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javascriptcompatibility

TypeOf undefined instead of comparing against undefined?

In JavaScript, why would people write typeof myVar == "undefined" instead of myVar == undefined?

Is it for compatibility reasons?

Solution

  • This is the main reason:

    if(a == undefined) console.log('test')
>> ReferenceError: a is not defined
if(typeof a == "undefined") console.log('test')
>> test

    But if you run this comparison:

    if(window.a == undefined) console.log('test')
>> test

    So if you use a as a standalone variable then you can't. Using window it's possible, and doesn't really matter what approach will you use, but as I stated in comment it's safer to use typeof as not every variable belongs to window scope.