Книга: Functional Programming in JavaScript
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Anonymous functions

Another benefit of treating functions as first-class objects is the advent of anonymous functions.

As the name might imply, anonymous functions are functions without names. But they are more than that. What they allow is the ability to define ad-hoc logic, on-the-spot and as needed. Usually, it's for the benefit of convenience; if the function is only referred to once, then a variable name doesn't need to be wasted on it.

Some examples of anonymous functions are as follows:

// The standard way to write anonymous functions function(){return "hello world"};  // Anonymous function assigned to variable var anon = function(x,y){return x+y};  // Anonymous function used in place of a named callback function,  // this is one of the more common uses of anonymous functions. setInterval(function(){console.log(new Date().getTime())}, 1000); // Output:  1413249010672, 1413249010673, 1413249010674, ...  // Without wrapping it in an anonymous function, it immediately // execute once and then return undefined as the callback: setInterval(console.log(new Date().getTime()), 1000) // Output:  1413249010671

A more involved example of anonymous functions used within higher-order functions:

function powersOf(x) {   return function(y) {     // this is an anonymous function!     return Math.pow(x,y);   }; } powerOfTwo = powersOf(2); console.log(powerOfTwo(1)); // 2 console.log(powerOfTwo(2)); // 4 console.log(powerOfTwo(3)); // 8  powerOfThree = powersOf(3); console.log(powerOfThree(3));  // 9 console.log(powerOfThree(10)); // 59049

The function that is returned doesn't need to be named; it can't be used anywhere outside the function, and so it is an anonymous function.

Remember our accumulator function? It can be re-written using anonymous functions.

var   obj1 = {value: 1},   obj2 = {value: 2},   obj3 = {value: 3};  var values = (function() {   // anonymous function   var values = [];   return function(obj) {     // another anonymous function!     if (obj) {       values.push(obj.value);       return values;     }     else {       return values;     }   } })(); // make it self-executing console.log(values(obj1)); // Returns: [obj.value] console.log(values(obj2)); // Returns: [obj.value, obj2.value]

Right on! A pure, high-order, anonymous function. How did we ever get so lucky? Actually, it's more than that. It's also self-executing as indicated by the structure, . The pair of parentheses following the anonymous function causes the function to be called right away. In the above example, the instance is assigned to the output of the self-executing function call.

Note

Anonymous functions are more than just syntactical sugar. They are the embodiment of Lambda calculus. Stay with me on this… Lambda calculus was invented long before computers or computer languages. It was just a mathematical notion for reasoning about functions. Remarkably, it was discovered that—despite the fact that it only defines three kinds of expressions: variable references, function calls, and anonymous functions—it was Turing-complete. Today, Lambda calculus lies at the core of all functional languages if you know how to find it, including JavaScript.

For this reason, anonymous functions are often called lambda expressions.

One drawback to anonymous functions remains. They're difficult to identify in call stacks, which makes debugging trickier. They should be used sparingly.

Назад: Pure functions
Дальше: Method chains

bsn
thank
Vesa Karvonen
I hope you don't mind, but I’d like to point you and your readers to my high-performance optics library for JavaScript that is in production use in multiple projects, has comprehensive support for partial optics and interactive documentation: https://calmm-js.github.io/partial.lenses/ (this takes a moment to load — be patient!)