How to check if variable is set && .length>x in same ()? - javascript

I want to check that a variable exists and that it its length is greater than a particular number within a single if statement. I can achieve the desired result by doing:
if(v){
if(v.length>3)
//do thing
}
But if I try to just do:
if(v.length>3)
// do thing
I bug out when v is not declared. Similarly if I try to do:
if(v&&v.length>3)
I bug out. How do I achieve the desired result most readably?

A final answer depends on how generic you want the solution. While .length is shorthand notation for looking up an object property named "length", it must be applied to an object value. But the typeof operator returns "function" for a function object, and "object" for null - which is not an object data type and will crash if you attempt to look up a property on it. Oh, and primitive string values promote to a String object with a length property.
So, a generic solution could look like
if(v && typeof v == "object" || typeof v == "function" || typeof v == "string" && v.length > 3)
{ // do stuff
}
where the test for v being truthy excludes v having a value of null. But notice that if you can exclude function objects (which have a length property giving the number of arguments they expect) and string values you could shorten this to
if(v && typeof v == "object" && v.length > 3)
Note that if the length property does not exist it will return a value of undefined and undefined > 3 is false.
Update after question comment:
if(v)
can fail in strict mode if v has never been defined. This could be the result of defining v using a let or const statement executed after the if statement. Either execute the let or const statement first or declare v using a var statement in scope of the testing code.

Related

Using .includes method in a function

I have a an object jsonRes[0] containing values which need to be removed based on a condition. The following works to remove null, missing values and those equal to zero in the stringified object:
function replacer(key, value) {
// Filtering out properties
if (value === null || value === 0 || value === "") {
return undefined;
}
return value;
}
JSON.stringify(jsonRes[0], replacer, "\t")
However, when I add a condition using the the includes method, I receive an error:
function replacer(key, value) {
// Filtering out properties
if (value === null || value === 0 || value === "" || value.includes("$")) {
return undefined;
}
return value;
}
Uncaught TypeError: value.includes is not a function
Why is this the case and is there a workaround?
You can use String.indexOf() instead of String.includes, As it is available in ES6 and not supported in IE at all.
typeof value == "string" && value.indexOf('$') > -1
Also note if value is not string type it will still raise an error boolean, Number doesn't the the method. You can use typeof to validate whether value is a string.
The .includes() API is part of the String and Array data type.
So what the error is trying to tell you is that the value for variable value, e.g. an integer or object, does not have the property .includes.
You could do checks like
typeof a_string === 'string'
an_array instanceof Array
before the .includes() api to prevent this.
Obviously this will make your if statement rather ugly due to the number of checks you have.
Based on the way your code is written I suspect you are more interested in checking "String" than array. So becareful of arrays. Your code may not work properly if it is array.
Anyway here is a refractored version of your code.
function replacer(key, value) {
// Filtering out properties
if (!value || typeof value === "string" && value.includes("$")) {
return undefined;
}
return value;
}
console.log("NULL returns:" + replacer('test', null));
console.log("$Test returns:" + replacer('test', '$test'));
console.log("Blah returns:" + replacer('test', 'Blah'));
Just one more possibility: Maybe your value is not a string type object.
(typeof(value) == "string" && value.includes("$"))
I solved this error, which I was getting when applying "includes" to a "window.location" value, by appending ".toString();"
var requestUrl = window.location.toString();
if (requestUrl.includes(urlBase + "#")) {
...
I actually am not sure what type of the variable named value is, but anyway, Array.prototype.includes and String.prototype.includes are only available in ES6. You need to use babel-polyfill or any other bundling modules like rollup.js, webpack with babel or something like that to use includes function.

What is the difference between "defined" and defined in javascript?

When writing this in javascript, I have seen it written in two different ways:
if (typeof x === "undefined") {
// execute code here
}
if (typeof x === undefined) {
// execute code here
}
my question here is:
what is the difference between "undefined" and undefined. One is enclosed in quotes and the other is not.
Can anybody clarify this for me?
Thanks!
undefined is a value, 'undefined' is a string literal. The typeof operator returns a string that gives you the type. So typeof x returns the string name of the type of x.
Use if( x === undefined ) or if( typeof x === 'undefined' ) but never if( typeof x === undefined ) because typeof x will always return a string (which will never equal undefined).
