| 5. 1. 8. The Undefined Type |
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The Undefined type has only one value, undefined. |
When a variable is declared and not initialized, it is given the value of undefined by default. |
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var oTemp;
alert(oTemp == undefined);
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You can also use the typeof operator to show that the variable has a value of undefined. |
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var oTemp;
alert(typeof oTemp);
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A variable having the value of undefined is different from a value being undefined. |
The typeof operator doesn't distinguish between the two. |
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var oTemp;
alert(typeof oTemp); //outputs "undefined"
alert(typeof oTemp2); //outputs "undefined"
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If you try to use oTemp2 with any operator other than typeof, it causes an error. |
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alert(oTemp2 == undefined); //causes error
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The value undefined is also returned when a function doesn't explicitly return a value: |
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function testFunc() {
//leave the function blank
}
alert(testFunc() == undefined); //outputs "true"
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