Javascript declare variable if not declared

Javascript if variable undefined javascript declare code example

For those of You who think that undefined can be overwriten, according to MDN In this article, we will learn about undefined or null values in JavaScript source code and how we can use a conditional statement to check undefined and null variables in JavaScript. Check Undefined Variable in JavaScript It is a primitive value that specifies that the value is not assigned, and the variable not assigned with any type of value is known as an undefined variable. Solution 3: You should use with operator and (to be sure that nobody overwrote undefined variable) to check if variable is undefined and when it is then assign value to it: Solution 1: remove the var.

Define a variable if its undefined using Javascript

If you have to do it from a name that’s in a javascript variable (that isn’t known ahead of time), then you can do it like this:

var str = "answer"; if (typeof window[str] == "undefined") < window[str] = <>; > 

This uses the fact that all global variables are properties of the window object (in a browser).

If you know the name of the variable ahead of time, then you can simply do this:

if (typeof answer == «undefined») var answer = <>;

Eval is executed in separate context.

You should use typeof with === operator and ‘undefined’ (to be sure that nobody overwrote undefined variable) to check if variable is undefined and when it is then assign value to it:

if (typeof answer === 'undefined')

Set a variable if undefined in JavaScript, 13 Answers. Yes, it can do that, but strictly speaking that will assign the default value if the retrieved value is falsey, as opposed to truly undefined. It would therefore not only match undefined but also null, false, 0, NaN, «» (but not «0» ). If you want to set to default only if the variable is strictly undefined then the … Usage examplevar x = (x === undefined) ? your_default_value : x;Feedback

If not declared, declare the variable?

if(typeof widget_width == 'undefined')

The Javascript has scoping rules in play — so when a variable is declared depending on the context where the declaration is made (if within function, it will be declared in the current scope) the variable is declared there.

By using a trick you can get variable declared in global scope if you leave out the ‘var’ keyword.

What would happen is that JS interpreter will try to find this variable in current scope (so local, within the function), and because it cannot find it, it will try to search all parent/enclosing scopes until it reaches Global (last) scope.

If the variable is not there either (and we know it is not due to the check you’re making), it will create it in the last scope searched, in this case, global.

So, remove the ‘var’ keyword and you’ll be fine.

This is probably beyond the scope of this question, but I wan’t to emphasize difference between undeclared and undefined , as I found myself being confused it many times.

Please look at the following example.

I thing using it for such scenarios is safe. For those of You who think that undefined can be overwriten, according to MDN

Please note: In modern browsers (JavaScript 1.8.5 / Firefox 4+), undefined is a non-configurable, non-writable property per the ECMAScript 5 specification. Even when this is not the case, avoid overriding it.

JavaScript Check if Undefined, An undefined variable or anything without a value will always return «undefined» in JavaScript. This is not the same as null, despite the fact that both imply an empty state. You’ll typically assign a value to a variable after you declare it, but this is not always the case.

Check Undefined and Null Variable in JavaScript

In this article, we will learn about undefined or null values in JavaScript source code and how we can use a conditional statement to check undefined and null variables in JavaScript.

Check Undefined Variable in JavaScript

It is a primitive value that specifies that the value is not assigned, and the variable not assigned with any type of value is known as an undefined variable. JavaScript directly sets the undefined value when programmers do not provide the value.

For example, if we declare a variable and try to display it in logs before initialization, it will get an undefined result by default. Non-existing elements for array returns are undefined also.

In JavaScript, if function arguments are not passed according to the parameters of that function, then the unpassed arguments are set to undefine.

let data console.log(data); //undefined value functionName(10); function functionName(x, y) < console.log(x) // it will print 10 console.log(y) //undefined argument >

let data checkVariable(data) // it will generate undefined result data = 10; // now initialized checkVariable(data) // it will generate defined result function checkVariable(data)< if(data === undefined)< console.log("variable is undefined") >else < console.log("variable is defined") >> 

"variable is undefined" // after initialzation "variable is defined" 

In the above code, we declared the data variable and passed that variable to function checkVariable as a parameter. That function is declared to get value as an argument and check with the conditional statement if else if the value is undefined then it will display «variable is undefined» result in logs, or else it will display defined.

We have reassigned the data variable with value 10 and passed it again to the function, and this time it will print «variable is defined» .

Check Null Variable in JavaScript

There is null which is treated as an object in JavaScript to represent an empty or unknown value. For example, let x = null the x variable is empty at the moment and may have any value later; it is a null variable.

boolean() function of JavaScript it will return false result.

let data = null console.log(data) // it will get null 

let data = null; checkNull(data) // will print null data = 10; // now initialized checkNull(data) // will print not null function checkNull(data)< if(data === null)< console.log("variable is null") >else < console.log("variable is not null") >> 

"variable is null" // after initialization "variable is not null" 

In the above code, we declared the data variable, assigned it with null , and passed that variable to function checkNull as a parameter. The function is declared to get value as an argument and check with the conditional statement if else if the value is equal to null, then it will display «variable is null» result in logs, or else it will display not null.

