What is the meaning of "callback.call( value, i, value )" in jQuery's each method?
each()
method in jQuery contains such a statement:
callback.call( value, i, value )
I couldn't understand what this sta开发者_Python百科tement means exactly.
I know what callback
and call
mean but I couldn't get the arguments of the function call: (value,i,value)
. What does this mean?
The statement is used in a for block of each()
but my question is independent of that context.
from the jQuery source:
for ( var value = object[0];
i < length &&
callback.call( value, i, value ) // <=== LOOK!
!== false;
value = object[++i] ) {}
The call
method exists on all functions in Javascript. It allows you to call the function and in doing so set the value of this
within that function.
function myFunc() {
console.log(this);
}
myFunc.call(document.body);
In this example, this
within myFunc
will be document.body
.
The first parameter of call
is the value to be set as this
; subsequent parameters are passed on to the function as normal parameters. So, in your example:
callback.call( value, i, value )
this is equivalent to
callback(i, value)
except that, within the callback, this
is now also set to value
.
The .each()
method calls the callback you pass it with the element (current iteration "target") as both the context object (the value of this
) and as the second parameter.
Thus, in one of those functions:
$('.foo').each(function(i, elem) {
var $this = $(this), $elem = $(elem);
The variables $this
and $elem
are interchangeable.
The first argument to .call()
is the value to which this
should be bound, if that wasn't clear. The rest of the arguments to .call()
are just passed as plain arguments to the function.
This calls the callback
method with this
set to value
(the first parameter to call
) and with the arguments i
and value
. (The other parameters to call
)
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