"undefined" is a string, and undefined is a variable containing the primitive value undefined (Thank you elclanrs).
if(typeof x === undefined) should only ever be able to return true if undefined is reassigned to a string matching the type of x.

How can I check if variable in java script is type of a particular object

How can I check if variable in java script is type of a particular object? What will be the result of this
var myvalue = "200"+50+44;
1) The typeof operator returns a string indicating the type of the unevaluated operand.
2) The result will be 2005044
I think you're trying like this
parseInt("200", 10)+50+44 // returns 294
Check parseInt(string, radix) for more information.
The type can be checked with the typeof operator.
typeof myvalue === "number"
The possible types are "number", "string", "object", "undefined". This has a few problems though.
typeof someArray === "object"
typeof null === "object"
The better way is compare constructors.
someArray.constructor === Array
someNumber.constructor === Number
You do however need to check if it's null or undefined, because neither have a constructor property.
someThing != null && someThing.constructor === SomeConstructor

What is the difference between null and undefined in JavaScript?

I want to know what the difference is between null and undefined in JavaScript.
undefined means a variable has been declared but has not yet been assigned a value :
var testVar;
console.log(testVar); //shows undefined
console.log(typeof testVar); //shows undefined
null is an assignment value. It can be assigned to a variable as a representation of no value :
var testVar = null;
console.log(testVar); //shows null
console.log(typeof testVar); //shows object
From the preceding examples, it is clear that undefined and null are two distinct types: undefined is a type itself (undefined) while null is an object.
Proof :
console.log(null === undefined) // false (not the same type)
console.log(null == undefined) // true (but the "same value")
console.log(null === null) // true (both type and value are the same)
and
null = 'value' // Uncaught SyntaxError: invalid assignment left-hand side
undefined = 'value' // 'value'
The difference can be explained with toilet tissue holder:
A non-zero value is like a holder with roll of toilet tissue and there's tissue still on the tube.
A zero value is like a holder with an empty toilet tissue tube.
A null value is like a holder that doesn't even have a tissue tube.
An undefined value is similar to the holder itself being missing.
I picked this from here
The undefined value is a primitive value used when a variable has not
been assigned a value.
The null value is a primitive value that represents the null, empty,
or non-existent reference.
When you declare a variable through var and do not give it a value, it will have the value undefined. By itself, if you try to WScript.Echo() or alert() this value, you won't see anything. However, if you append a blank string to it then suddenly it'll appear:
var s;
WScript.Echo(s);
WScript.Echo("" + s);
You can declare a variable, set it to null, and the behavior is identical except that you'll see "null" printed out versus "undefined". This is a small difference indeed.
You can even compare a variable that is undefined to null or vice versa, and the condition will be true:
undefined == null
null == undefined
They are, however, considered to be two different types. While undefined is a type all to itself, null is considered to be a special object value. You can see this by using typeof() which returns a string representing the general type of a variable:
var a;
WScript.Echo(typeof(a));
var b = null;
WScript.Echo(typeof(b));
Running the above script will result in the following output:
undefined
object
Regardless of their being different types, they will still act the same if you try to access a member of either one, e.g. that is to say they will throw an exception. With WSH you will see the dreaded "'varname' is null or not an object" and that's if you're lucky (but that's a topic for another article).
You can explicitely set a variable to be undefined, but I highly advise against it. I recommend only setting variables to null and leave undefined the value for things you forgot to set. At the same time, I really encourage you to always set every variable. JavaScript has a scope chain different than that of C-style languages, easily confusing even veteran programmers, and setting variables to null is the best way to prevent bugs based on it.
Another instance where you will see undefined pop up is when using the delete operator. Those of us from a C-world might incorrectly interpret this as destroying an object, but it is not so. What this operation does is remove a subscript from an Array or a member from an Object. For Arrays it does not effect the length, but rather that subscript is now considered undefined.
var a = [ 'a', 'b', 'c' ];
delete a[1];
for (var i = 0; i < a.length; i++)
WScript.Echo((i+".) "+a[i]);
The result of the above script is:
0.) a
1.) undefined
2.) c
You will also get undefined returned when reading a subscript or member that never existed.
