, undefined, 0, NaN, false, or ""
}
Note 3
In general it is recommended to use === instead of ==.
The proposed solution is an exception to this rule. The JSHint syntax checker even provides the eqnull option for this reason.
From the jQuery style guide:
Strict equality checks (===) should be used in favor of ==. The only
exception is when checking for undefined and null by way of null.
// Check for both undefined and null values, for some important reason.
undefOrNull == null;
EDIT 2021-03:
Nowadays most browsers
support the Nullish coalescing operator (??)
and the Logical nullish assignment (??=), which allows a more concise way to
assign a default value if a variable is null or undefined, for example:
if (a.speed == null) {
// Set default if null or undefined
a.speed = 42;
}
can be written as any of these forms
a.speed ??= 42;
a.speed ?? a.speed = 42;
a.speed = a.speed ?? 42;
You have to differentiate between cases:
Variables can be undefined or undeclared. You'll get an error if you access an undeclared variable in any context other than typeof.
if(typeof someUndeclaredVar == whatever) // works
if(someUndeclaredVar) // throws error
A variable that has been declared but not initialized is undefined.
let foo;
if (foo) //evaluates to false because foo === undefined
Undefined properties , like someExistingObj.someUndefProperty. An undefined property doesn't yield an error and simply returns undefined, which, when converted to a boolean, evaluates to false. So, if you don't care about
0 and false, using if(obj.undefProp) is ok. There's a common idiom based on this fact:
value = obj.prop || defaultValue
which means "if obj has the property prop, assign it to value, otherwise assign the default value defautValue".
Some people consider this behavior confusing, arguing that it leads to hard-to-find errors and recommend using the in operator instead
value = ('prop' in obj) ? obj.prop : defaultValue
Checking null with normal equality will also return true for undefined.
if (window.variable == null) alert('variable is null or undefined');
This is the only case in which == and != should be used:
if (val == null) console.log('val is null or undefined')
if (val != null) console.log('val is neither null nor undefined')
For any other comparisons, the strict comparators (=== and !==) should be used.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Equality_comparisons_and_sameness
https://2ality.com/2011/12/strict-equality-exemptions.html
In newer JavaScript standards like ES5 and ES6 you can just say
> Boolean(0) //false
> Boolean(null) //false
> Boolean(undefined) //false
all return false, which is similar to Python's check of empty variables.
So if you want to write conditional logic around a variable, just say
if (Boolean(myvar)){
// Do something
}
here "null" or "empty string" or "undefined" will be handled efficiently.
If you try and reference an undeclared variable, an error will be thrown in all JavaScript implementations.
Properties of objects aren't subject to the same conditions. If an object property hasn't been defined, an error won't be thrown if you try and access it. So in this situation you could shorten:
if (typeof(myObj.some_property) != "undefined" && myObj.some_property != null)
to
if (myObj.some_property != null)
With this in mind, and the fact that global variables are accessible as properties of the global object (window in the case of a browser), you can use the following for global variables:
if (window.some_variable != null) {
// Do something with some_variable
}
In local scopes, it always useful to make sure variables are declared at the top of your code block, this will save on recurring uses of typeof.
Firstly you have to be very clear about what you test. JavaScript has all sorts of implicit conversions to trip you up, and two different types of equality comparator: == and ===.
A function, test(val) that tests for null or undefined should have the following characteristics:
test(null) => true
test(undefined) => true
test(0) => false
test(1) => false
test(true) => false
test(false) => false
test('s') => false
test([]) => false
Let's see which of the ideas here actually pass our test.
These work:
val == null
val === null || val === undefined
typeof(val) == 'undefined' || val == null
typeof(val) === 'undefined' || val === null
These do not work:
typeof(val) === 'undefined'
!!val
I created a jsperf entry to compare the correctness and performance of these approaches. Results are inconclusive for the time being as there haven't been enough runs across different browsers/platforms. Please take a minute to run the test on your computer!
At present, it seems that the simple val == null test gives the best performance. It's also pretty much the shortest. The test may be negated to val != null if you want the complement.
here's another way using the Array includes() method:
[undefined, null].includes(value)
Since there is no single complete and correct answer, I will try to summarize:
In general, the expression:
if (typeof(variable) != "undefined" && variable != null)
cannot be simplified, because the variable might be undeclared so omitting the typeof(variable) != "undefined" would result in ReferenceError. But, you can simplify the expression according to the context:
If the variable is global, you can simplify to:
if (window.variable != null)
If it is local, you can probably avoid situations when this variable is undeclared, and also simplify to:
if (variable != null)
If it is object property, you don't have to worry about ReferenceError:
if (obj.property != null)
This is an example of a very rare occasion where it is recommended to use == instead of ===. Expression somevar == null will return true for undefined and null, but false for everything else (an error if variable is undeclared).
Using the != will flip the result, as expected.
Modern editors will not warn for using == or != operator with null, as this is almost always the desired behavior.
