In JavaScript, null is an object. There's another value for things that don't exist, undefined. The DOM returns null for almost all cases where it fails to find some structure in the document, but in JavaScript itself undefined is the value used.
Second, no, there is not a direct equivalent. If you really want to check for specifically for null, do:
if (yourvar === null) // Does not execute if yourvar is `undefined`
If you want to check if a variable exists, that can only be done with try/catch, since typeof will treat an undeclared variable and a variable declared with the value of undefined as equivalent.
But, to check if a variable is declared and is not undefined:
if (yourvar !== undefined) // Any scope
Previously, it was necessary to use the typeof operator to check for undefined safely, because it was possible to reassign undefined just like a variable. The old way looked like this:
if (typeof yourvar !== 'undefined') // Any scope
The issue of undefined being re-assignable was fixed in ECMAScript 5, which was released in 2009. You can now safely use === and !== to test for undefined without using typeof as undefined has been read-only for some time.
If you want to know if a member exists independent but don't care what its value is:
if ('membername' in object) // With inheritance
if (object.hasOwnProperty('membername')) // Without inheritance
If you want to to know whether a variable is truthy:
if (yourvar)
Source
Answer from Natrium on Stack OverflowHi, all, I often do stuff like this in my code, to check for a variable being not null and not undefined.
// check if value is not null and not undefined
if (value) {
...
}
However, I'm now thinking this can leads to bugs, because of 0, "", false and NaN also being falsy.
What is a better way to check a variable is not null and not undefined? I could use this I think, wondering if there is something shorter than this:
if (typeof value !== 'undefined' || value !== null) {
...
}In JavaScript, null is an object. There's another value for things that don't exist, undefined. The DOM returns null for almost all cases where it fails to find some structure in the document, but in JavaScript itself undefined is the value used.
Second, no, there is not a direct equivalent. If you really want to check for specifically for null, do:
if (yourvar === null) // Does not execute if yourvar is `undefined`
If you want to check if a variable exists, that can only be done with try/catch, since typeof will treat an undeclared variable and a variable declared with the value of undefined as equivalent.
But, to check if a variable is declared and is not undefined:
if (yourvar !== undefined) // Any scope
Previously, it was necessary to use the typeof operator to check for undefined safely, because it was possible to reassign undefined just like a variable. The old way looked like this:
if (typeof yourvar !== 'undefined') // Any scope
The issue of undefined being re-assignable was fixed in ECMAScript 5, which was released in 2009. You can now safely use === and !== to test for undefined without using typeof as undefined has been read-only for some time.
If you want to know if a member exists independent but don't care what its value is:
if ('membername' in object) // With inheritance
if (object.hasOwnProperty('membername')) // Without inheritance
If you want to to know whether a variable is truthy:
if (yourvar)
Source
The only way to truly test if a variable is undefined is to do the following. Remember, undefined is an object in JavaScript.
if (typeof someVar === 'undefined') {
// Your variable is undefined
}
Some of the other solutions in this thread will lead you to believe a variable is undefined even though it has been defined (with a value of NULL or 0, for instance).
What's the best method to detect undefined variables in JS?
How to detect that a pointer is undefined?
jQuery / Javascript code check, if not undefined - Stack Overflow
javascript - Check if variable is false and not either true or undefined - Stack Overflow
If the variable is declared then:
if (myvar === false) {
will work fine. === won't consider false to be undefined.
If it is undefined and undeclared then you should check its type before trying to use it (otherwise you will get a reference error).
if(typeof myvar === 'boolean' && myvar === false) {
That said, you should ensure that the variable is always declared if you plan to try to use it.
var myvar;
// ...
// some code that may or may not assign a value to myvar
// ...
if (myvar === false) {
if(typeof myvar === 'boolean' && !myvar)