Jquery ajax cannot pass the value to the background using the get method, why

Jquery ajax cannot pass the value to the background using the get method. why is as follows:
// The following is the js method
Function login (){
$. Ajax ({
Type: "get ",
Url: 'valid. php ',
Data: "username = admin ",
DataType: "json ",
Success: ajaxComplete (result ),
Error: ajaxError (response )});
});
}

// The following is valid. php
Echo $ _ GET ['username'];
?>

After the login () method is triggered, the page reports the error Notice: Undefined index: username, which means this parameter cannot be found. why !!

Reply to discussion (solution)

Data: "username = admin", the error here is either your data "username: admin"
Or serialize it first.

You have written an error in JavaScript. how can you get the result?

Function login () {$. ajax ({type: "get", url: 'valid. php ', data: "username = admin", dataType: "json", success: ajaxComplete, error: ajaxError });}

Both success and error require callback functions.
You can either assign a function name or directly define an anonymous function.