This question already has answers here:
Parameters inside callback function in Javascript
(1 answer)
Higher-Order Functions in JS
(1 answer)
Closed 2 years ago.
I have this code that filters an array using the .filter method.
I'm extremely confused about the function's parameters that is being specified in the .filter method.
Where does the parameter come from? How do I know when to add a parameter like 'value', and what is the value of the parameter 'value'?
var newArray = [1,2,3,4,5,6,7,8,9,10];
newArray = newArray.filter(function(value) {
return value < 6;
});
I'm not too sure if it is the right term to use.
Where does the parameter come from?
It gets passed to the function when the function is called … which will happen somewhere inside filter or a function called by filter.
How do I know when to add a parameter like 'value',
Generally by reading the documentation
what is the value of the parameter 'value'
From the documentation, the first argument is:
element:
The current element being processed in the array.
Array is a JavaScript class and each JavaScript class can have prototype. Filter method is a part of Array prototype and this function is getting as an argument callback function with one argument.
Inside Filter function, your callback is being called on each array item.
You can find more informations about this function in JavaScript docs.
Related
This question already has answers here:
Why provide an array argument in Javascript's array.forEach callback?
(2 answers)
Closed 1 year ago.
I know that forEach in JavaScript calls my callback function with three parameters:
arr.forEach(function callback(currentValue, index, array) {
//your iterator
})
In the above example arr and array are same array and arr exists in the callback function closure.
Now the question is what is the point of passing array to the callback function?
If your callback function were declared elsewhere:
function forEachCallback(value, i, array) {
// ...
}
Then it has no idea what array it's being used for:
someArray.forEach(forEachCallback);
Because the array is passed as the last argument, such a callback has access to it.
The callback need not be in the same scope as the forEach call. In a case like this, the third parameter would ensure that the callback has some reference to that array.
This question already has answers here:
Is there a way to provide named parameters in a function call in JavaScript?
(12 answers)
How to get function parameter names/values dynamically?
(34 answers)
Closed 4 years ago.
I have many functions defined elsewhere that look like this
export function foo(bar1, bar2, bar3, ...) {
// does stuff
}
In the module I am working on I am passed both the function above and an object containing the parameters like this:
// params = {
// bar1: bar1Value,
// bar2: bar2Value,
// bar3: bar3Value,
// ...
// }
function myFunc(foo, params) {
}
I need to call func with the params in the correct order. It is an object not an array so I can't rely on the property order, I need to match up the params to the signature of the function. Is this possible?
Edit to add: There are hundreds of functions that can be passed in, I am trying to avoid refactoring them all, but it seems that it is impossible to look up the parameter names directly. However I've found this which shows it can be done by breaking down the function as a string.
You can access the properties of an object (like "params") either like
params.bar1 or like params["bar1"] and you can get the names of the properties by using Object.keys(params), if thats what your asking for.
See also https://www.w3schools.com/js/js_object_properties.asp and https://developer.mozilla.org/de/docs/Web/JavaScript/Reference/Global_Objects/Object/keys
This question already has answers here:
JavaScript variable number of arguments to function
(12 answers)
Closed 5 years ago.
I would like to call a function, lets say test_func that accepts variable number of arguments. (like this function http://locutus.io/php/array/array_intersect_assoc/)
Please note I would like to avoid modifying the receiver function! All the answers so far require modifying the receiver function, just as the linked "possible dupes". I would like to programmatically generate the argument list itself. (eg pass variable number of objects)
I do not know in advance how many arguments I will have. How can I generate a variable length argument with objects?
var v1 = {test:1};
var v2 = {test2:2};
var obj_arr = [v1,v2];
console.log(test_func (obj_arr.join(",")));
//in my case this should be the equivalent of test_func (v1,v2);
function test_func (object_arg) {
return(arguments.length);
}
//should return 2!
If you're in an ES5 environment you can use arguments:
function test() {
var param1 = arguments[0];
var param2 = arguments[1];
// arguments is array like, you can iterate over it with a loop
}
If you are in an ES6 environment you can either use the rest operator as suggested by Suren or also use the arguments variant as above - depending on the convention of you're team.
This question already has answers here:
Why provide an array argument in Javascript's array.forEach callback?
(2 answers)
Closed 1 year ago.
I know that forEach in JavaScript calls my callback function with three parameters:
arr.forEach(function callback(currentValue, index, array) {
//your iterator
})
In the above example arr and array are same array and arr exists in the callback function closure.
Now the question is what is the point of passing array to the callback function?
If your callback function were declared elsewhere:
function forEachCallback(value, i, array) {
// ...
}
Then it has no idea what array it's being used for:
someArray.forEach(forEachCallback);
Because the array is passed as the last argument, such a callback has access to it.
The callback need not be in the same scope as the forEach call. In a case like this, the third parameter would ensure that the callback has some reference to that array.
This question already has answers here:
What's the difference between an argument and a parameter?
(38 answers)
Closed 7 years ago.
I know the difference between arguments and parameters, but I'am a bit confused about how jQuery uses these terms.
For instance, jQuery defines as "parameters" the options passed to the load method. These shouldn't be declared as arguments because you pass your own values?
Then, on ajax function you can find many times the terms "parameters" and "arguments". For example, if you search for the beforeSend property, you'll see:
The jqXHR and settings objects are passed as arguments.
But, in another section you'll also see:
The callback hooks provided by $.ajax() are as follows:
beforeSend callback option is invoked; it receives the jqXHR object
and the settings object as parameters.
Does it mean that jQuery uses equally the terms "parameters" and "arguments"?
"A parameter is a variable in a method definition. When a method is called, the arguments are the data you pass into the method's parameters."
What's the difference between an argument and a parameter?
"parameters" are passed , "arguments" are received
See
What does passing a parameter mean? in C#
Parameters and arguments
What is an 'Argument' in Programming means?