If it is undefined, it will not be equal to a string that contains the characters "undefined", as the string is not undefined. You can check the type of the variable: May 7, 2009 · Assigning undefined to a variable though is probably confusing, since it's a bit of a paradox (you've defined the variable as undefined) and it's not possible to distinguish that variable . While literally using the keyword undefined, Boolean(undefined) works, trying that with an undefined variable doesn't work, and that is the whole point of doing the check for null or undefined.
Dec 31, 2009 · typeof foo !== 'undefined' window.foo !== undefined 'foo' in window The first two should be equivalent (as long as foo isn't shadowed by a local variable), whereas the last one will return true . Feb 24, 2011 · How do I check a variable if it's null or undefined and what is the difference between the null and undefined? What is the difference between == and === (it's hard to search Google for "===" )? Deseo determinar si una variable no está definida o si tiene el valor nulo. En el siguiente ejemplo, quiero que se ejecute la clásula correspondiente a la condición verdadera, es decir se imprima .
Feb 22, 2011 · NaN. See for yourself. console.log (null-undefined). The difference between null and undefined is NaN. (Note that this is an attempt at humour, before you flame me for misunderstanding . I can't find any difference between typeof somevar == 'undefined' and typeof somevar === 'undefined', because typeof always returns string. For null it will return 'object'. Or could be that I am wrong?
Assigning undefined to a variable though is probably confusing, since it's a bit of a paradox (you've defined the variable as undefined) and it's not possible to distinguish that variable.
typeof foo!== 'undefined' window.foo!== undefined 'foo' in window The first two should be equivalent (as long as foo isn't shadowed by a local variable), whereas the last one will return true.
How do I check a variable if it's null or undefined and what is the difference between the null and undefined?
- The difference between
typeof x!== "undefined"andx!= null.
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Sources
- https://stackoverflow.com/questions/3390396/how-can-i-check-for-undefined-in-javascript
- https://stackoverflow.com/questions/833661/what-is-the-difference-in-javascript-between-undefined-and-not-defined
- https://stackoverflow.com/questions/2559318/how-can-i-check-for-an-undefined-or-null-variable-in-javascript
- https://stackoverflow.com/questions/1984721/how-to-handle-undefined-in-javascript