The difference between null and undefined is: JavaScript will never set anything to null, that's usually what we do. While we can set variables to undefined, we prefer null because it's not something that is ever done for us. When you're debugging this means that anything set to null is of your own doing and not JavaScript. Beyond that, these two special values are nearly equivalent.
Please read the following carefully. It should remove all your doubts regarding the difference between null and undefined in JavaScript. Also, you can use the utility function at the end of this answer to get more specific types of variables.
In JavaScript we can have the following types of variables:
Undeclared Variables
Declared but Unassigned Variables
Variables assigned with literal undefined
Variables assigned with literal null
Variables assigned with anything other than undefined or null
The following explains each of these cases one by one:
Undeclared Variables
Can only be checked with the typeof operator which returns string 'undefined'
Cannot be checked with the loose equality operator ( == undefined ), let alone the strict equality operator ( === undefined ),
as well as if-statements and ternary operators ( ? : ) — these throw Reference Errors
Declared but Unassigned Variables
typeof returns string 'undefined'
== check with null returns true
== check with undefined returns true
=== check with null returns false
=== check with undefined returns true
Is falsy to if-statements and ternary operators ( ? : )
Variables assigned with literal undefined
These variables are treated exactly the same as Declared But Unassigned Variables.
Variables assigned with literal null
typeof returns string 'object'
== check with null returns true
== check with undefined returns true
=== check with null returns true
=== check with undefined returns false
Is falsy to if-statements and ternary operators ( ? : )
Variables assigned with anything other than undefined or null
typeof returns one of the following strings: 'bigint', 'boolean', 'function', 'number', 'object', 'string', 'symbol'
Following provides the algorithm for correct type checking of a variable:
Get the typeof our variable and return it if it isn't 'object'
Check for null, as typeof null returns 'object' as well
Evaluate Object.prototype.toString.call(o) with a switch statement to return a more precise value. Object's toString method returns strings that look like '[object ConstructorName]' for native/host objects. For all other objects (user-defined objects), it always returns '[object Object]'
If that last part is the case (the stringified version of the variable being '[object Object]') and the parameter returnConstructorBoolean is true, it will try to get the name of the constructor by toString-ing it and extracting the name from there. If the constructor can't be reached, 'object' is returned as usual. If the string doesn't contain its name, 'anonymous' is returned
(supports all types up to ECMAScript 2020)
function TypeOf(o, returnConstructorBoolean) {
const type = typeof o
if (type !== 'object') return type
if (o === null) return 'null'
const toString = Object.prototype.toString.call(o)
switch (toString) {
// Value types: 6
case '[object BigInt]': return 'bigint'
case '[object Boolean]': return 'boolean'
case '[object Date]': return 'date'
case '[object Number]': return 'number'
case '[object String]': return 'string'
case '[object Symbol]': return 'symbol'
// Error types: 7
case '[object Error]': return 'error'
case '[object EvalError]': return 'evalerror'
case '[object RangeError]': return 'rangeerror'
case '[object ReferenceError]': return 'referenceerror'
case '[object SyntaxError]': return 'syntaxerror'
case '[object TypeError]': return 'typeerror'
case '[object URIError]': return 'urierror'
// Indexed Collection and Helper types: 13
case '[object Array]': return 'array'
case '[object Int8Array]': return 'int8array'
case '[object Uint8Array]': return 'uint8array'
case '[object Uint8ClampedArray]': return 'uint8clampedarray'
case '[object Int16Array]': return 'int16array'
case '[object Uint16Array]': return 'uint16array'
case '[object Int32Array]': return 'int32array'
case '[object Uint32Array]': return 'uint32array'
case '[object Float32Array]': return 'float32array'
case '[object Float64Array]': return 'float64array'
case '[object ArrayBuffer]': return 'arraybuffer'
case '[object SharedArrayBuffer]': return 'sharedarraybuffer'
case '[object DataView]': return 'dataview'
// Keyed Collection types: 2
case '[object Map]': return 'map'
case '[object WeakMap]': return 'weakmap'
// Set types: 2
case '[object Set]': return 'set'
case '[object WeakSet]': return 'weakset'
// Operation types: 3
case '[object RegExp]': return 'regexp'
case '[object Proxy]': return 'proxy'
case '[object Promise]': return 'promise'
// Plain objects
case '[object Object]':
if (!returnConstructorBoolean)
return type
const _prototype = Object.getPrototypeOf(o)
if (!_prototype)
return type
const _constructor = _prototype.constructor
if (!_constructor)
return type
const matches = Function.prototype.toString.call(_constructor).match(/^function\s*([^\s(]+)/)
return matches ? matches[1] : 'anonymous'
default: return toString.split(' ')[1].slice(0, -1)
}
}
null is a special keyword that indicates an absence of value.
think about it as a value, like:
"foo" is string,
true is boolean ,
1234 is number,
null is undefined.
undefined property indicates that a variable has not been assigned a value including null too .
