Is there a universal JavaScript function that checks that a variable has a value and ensures that it's not undefined or null? I've got this code, but I'm not sure if it covers all cases:
function isEmpty(val){
return (val === undefined || val == null || val.length <= 0) ? true : false;
}
转载于:https://stackoverflow.com/questions/5515310/is-there-a-standard-function-to-check-for-null-undefined-or-blank-variables-in
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You can just check if the variable has a truthy value or not. That means
if( value ) {
}
will evaluate to true if value is not:
The above list represents all possible falsy values in ECMA-/Javascript. Find it in the specification at the ToBoolean section.
Furthermore, if you do not know whether a variable exists (that means, if it was declared) you should check with the typeof operator. For instance
if( typeof foo !== 'undefined' ) {
// foo could get resolved and it's defined
}
If you can be sure that a variable is declared at least, you should directly check if it has a truthy value like shown above.
Further read: http://typeofnan.blogspot.com/2011/01/typeof-is-fast.html
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