javascript tutorial - [Solved-5 Solutions] How to check for “undefined” in javascript ? - javascript - java script - javascript array
Problem:
What is the most appropriate way to test if a variable is undefined in JavaScript ? I've seen several possible ways:
if (window.myVariable)
Or
if (typeof(myVariable) != "undefined")
Or
if (myVariable) //This throws an error if undefined. Should this be in Try/Catch?
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Solution 1:
If we are interested in finding out whether a variable has been declared regardless of its value, then using the in operator is the safest way to go. Consider this example.
// global scope
var theFu; // theFu has been declared, but its value is undefined
typeof theFu; // "undefined"
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But this may not be the intended result for some cases, since the variable or property was declared but just not initialized. Use the in operator for a more robust check.
"theFu" in window; // true
"theFoo" in window; // false
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If we are interested in knowing whether the variable hasn't been declared or has the value undefined, then use the typeof operator.
if (typeof myVar != 'undefined')
The typeof operator is guaranteed to return a string. Direct comparisons against undefined are troublesome as undefined can be overwritten.
window.undefined = "omg";
"omg" == undefined // true
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As @CMS pointed out, this has been patched in ECMAScript 5th ed., and undefined is non-writable.
if (window.myVar) will also include these falsy values, so it's not very robust:
false
0
""
NaN
null
undefined
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Thanks to @CMS for pointing out that our third case - if (myVariable) can also throw an error in two cases. The first is when the variable hasn't been defined which throws a ReferenceError.
// abc was never declared.
if (abc) {
// ReferenceError: abc is not defined
}
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The other case is when the variable has been defined, but has a getter function which throws an error when invoked. For example,
// or it's a property that can throw an error
Object.defineProperty(window, "myVariable", {
get: function() { throw new Error("W00t?"); },
set: undefined
});
if (myVariable) {
// Error: W00t?
}
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Solution 2:
Using typeof is my preference. It will work when the variable has never been declared, unlike any comparison with the == or === operators or type coercion using if. (undefined, unlike null, may also be redefined in ECMAScript 3 environments, making it unreliable for comparison, although nearly all common environments now are compliant with ECMAScript 5 or above).
if (typeof someUndeclaredVariable == "undefined") {
// Works
}
if (someUndeclaredVariable === undefined) {
// Throws an error
}
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Solution 3:
We need to use typeof .
if (typeof something != "undefined") {
// ...
}
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Solution 4:
If it is undefined, it will not be equal to a string that contains the characters "undefined", as the string is not undefined. We can check the type of the variable:
if (typeof(something) != "undefined") ...
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Sometimes we don't even have to check the type. If the value of the variable can't evaluate to false when it's set (for example if it's a function), then we can just evalue the variable. Example:
if (something) {
something(param);
}
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Solution 5:
if (typeof foo == 'undefined') {
// Do something
};
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Note that strict comparison (!==) is not necessary in this case, since typeof will always return a string.