Like
var foo;
defined empty variable is null of datatype undefined
Both of them are representing a value of a variable with no value
AND
null doesn't represent a string that has no value - empty string-
Like
var a = '';
console.log(typeof a); // string
console.log(a == null); //false
console.log(a == undefined); // false
Now if
var a;
console.log(a == null); //true
console.log(a == undefined); //true
BUT
var a;
console.log(a === null); //false
console.log(a === undefined); // true
SO each one has it own way to use
undefined use it to compare the variable data type
null use it to empty a value of a variable
var a = 'javascript';
a = null ; // will change the type of variable "a" from string to object
null: absence of value for a variable; undefined: absence of variable itself;
..where variable is a symbolic name associated with a value.
JS could be kind enough to implicitly init newly declared variables with null, but it does not.
You might consider undefined to represent a system-level, unexpected, or error-like absence of value and null to represent program-level, normal, or expected absence of value.
via JavaScript:The Definitive Guide
The best way to understand the difference is to first clear your mind of the inner workings of JavaScript and just understand the differences in meaning between:
let supervisor = "None"
// I have a supervisor named "None"
let supervisor = null
// I do NOT have a supervisor. It is a FACT that I do not.
let supervisor = undefined
// I may or may not have a supervisor. I either don't know
// if I do or not, or I am choosing not to tell you. It is
// irrelevant or none of your business.
There is a difference in meaning between these three cases, and JavaScript distinguishes the latter two cases with two different values, null and undefined. You are free to use those values explicitly to convey those meanings.
So what are some of the JavaScript-specific issues that arise due to this philosophical basis?
A declared variable without an initializer gets the value undefined because you never said anything about the what the intended value was.
let supervisor;
assert(supervisor === undefined);
A property of an object that has never been set evaluates to undefined because no one ever said anything about that property.
const dog = { name: 'Sparky', age: 2 };
assert(dog.breed === undefined);
null and undefined are "similar" to each other because Brendan Eich said so. But they are emphatically not equal to each other.
assert(null == undefined);
assert(null !== undefined);
null and undefined thankfully have different types. null belongs to the type Null and undefined to the type Undefined. This is in the spec, but you would never know this because of the typeof weirdness which I will not repeat here.
A function reaching the end of its body without an explicit return statement returns undefined since you don't know anything about what it returned.
By the way, there are other forms of "nothingness" in JavaScript (it's good to have studied Philosophy....)
NaN
Using a variable that has never been declared and receiving a ReferenceError
Using a let or const defined local variable in its temporal dead zone and receiving a ReferenceError
Empty cells in sparse arrays. Yes these are not even undefined although they compare === to undefined.
$ node
> const a = [1, undefined, 2]
> const b = [1, , 2]
> a
[ 1, undefined, 2 ]
> b
[ 1, <1 empty item>, 2 ]
A lot of "technical" answers have been given, all of them mostly correct from the limited point of view of JS as a mere programming language.
However, I would like to add the following thoughts, especially when you're writing TypeScript code as part of a bigger project / (enterprise) application:
When talking with a Backend of some kind you'll most probably receive JSON
While some backends correctly avoid the use of "null" in their JSON (removing those properties), others do not
Now, while "null" may mean that the value is missing deliberately, more often it does not convey this meaning. Most databases use "null" just because they don't have an "undefined" type. But the meaning really just is "undefined".
Because of that, you can never know if a "null" value really means deliberate absence. Therefore "null" cannot really mean the deliberate choice of "missing value". It is undecidable in general.
As a consequence, semantically, "null" and "undefined" are exactly the same thing in practice.
