Search code examples
javascriptfunctionecmascript-5

What precisely is a function value?

In JavaScript (ECMAScript 5), functions are valued (they are told to be "first-class functions").

This allows us to use them as expressions (an expression is everything which produces a value, and can contain other expressions : var exp0 = (exp1) + exp2 - exp3.function(); is a grammar-correct statement).

In the code above, there are 8 expressions : exp0, exp1, (exp1), exp2, (exp1) + exp2, exp3, exp3.function() and (exp1) + exp2 - exp3.function().

Because functions can be used as expressions, the following code is correct :

var my_function_0 = function a() {} is a named function expression.

The following code is also correct :

var my_function_1 = function(){}` is an anonymous function expression.

Both are valued, both are values.

Now, consider the code below :

function requiredIdentifier() {}

It is NOT a "named or anonymous function expression", but a function declaration.

My question is :

Does a declared function have/produce a value ?

This question is equivalent to this one : Is a declared function an expression ? (even if it's not a named or anonymous function expression ?!)

Solution

  • Does a declared function have/produce a value?

    Yes. Regardless what syntax is used to create the function, a function is a callable object (i.e. it implements an internal interface that makes it callable):

    function a() {}
var b = function() {}
var c = (new Function()) // or some other expression that returns a function

    All of the variables a, b and c hold a function value.

    The difference between the syntaxes is only when the value is created and whether/when it is bound to a variable. See var functionName = function() {} vs function functionName() {} for those details.