We reassigned the data variable and passed it to the function; this time, it will print «variable is not null» .

JavaScript Variable

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Undeclared variables usage in javascript

That’s just how the language works. In non-strict mode, an assignment to an undeclared symbol is implicitly treated as creating a global variable. In strict mode, that’s an error.

To use strict mode, a global script block or a function should start with the statement:

That is, a simple expression statement consisting of the string «use strict» . That will prevent the accidental creation of global variables (which can still be created explicitly), and impose a few other restrictions.

According to the documentation :

1. Declared variables are constrained in the execution context in which they are declared. Undeclared variables are always global.
2. Declared variables are created before any code is executed. Undeclared variables do not exist until the code assigning to them is executed.

console.log(a); // Throws a ReferenceError. console.log('still going. '); // Never executes. 

var a; console.log(a); // logs "undefined" or "" depending on browser. console.log('still going. '); // logs "still going. ". 

1. Declared variables are a non-configurable property of their execution context (function or global). Undeclared variables are configurable (e.g. can be deleted).

a = 10;
internally declares var a i.e. in background it first declares the variable globally and then set the value for that variable.

So whenever we write a=10 what actually happens:
var a;
a=10;
hence a is never undefined ;

but alert(b) in this case b has no values defined so it remains undefined

In non-strict mode, an assignment to an undeclared symbol is implicitly treated as creating a global variable. In strict mode, that’s an error.

Its always a better practice to use use strict

Javascript — How can I determine if a variable is, I have run into a little null checking problem. I want to check if a parameter being passed is null, or an empty object < >.This is a common and silly language problem, but I had forgotten about it.

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how to force variable declaration in javascript or how to check it?

Though you can use a variable without declaring it in javascript, the misuse of variable will cause hard-to-solve errors. For example, the following code will cause endless loop.

 for(i=0;i <100;i++)< document.write(fiveTimes(i)); >. function fiveTimes(x)

I’d like to know if there is a way to force every variable in the javascript to be declared before use. Or anybody knows how to check for variable declaration-before-using in tons of javascript files and blocks in a huge web server.

Technically the code i=5 is a valid declaration of a javascript variable. It’s just declared onto the global scope instead of the local.

2 Answers 2

You can enable «Strict Mode» and you can pass your code through jsLint (Or jsHint if you have sensitive feelings :))These steps will go a long way toward making your code execute predictably.

I think a lot of people feel that way. 😛 Plus, you can configure jsHint a lot more than jsLint—you can even get them to behave the same way if you want, but it’s easy to change anything you find annoying, unlike with Lint.

Another fun tool, that may do more than needed here, is Google’s Closure Compiler — developers.google.com/closure/compiler . I have yet to look into it or use it 🙂

The main architecture behind JavaScript doesn’t allow you to achieve this without parsing the code with another compiler ( jsHint, etc . ). What will happen there is that the library will double check your code for some dangerous ( unwanted ) results. If you are good enough and know what you are doing, you can avoid using those and just be careful of what you are doing.

Also, there are a lot of languages that compile to JS ( CoffeeScript, etc. ) that has automatic variable declaration for every function and more stuff you might be interested in.

This solution has to tell you that JavaScript isn’t supposed to do that internally and there is no forced use of ‘strict’ mode at least at this time.

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Undeclared variables usage in javascript

because in case a = 10 it sets the variable, and in case of alert(b) it searches variable and can not find it.

3 Answers 3

That’s just how the language works. In non-strict mode, an assignment to an undeclared symbol is implicitly treated as creating a global variable. In strict mode, that’s an error.

To use strict mode, a global script block or a function should start with the statement:

That is, a simple expression statement consisting of the string «use strict» . That will prevent the accidental creation of global variables (which can still be created explicitly), and impose a few other restrictions.

1. Declared variables are constrained in the execution context in which they are declared. Undeclared variables are always global.
2. Declared variables are created before any code is executed. Undeclared variables do not exist until the code assigning to them is executed.

console.log(a); // Throws a ReferenceError. console.log('still going. '); // Never executes. 

var a; console.log(a); // logs "undefined" or "" depending on browser. console.log('still going. '); // logs "still going. ". 

1. Declared variables are a non-configurable property of their execution context (function or global). Undeclared variables are configurable (e.g. can be deleted).

a = 10;
internally declares var a i.e. in background it first declares the variable globally and then set the value for that variable.

So whenever we write a=10 what actually happens:
var a;
a=10;
hence a is never undefined ;

but alert(b) in this case b has no values defined so it remains undefined

In non-strict mode, an assignment to an undeclared symbol is implicitly treated as creating a global variable. In strict mode, that’s an error.

Its always a better practice to use use strict

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