Therefore, in an effort to harmonize things I'm strictly against using "null" and want to encourage you to stop using "null" in your code. It's far easier than you might think. Don't get me wrong. I'm not talking about not handling "null" values, only to avoid explicitly using them in your code. Put differently: your code should still be able to work with accidentally passed "null" values coming from outside your application, e.g. via a 3rd party lib like Angular, or a 3rd party backend.
Here are the guidelines that make it possible:
avoid direct undefined type guards (e.g. if (value === undefined) { ... }.
Instead, use indirect type guards (aka truthiness checks) e.g. if (value) { ... }
Whenever 0 or empty strings are meaningful, use either
an explicit helper method like Lodash's isNil
or include the meaningful value in the comparison (e.g. if (!value && value !== 0) { ... })
Consider using a lint rule that disallows the usage of null
null is a special value meaning "no value". null is a special object because typeof null returns 'object'.
On the other hand, undefined means that the variable has not been declared, or has not been given a value.
I'll explain undefined, null and Uncaught ReferenceError:
1 - Uncaught ReferenceError : variable has not been declared in your script, there is no reference to this varaible
2 - undefined: Variable declared but does not initialised
3 - null : Variable declared and is an empty value
null and undefined are two distinct object types which have the following in common:
both can only hold a single value, null and undefined respectively;
both have no properties or methods and an attempt to read any properties of either will result in a run-time error (for all other objects, you get value undefined if you try to read a non-existent property);
values null and undefined are considered equal to each other and to nothing else by == and != operators.
The similarities however end here. For once, there is a fundamental difference in the way how keywords null and undefined are implemented. This is not obvious, but consider the following example:
var undefined = "foo";
WScript.Echo(undefined); // This will print: foo
undefined, NaN and Infinity are just names of preinitialized "superglobal" variables - they are initialized at run-time and can be overridden by normal global or local variable with the same names.
Now, let's try the same thing with null:
var null = "foo"; // This will cause a compile-time error
WScript.Echo(null);
Oops! null, true and false are reserved keywords - compiler won't let you use them as variable or property names
Another difference is that undefined is a primitive type, while null is an object type (indicating the absense of an object reference). Consider the following:
WScript.Echo(typeof false); // Will print: boolean
WScript.Echo(typeof 0); // Will print: number
WScript.Echo(typeof ""); // Will print: string
WScript.Echo(typeof {}); // Will print: object
WScript.Echo(typeof undefined); // Will print: undefined
WScript.Echo(typeof null); // (!!!) Will print: object
Also, there is an important difference in the way null and undefined are treated in numeric context:
var a; // declared but uninitialized variables hold the value undefined
WScript.Echo(a === undefined); // Prints: -1
var b = null; // the value null must be explicitly assigned
WScript.Echo(b === null); // Prints: -1
WScript.Echo(a == b); // Prints: -1 (as expected)
WScript.Echo(a >= b); // Prints: 0 (WTF!?)
WScript.Echo(a >= a); // Prints: 0 (!!!???)
WScript.Echo(isNaN(a)); // Prints: -1 (a evaluates to NaN!)
WScript.Echo(1*a); // Prints: -1.#IND (in Echo output this means NaN)
WScript.Echo(b >= b); // Prints: -1 (as expected)
WScript.Echo(isNaN(b)); // Prints: 0 (b evaluates to a valid number)
WScript.Echo(1*b); // Prints: 0 (b evaluates to 0)
WScript.Echo(a >= 0 && a <= 0); // Prints: 0 (as expected)
WScript.Echo(a == 0); // Prints: 0 (as expected)
WScript.Echo(b >= 0 && b <= 0); // Prints: -1 (as expected)
WScript.Echo(b == 0); // Prints: 0 (!!!)
null becomes 0 when used in arithmetic expressions or numeric comparisons - similarly to false, it is basically just a special kind of "zero". undefined, on the other hand, is a true "nothing" and becomes NaN ("not a number") when you try to use it in numeric context.
Note that null and undefined receive a special treatment from == and != operators, but you can test true numeric equality of a and b with the expression (a >= b && a <= b).
Undefined means a variable has been declared but has no value:
var var1;
alert(var1); //undefined
alert(typeof var1); //undefined
Null is an assignment:
var var2= null;
alert(var2); //null
alert(typeof var2); //object
tl;dr
Use null for set a variable you know it is an Object.
Use undefined for set a variable whose type is mixed.
