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Every function in JavaScript is actually a Function object.
Function objects created with the Function constructor are parsed when the
function is created. This is less efficient than declaring a function and
calling it within your code, because functions declared with the function
statement are parsed with the rest of the code.
All arguments passed to the function are treated as the names of the identifiers of the parameters in the function to be created, in the order in which they are passed.
Invoking the Function constructor as a function (without using the new
operator) has the same effect as invoking it as a constructor.
Function constructorThe following code creates a Function object that takes two arguments.
// Example can be run directly in your JavaScript console
// Create a function that takes two arguments and returns the sum of those
arguments
var adder = new Function("a", "b", "return a + b");
// Call the function
adder(2, 6);
// > 8
The arguments "a" and "b" are formal argument names that are used in the function body, "return a + b".
Creates new Function object.
Names to be used by the function as formal argument names. Each must be a
string that corresponds to a valid JavaScript identifier or a list of such
strings separated with a comma; for example "x", "theValue", or "a,b".
A string containing the JavaScript statements comprising the function definition.
Applies the method of another object in the context of a different object (the calling object); arguments can be passed as an Array object.
You can assign a different this object when calling an existing function. this refers to the
current object, the calling object. With apply, you can write a method once and then inherit it
in another object, without having to rewrite the method for the new object.
apply is very similar to call, except for the type of arguments it supports. You can use an
arguments array instead of a named set of parameters. With apply, you can use an array literal, for
example, fun.apply(this, ['eat', 'bananas']), or an Array object, for example, fun.apply(this,
new Array('eat', 'bananas')).
You can also use arguments for the argsArray parameter. arguments is a local variable of a
function. It can be used for all unspecified arguments of the called object. Thus, you do not have
to know the arguments of the called object when you use the apply method. You can use arguments
to pass all the arguments to the called object. The called object is then responsible for handling
the arguments.
Since ECMAScript 5th Edition you can also use any kind of object which is array like, so in
practice this means it's going to have a property length and integer properties in the range
[0...length). As an example you can now use a NodeList or a own custom object like {'length': 2,
'0': 'eat', '1': 'bananas'}.
You can use apply to chain constructors for an object, similar to Java. In the following example,
the constructor for the Product object is defined with two parameters, name and value. Two
other functions Food and Toy invoke Product passing this and arguments. Product
initializes the properties name and price, both specialized functions define the category. In
this example, the arguments object is fully passed to the product constructor and corresponds to
the two defined parameters.
function Product(name, price) {
this.name = name;
this.price = price;
if (price < 0)
throw RangeError('Cannot create product "' + name + '" with a negative price');
return this;
}
function Food(name, price) {
Product.apply(this, arguments);
this.category = 'food';
}
Food.prototype = new Product();
function Toy(name, price) {
Product.apply(this, arguments);
this.category = 'toy';
}
Toy.prototype = new Product();
var cheese = new Food('feta', 5);
var fun = new Toy('robot', 40);
Clever usage of apply allows you to use built-ins functions for some tasks that otherwise
probably would have been written by looping over the array values. As an example here we are going
to use Math.max/Math.min to find out the maximum/minimum value in an array.
//min/max number in an array
var numbers = [5, 6, 2, 3, 7];
//using Math.min/Math.max apply
var max = Math.max.apply(null, numbers); // This about equal to Math.max(numbers[0], ...) or
// Math.max(5, 6, ..)
var min = Math.min.apply(null, numbers);
//vs. simple loop based algorithm
max = -Infinity, min = +Infinity;
for (var i = 0; i < numbers.length; i++) {
if (numbers[i] > max)
max = numbers[i];
if (numbers[i] < min)
min = numbers[i];
}
But beware: in using apply this way, you run the risk of exceeding the JavaScript engine's
argument length limit. The consequences of applying a function with too many arguments (think more
than tens of thousands of arguments) vary across engines, because the limit (indeed even the nature
of any excessively-large-stack behavior) is unspecified. Some engines will throw an exception. More
perniciously, others will arbitrarily limit the number of arguments actually passed to the applied
function. (To illustrate this latter case: if such an engine had a limit of four arguments [actual
limits are of course significantly higher], it would be as if the arguments 5, 6, 2, 3 had been
passed to apply in the examples above, rather than the full array.) If your value array might grow
into the tens of thousands, use a hybrid strategy: apply your function to chunks of the array at a
time:
function minOfArray(arr)
{
var min = Infinity;
var QUANTUM = 32768;
for (var i = 0, len = arr.length; i < len; i += QUANTUM)
{
var submin = Math.min.apply(null, numbers.slice(i, Math.min(i + QUANTUM, len)));
min = Math.min(submin, min);
}
return min;
}
var min = minOfArray([5, 6, 2, 3, 7]);
The value of this provided for the call to fun. Note that this may not be the actual value seen by the method: if the method is a function in non-strict mode code, null and undefined will be replaced with the global object, and primitive values will be boxed.
An array like object, specifying the arguments with which fun should be called, or null or undefined if no arguments should be provided to the function.
Returns what the function returns.
Calls (executes) a method of another object in the context of a different object (the calling object); arguments can be passed as they are.
You can assign a different this object when calling an existing function. this refers to the
current object, the calling object.
With call, you can write a method once and then inherit it in another object, without having to
rewrite the method for the new object.
You can use call to chain constructors for an object, similar to Java. In the following example,
the constructor for the product object is defined with two parameters, name and value. Another
object, prod_dept, initializes its unique variable (dept) and calls the constructor for
product in its constructor to initialize the other variables.
function Product(name, price) {
this.name = name;
this.price = price;
if (price < 0)
throw RangeError('Cannot create product "' + name + '" with a negative price');
return this;
}
function Food(name, price) {
Product.call(this, name, price);
this.category = 'food';
}
Food.prototype = new Product();
function Toy(name, price) {
Product.call(this, name, price);
this.category = 'toy';
}
Toy.prototype = new Product();
var cheese = new Food('feta', 5);
var fun = new Toy('robot', 40);
In this purely constructed example, we create anonymous function and use call to invoke it on
every object in an array. The main purpose of the anonymous function here is to add a print
function to every object, which is able to print the right index of the object in the array.
Passing the object as this value was not strictly necessary, but is done for explanatory purpose.
var animals = [
{species: 'Lion', name: 'King'},
{species: 'Whale', name: 'Fail'}
];
for (var i = 0; i < animals.length; i++) {
(function (i) {
this.print = function () {
console.log('#' + i + ' ' + this.species + ': ' + this.name);
}
}).call(animals[i], i);
}
The value of this provided for the call to fun.Note that this may not be
the actual value seen by the method: if the method is a function in non-strict mode code, null
and undefined will be replaced with the global object, and primitive values will be boxed.
Arguments for the object.
Returns what the function returns.
Returns a string representing the source code of the function. Overrides the
Object.toString method.
The Function object overrides the toString method of the Object object; it does
not inherit Object.toString. For Function objects, the toString method returns a string
representation of the object.
JavaScript calls the toString method automatically when a Function is to be represented as a
text value or when a Function is referred to in a string concatenation.
For Function objects, the built-in toString method decompiles the function back into the
JavaScript source that defines the function. This string includes the function keyword, the
argument list, curly braces, and function body.
The function as a string.