Most common comparisions:
undeffinedVar == null // true
obj.undefinedProp == null // true
null == null // true
0 == null // false
'0' == null // false
'' == null // false
Try it yourself:
let undefinedVar;
console.table([
{ test : undefinedVar, result: undefinedVar == null },
{ test : {}.undefinedProp, result: {}.undefinedProp == null },
{ test : null, result: null == null },
{ test : false, result: false == null },
{ test : 0, result: 0 == null },
{ test : '', result: '' == null },
{ test : '0', result: '0' == null },
]);
You can just check if the variable has a value or not. Meaning,
if( myVariable ) {
//mayVariable is not :
//null
//undefined
//NaN
//empty string ("")
//0
//false
}
If you do not know whether a variable exists (that means, if it was declared) you should check with the typeof operator. e.g.
if( typeof myVariable !== 'undefined' ) {
// myVariable will get resolved and it is defined
}
Similar to what you have, you could do something like
if (some_variable === undefined || some_variable === null) {
do stuff
}
This is also a nice (but verbose) way of doing it:
if((someObject.someMember ?? null) === null) {
// bladiebla
}
It's very clear what's happening and hard to misunderstand. And that can be VERY important! :-)
This uses the ?? operator (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing_operator). If the value of someObject.someMember is null or undefined, the ?? operator kicks in and will make the value null.
TBH, I like the explicitness of this thing, but I usualle prefer someObject.someMember == null, it's more readable and skilled JS developers probably know what's going on here.
whatever yyy is undefined or null, it will return true
if (typeof yyy == 'undefined' || !yyy) {
console.log('yes');
} else {
console.log('no');
}
yes
if (!(typeof yyy == 'undefined' || !yyy)) {
console.log('yes');
} else {
console.log('no');
}
no
Open the Developer tools in your browser and just try the code shown in the below image.
If the purpose of the if statement is to check for null or undefined values before assigning a value to a variable, you can make use of the Nullish Coalescing Operator. According to the data from caniuse, it should be supported by around 85% of the browsers(as of January 2021). An example of the operator is shown below:
const a = some_variable ?? '';
This will ensure that the variable will be assigned to an empty string (or any other default value) if some_variable is null or undefined.
This operator is most suited for your use case, as it does not return the default value for other types of falsy value such as 0 and ''.
As mentioned in one of the answers, you can be in luck if you are talking about a variable that has a global scope. As you might know, the variables that you define globally tend to get added to the windows object. You can take advantage of this fact so lets say you are accessing a variable called bleh, just use the double inverted operator (!!)
!!window['bleh'];
This would return a false while bleh has not been declared AND assigned a value.
In order to understand, Let's analyze what will be the value return by the Javascript Engine when converting undefined , null and ''(An empty string also). You can directly check the same on your developer console.
You can see all are converting to false , means All these three are assuming ‘lack of existence’ by javascript. So you no need to explicitly check all the three in your code like below.
if (a === undefined || a === null || a==='') {
console.log("Nothing");
} else {
console.log("Something");
}
Also I want to point out one more thing.
What will be the result of Boolean(0)?
Of course false. This will create a bug in your code when 0 is a valid value in your expected result. So please make sure you check for this when you write the code.
With Ramda, you can simply do R.isNil(yourValue)
Lodash and other helper libraries have the same function.
I have done this using this method
save the id in some variable
var someVariable = document.getElementById("someId");
then use if condition
if(someVariable === ""){
//logic
} else if(someVariable !== ""){
//logic
}
In ES5 or ES6 if you need check it several times you cand do:
const excluded = [null, undefined, ''];
if (!exluded.includes(varToCheck) {
// it will bee not null, not undefined and not void string
}
let varToCheck = ""; //U have to define variable firstly ,or it throw error
const excluded = [null, undefined, ""];
if (!excluded.includes(varToCheck)) {
// it will bee not null, not undefined and not void string
console.log("pass");
} else {
console.log("fail");
}
for example I copy vladernn's answer to test, u can just click button "Copy snippets to answer" to test too .
Testing nullity (if (value == null)) or non-nullity (if (value != null)) is less verbose than testing the definition status of a variable.
Moreover, testing if (value) (or if( obj.property)) to ensure the existence of your variable (or object property) fails if it is defined with a boolean false value. Caveat emptor :)
Best way to compare undefined or null or 0 with ES5 and ES6 standards
if ((Boolean(some_variable_1) && Boolean(some_variable_2)) === false) {
// do something
}
You can make use of lodash library.
_.isNil(value) gives true for both null and undefined
Test on - https://bazinga.tools/lodash
You must define a function of this form:
validate = function(some_variable){
return(typeof(some_variable) != 'undefined' && some_variable != null)
}
Both values can be easily distinguished by using the strict comparison operator.
Sample Code:
function compare(){
var a = null; //variable assigned null value
var b; // undefined
if (a === b){
document.write("a and b have same datatype.");
}
else{
document.write("a and b have different datatype.");
}
}