This is my usage of both 5 primitives and Object type, and that explain the difference between « use case » of undefined or null.
String
If you know a variable is only a string while all lifecycle, by convention, you could initialize it, to "":
("") ? true : false; // false
typeof ""; // "string";
("Hello World") ? true : false; // true
typeof "Hello World"; // "string"
Number
If you know a variable is only a number while all lifecycle, by convention, you could initialize it, to 0 (or NaN if 0 is an important value in your usage):
(0) ? true : false; // false
typeof 0; // "number";
(16) ? true : false; // true
typeof 16; // "number"
or
(NaN) ? true : false; // false
typeof NaN; // "number";
(16) ? true : false; // true
typeof 16; // "number"
Boolean
If you know a variable is only a boolean while all lifecycle, by convention, you could initialize it, to false:
(false) ? true : false; // false
typeof false; // "boolean";
(true) ? true : false; // true
typeof true; // "boolean"
Object
If you know a variable is only an Object while all lifecycle, by convention, you could initialize it, to null:
(null) ? true : false; // false
typeof null; // "object";
({}) ? true : false; // true
typeof {}; // "object"
Note: the smart usage off null is to be the falsy version of an Object because an Object is always true, and because typeof null return object. That means typeof myVarObject return consistent value for both Object and null type.
All
If you know a variable has a mixed type (any type while all lifecycle), by convention, you could initialize it, to undefined.
In addition to a different meaning there are other differences:
Object destructuring works differently for these two values:
const { a = "default" } = { a: undefined }; // a is "default"
const { b = "default" } = { b: null }; // b is null
JSON.stringify() keeps null but omits undefined
const json = JSON.stringify({ undefinedValue: undefined, nullValue: null });
console.log(json); // prints {"nullValue":null}
typeof operator
console.log(typeof undefined); // "undefined"
console.log(typeof null); // "object" instead of "null"
In JavasScript there are 5 primitive data types: String, Number, Boolean, null and undefined.
I will try to explain with some simple examples.
Let's say we have a simple function
function test(a) {
if(a == null) {
alert("a is null");
} else {
alert("The value of a is " + a);
}
}
Also, in above function if(a == null) is the same as if(!a).
Now when we call this function without passing the parameter a
test(); // will alert "a is null";
test(4); // will alert "The value of a is " + 4;
also
var a;
alert(typeof a);
This will give undefined; we have declared a variable but we have not asigned any value to this variable;
but if we write
var a = null;
alert(typeof a); // will give alert as object
so null is an object. In a way we have assigned a value null to 'a'
When you declare a variable in javascript, it is assigned the value undefined. This means the variable is untouched and can be assigned any value in future. It also implies that you don't know the value that this variable is going to hold at the time of declaration.
Now you can explicitly assign a variable null. It means that the variable does not have any value. For example - Some people don't have a middle name. So in such a case its better to assign the value null to the middlename variable of a person object.
Now suppose that someone is accessing the middlename variable of your person object and it has the value undefined. He wouldn't know if the developer forgot to initialize this variable or if it didn't have any value. If it has the value null, then the user can easily infer that middlename doesn't have any value and it is not an untouched variable.
OK, we may get confused when we hear about null and undefined, but let's start it simple, they both are falsy and similar in many ways, but weird part of JavaScript, make them a couple of significant differences, for example, typeof null is 'object' while typeof undefined is 'undefined'.
typeof null; //"object"
typeof undefined; //"undefined";
But if you check them with == as below, you see they are both falsy:
null==undefined; //true
Also you can assign null to an object property or to a primitive, while undefined can simply be achieved by not assigning to anything.
I create a quick image to show the differences for you at a glance.
For the undefined type, there is one and only one value: undefined.
For the null type, there is one and only one value: null.
So for both of them, the label is both its type and its value.
The difference between them. For example:
null is an empty value
undefined is a missing value
Or:
undefined hasn't had a value yet
null had a value and doesn't anymore
Actually, null is a special keyword, not an identifier, and thus you cannot treat it as a variable to assign to.
However, undefined is an identifier. In both non-strict mode and strict mode, however, you can create a local variable of the name undefined. But this is one terrible idea!
function foo() {
undefined = 2; // bad idea!
}
foo();
function foo() {
"use strict";
undefined = 2; // TypeError!
