JavaScript undefined Property

　JavaScript Global Properties/Function

Globalundefined representing that a value has not been assigned to a variable, or that the variable has not been declared at all.

It is a property of JavaScriptOne of the primitive types.

If the variable to be calculated has not been assigned a value, the method or statement will also return undefined.

If the function does not return a value, it returns undefined.

Syntax:

undefined

var str;
if (str === undefined) {
　　　//　str is undefined
} else {
　　　//　str is defined
}

Browser Compatibility

All browsers fully support the undefined property:

Technical Details

ECMAScript

More Examples

Check if the variable str is defined or undefined:39;if (typeof str === 'undefined&#
　　　//　str is undefined
} else {
　　　//　str is defined
}

If no value is returned, the function returns undefined:

function demo() {
　　　return;　//　return nothing....
}
;document.getElementById('result').innerHTML = demo();

　JavaScript Global Properties/Function