Menu
I'm looking for the javascript equivalent of the php function isset(). I've tried the method described here at JavaScript isset() equivalent but at firebug, error comes up saying
data.del is undefined //Firebug warning/error
if(typeof data.del[0].node != 'undefined') { // codes in my js file
And in some cases
data is null //Firebug warning/error
if(typeof data.storyLine != 'undefined') { // codes in my js file
The logic seems to work but I'm wondering why is there an error then??
Basically, I want to check whether data.del[0].node or data.storyLine isset or not??
400
PHP provides a function isset that determines if a variable is set; this means if a variable is declared and assigned a value other than null. When a variable is assigned to null, isset() will return false. In JavaScript, we need to determine whether a variable is set or not.
The equivalent of isset($var) for a function return value is func() === null . isset basically does a !== null comparison, without throwing an error if the tested variable does not exist.
Use the typeof operator to check if a variable is defined or initialized, e.g. if (typeof a !== 'undefined') {} . If the the typeof operator doesn't return a string of "undefined" , then the variable is defined.
The isset() function is an inbuilt function in PHP which checks whether a variable is set and is not NULL. This function also checks if a declared variable, array or array key has null value, if it does, isset() returns false, it returns true in all other possible cases.
isset() makes two checks: first if the variable is defined, and second if it is null.
You will have to check for both the 'undefined' case and the null case, for example:
if (typeof data !== 'undefined' && data !== null)
128
ECMAScript defines the hasOwnProperty method for checking if an object has a property of a given name:
var foo = {'bar':'bar'}
alert( foo.hasOwnProperty( 'bar' ) ); //true
alert( foo.hasOwnProperty( 'baz' ) ); //false
EDIT: This doesn't fully answer your question
It's possible for a property to be set as undefined
foo.bar = undefined;
alert( foo.hasOwnProperty( 'bar' ) ); //still true
The important question is: What do you need your truth table to be?
In php:
type | isset() | == true
------+---------+----------
null | false | false
false | true | false
true | true | true
"" | true | false
"a" | true | true
0 | true | false
1 | true | true
In JS:
type | isset() | truthy
----------+---------+--------
NaN | ? | false
undefined | ? | false
null | false | false
true | true | true
false | true | false
"" | true | false
"a" | true | true
0 | true | false
1 | true | true
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