}
foo();
I want to add a knowledge point which pertains to a subtle difference between null and undefined. This is good to know when you are trying to learn Vanilla JavaScript(JS) from ground up:
null is a reserved keyword in JS while undefined is a property on
the global object of the run-time environment you're in.
While writing code, this difference is not identifiable as both null and undefined are always used in right hand side (RHS) of a JavaScript statement. But when you use them in left hand side (LHS) of an expression then you can observe this difference easily. So JS interpreter interprets the below code as error:
var null = 'foo'
It gives below error:
Uncaught SyntaxError: Unexpected token null
At the same time, below code runs successfully although I won't recommend doing so in real life:
var undefined = 'bar'
This works because undefined is a property on the global object (window object in case of JavaScript running in a browser)
null and undefined are both are used to represent the absence of some value.
var a = null;
a is initialized and defined.
typeof(a)
//object
null is an object in JavaScript
Object.prototype.toString.call(a) // [object Object]
var b;
b is undefined and uninitialized
undefined object properties are also undefined. For example "x" is not defined on object c and if you try to access c.x, it will return undefined.
Generally we assign null to variables not undefined.
Per Ryan Morr's thorough article on this subject...
"Generally, if you need to assign a non-value to a variable or property, pass it to a function, or return it from a function, null is almost always the best option. To put it simply, JavaScript uses undefined and programmers should use null."
See Exploring the Eternal Abyss of Null and Undefined
In javascript all variables are stored as key value pairs. Each variable is stored as variable_name : variable_value/reference.
undefined means a variable has been given a space in memory, but no value is assigned to it. As a best practice, you should not use this type as an assignment.
In that case how to denote when you want a variable to be without value at a later point in the code? You can use the type
null ,which is also a type that is used to define the same thing, absence of a value, but it is not the same as undefined, as in this case you actually have the value in memory. That value is null
Both are similar but usage and meaning are different.
The difference in meaning between undefined and null is an accident of JavaScript’s design, and it doesn’t matter most of the time. In cases where you actually have to concern yourself with these values, I recommend treating them as mostly interchangeable.
From the Eloquent Javascript book
As typeof returns undefined, undefined is a type where as null is an initializer indicates the variable points to no object(virtually everything in Javascript is an object).
null - It is an assignment value, which is used with variable to represent no value (it's an object).
undefined - It is a variable which does not have any value assigned to it, so JavaScript will assign an undefined to it (it's a data type).
undeclared - If a variable is not created at all, it is known as undeclared.
Check this out. The output is worth thousand words.
var b1 = document.getElementById("b1");
checkif("1, no argument" );
checkif("2, undefined explicitly", undefined);
checkif("3, null explicitly", null);
checkif("4, the 0", 0);
checkif("5, empty string", '');
checkif("6, string", "string");
checkif("7, number", 123456);
function checkif (a1, a2) {
print("\ncheckif(), " + a1 + ":");
if (a2 == undefined) {
print("==undefined: YES");
} else {
print("==undefined: NO");
}
if (a2 === undefined) {
print("===undefined: YES");
} else {
print("===undefined: NO");
}
if (a2 == null) {
print("==null: YES");
} else {
print("==null: NO");
}
if (a2 === null) {
print("===null: YES");
} else {
print("===null: NO");
}
if (a2 == '') {
print("=='': YES");
} else {
print("=='': NO");
}
if (a2 === '') {
print("==='': YES");
} else {
print("==='': NO");
}
if (isNaN(a2)) {
print("isNaN(): YES");
} else {
print("isNaN(): NO");
}
if (a2) {
print("if-?: YES");
} else {
print("if-?: NO");
}
print("typeof(): " + typeof(a2));
}
function print(v) {
b1.innerHTML += v + "\n";
}
<!DOCTYPE html>
<html>
<body>
<pre id="b1"></pre>
</body>
</html>
See also:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/undefined
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/NaN
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/null
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Comparison_Operators
Cheers!
The difference between undefined and null is minimal, but there is a difference. A variable whose value is undefined has never been initialized. A variable whose value is null was explicitly given a value of null, which means that the variable was explicitly set to have no value. If you compare undefined and null by using the null==undefined expression, they will be equal.
Basically, Undefined is a global variable that javascript create at the run time whether null means that no value has assigned to the variable (actually null is itself an object).
