如何在 JavaScript 中检查"未定义"？ [复制]

Question

This question already has an answer here:

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• How to check a not-defined variable in JavaScript 13 answers
• How to handle 'undefined' in javascript [duplicate] 3 answers
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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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转载于:https://stackoverflow.com/questions/3390396/how-to-check-for-undefined-in-javascript

Answer 1

If you 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"

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

If you 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

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

Thanks to @CMS for pointing out that your 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
} 

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?
}