how to check for null and undefined in js?
3303
Comparisons can be made: null === null to check strictly for null or null == undefined to check loosely for either null or undefined. The value null is falsy, but empty objects are truthy, so typeof maybeNull === "object" && ! maybeNull is an easy way to check that a value is not null .
1257401
An offset is undefined if it doesn't exist in the array. The notice is thrown the first time you reference the element, then PHP initializes the element and sets it to NULL. ... You can test if an offset exists either through array_key_exists or using isset.
172093
In JavaScript, undefined is a type, whereas null an object. It means a variable declared, but no value has been assigned a value. Whereas, null in JavaScript is an assignment value. You can assign it to a variable.
92974
var sel = document. getElementsByName('item'); var sv = sel. value; alert(sv); The only reason your code might fail is when there is no item selected, then the selectedIndex returns -1 and the code breaks.
733593
To establish whether or not a checkbox or radio button was checked, use the empty() function. Testing the value of a non-existent variable spawns a warning message. If empty() returns false, the field was checked and the field's value can be accessed.
167916
Strict Equality. So undefined really means undefined. Not False, not True, not 0, not empty string. So when you compare undefined to anything, the result is always false, it is not equal to that.
361497
An offset is undefined if it doesn't exist in the array. The notice is thrown the first time you reference the element, then PHP initializes the element and sets it to NULL.
134544
A rational expression is undefined when the denominator is equal to zero. To find the values that make a rational expression undefined, set the denominator equal to zero and solve the resulting equation. Example: 0 7 2 3 x x − Is undefined because the zero is in the denominator.
1340910
['Boolean.', 'Null.', 'Undefined.', 'Number.', 'String.', 'Symbol.', 'Object.']
1222666
['Compare with typeof and undefined .', 'Use hasOwnProperty method.', 'Use in keyword.']
184437
An offset is undefined if it doesn't exist in the array. The notice is thrown the first time you reference the element, then PHP initializes the element and sets it to NULL. To avoid the notice you must initialize the offset. You can test if an offset exists either through array_key_exists or using isset.
217070
You can use the Oracle IS NULL condition in PLSQL to check if a value is null. For example, IF Lvalue IS NULL then ... ... If Lvalue contains a null value, the "IF" expression will evaluate to TRUE.
1232699
Summary. null is an assigned value. It means nothing. undefined typically means a variable has been declared but not defined yet.
1195874
In order for Javascript to compare the value of the x variable it must look it up; since it is not yet defined it throws an error message. This error is happening before the runtime even attempts to compare the value to undefined . ... use typeof x === 'undefined' instead.
1009682
What are undeclared and undefined variables in JavaScript? Undefined: It occurs when a variable has been declared but has not been assigned with any value. ... Undeclared: It occurs when we try to access any variable that is not initialized or declared earlier using var or const keyword.
526391
If you are using LEN(...) to determine whether the field is NULL or EMPTY then you need to use it as follows: ... WHEN LEN(ISNULL(MyField, '')) < 1 THEN NewValue...
1136958
undefined means a variable has been declared but has not yet been assigned a value. On the other hand, null is an assignment value. It can be assigned to a variable as a representation of no value. Also, undefined and null are two distinct types: undefined is a type itself (undefined) while null is an object.