Let's take an example:
var x; //we declared a variable x, but no value has been assigned to it.
document.write(x) //let's print the variable x
Undefined that's what you will get as output.
Now,
x=5;
y=null;
z=x+y;
and you will get 5 as output. That's the main difference between the Undefined and null
Both special values imply an empty state.
The main difference is that undefined represents the value of a variable that wasn’t yet initialized, while null represents an intentional absence of an object.
The variable number is defined, however, is not assigned with an initial value:
let number;
number; // => undefined
number variable is undefined, which clearly indicates an uninitialized variable
The same uninitialized concept happens when a non-existing object property is accessed:
const obj = { firstName: 'Dmitri' };
obj.lastName; // => undefined
Because lastName property does not exist in obj, JavaScript correctly evaluates obj.lastName to undefined.
In other cases, you know that a variable expects to hold an object or a function to return an object. But for some reason, you can’t instantiate the object. In such a case null is a meaningful indicator of a missing object.
For example, clone() is a function that clones a plain JavaScript object. The function is expected to return an object:
function clone(obj) {
if (typeof obj === 'object' && obj !== null) {
return Object.assign({}, obj);
}
return null;
}
clone({name: 'John'}); // => {name: 'John'}
clone(15); // => null
clone(null); // => null
However, clone() might be invoked with a non-object argument: 15 or null (or generally a primitive value, null or undefined). In such case, the function cannot create a clone, so it returns null - the indicator of a missing object.
typeof operator makes the distinction between the two values:
typeof undefined; // => 'undefined'
typeof null; // => 'object'
The strict quality operator === correctly differentiates undefined from null:
let nothing = undefined;
let missingObject = null;
nothing === missingObject; // => false

Javascript array value is undefined ... how do I test for that [duplicate]

This question already has answers here:
JavaScript check if variable exists (is defined/initialized)
(32 answers)
Closed 6 years ago.
I am trying to test to see whether a Javascript variable is undefined.
You will see that I am not expecting the value of predQuery[preId] to be 'undefined' if I don't first get an alert saying "its unbelievable". But I often do, so I am guessing that my statement
predQuery[preId]=='undefined')
is not matching the undefined elements properly.
if((predQuery.length < preId) || (predQuery[preId]=="") || (predQuery[preId]=='undefined')){
alert("its unbelievable");
alert(predQuery[preId]);
queryPreds[variables] = preId;
queryObjs[variables] = objId;
predQuery[preId] = variables;
}
else {
alert(predQuery[preId]);
var predIndex = predQuery[preId];
queryPreds[predIndex] = preId;
queryObjs[predIndex] = objId;
}
I can add more code if needed.
array[index] == 'undefined' compares the value of the array index to the string "undefined".
You're probably looking for typeof array[index] == 'undefined', which compares the type.
You are checking it the array index contains a string "undefined", you should either use the typeof operator:
typeof predQuery[preId] == 'undefined'
Or use the undefined global property:
predQuery[preId] === undefined
The first way is safer, because the undefined global property is writable, and it can be changed to any other value.
predQuery[preId]=='undefined'
You're testing against the string 'undefined'; you've confused this test with the typeof test which would return a string. You probably mean to be testing against the special value undefined:
predQuery[preId]===undefined
Note the strict-equality operator to avoid the generally-unwanted match null==undefined.
However there are two ways you can get an undefined value: either preId isn't a member of predQuery, or it is a member but has a value set to the special undefined value. Often, you only want to check whether it's present or not; in that case the in operator is more appropriate:
!(preId in predQuery)
There are more (many) ways to Rome:
//=>considering predQuery[preId] is undefined:
predQuery[preId] === undefined; //=> true
undefined === predQuery[preId] //=> true
predQuery[preId] || 'it\'s unbelievable!' //=> it's unbelievable
var isdef = predQuery[preId] ? predQuery[preId] : null //=> isdef = null
cheers!
Check for
if (predQuery[preId] === undefined)
Use the strict equal to operator. See comparison operators
try: typeof(predQuery[preId])=='undefined'
or more generally: typeof(yourArray[yourIndex])=='undefined'
You're comparing "undefined" to undefined, which returns false =)
This code works very well
function isUndefined(array, index) {
return ((String(array[index]) == "undefined") ? "Yes" : "No");
}

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