how to check if a variable exist in javascript?
I am not asking if the variable is undefined or if it is null . I want to check if the variable exists or not. Is this possible?
The typeof techniques don't work because they don't distinguish between when a variable has not been declared at all and when a variable has been declared but not assigned a value, or declared and set equal to undefined.
But if you try to use a variable that hasn't been declared in an if condition (or on the right-hand side of an assignment) you get an error. So this should work:
var exists = true;
try {
if (someVar)
exists = true;
} catch(e) { exists = false; }
if (exists)
// do something - exists only == true if someVar has been declared somewhere.
if ('bob' in window) console.log(bob);
请记住，即使你用var声明了一个变量，这也意味着它存在。
I use this function:
function exists(varname){
try {
var x = eval(varname);
return true;
} catch(e) { return false; }
}
Hope this helps.
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