Javascript's ternary operator semantic

Question

I'm trying to dive into the src code of jQuery, and at the very beginning of the the src code in the development version(3.2.1) of jQuery we find:

( function( global, factory ) {
"use strict";
if ( typeof module === "object" && typeof module.exports === "object" ) {
    module.exports = global.document ?
        factory( global, true ) :
        function( w ) {
            if ( !w.document ) {
                throw new Error( "jQuery requires a window with a document" );
            }
            return factory( w );
        };
} else {
    factory( global );
}

As far as my knowledge is concerned, I know that ternary operator comes after some condition, but here it comes after an assignment operator module.exports = global.document ?. Could you explain this? What it does in the context of the previous code?

ternary operator goes after anything really. If its not a boolean, it will be converted as such. So if global.document is null or undefined or "" or any other falsy value, it will be considered as false and if not, true — juvian, May 31 '17 at 15:43
That means if global.document is true then module.exports is assigned global.document. If global.document is falsy then function( w ) is assigned to module.exports which is being exported — Chris, May 31 '17 at 15:44
Possible duplicate of [How do you use the ? : (conditional) operator in JavaScript?](https://stackoverflow.com/questions/6259982/how-do-you-use-the-conditional-operator-in-javascript) — juvian, May 31 '17 at 15:47

Ilario Pierbattista · Accepted Answer · 2017-05-31T15:59:42.697

The ternary operator allow to conditionally evaluate two expressions (the first in case of the condition is true, the second one otherwise).

module.exports = global.document ?
        factory( global, true ) :
        function( w ) {
            if ( !w.document ) {
                throw new Error( "jQuery requires a window with a document" );
            }
            return factory( w );
        };

The assignment means that, the result of the conditional evaluation is assigned to module.export.

The ternary operator is a syntactic sugar and works like this:

function ternary_operator(condition, expression1, expression2) {
    if (condition) {
        return expression1;
    } else {
        return expression2;
    }
}

so you can translate the code to:

module.exports = ternary_operator(global.document,
        factory( global, true ),
        function( w ) {
            if ( !w.document ) {
                throw new Error( "jQuery requires a window with a document" );
            }
            return factory( w );
        });

Please, pay attention to:

factory( global, true )

and

    function( w ) {
        if ( !w.document ) {
            throw new Error( "jQuery requires a window with a document" );
        }
        return factory( w );
    };

They are both expressions: the result of the activation of the factory function is an expression; the definition of a function value is an expression too.

Lazy evaluation

The function ternary_operator provides an eager ternary operator. The expressions are evaluated within the condition, and then one of them are returned. For example:

var l = ternary_operator(C, A, B);
1. evaluate C
2. evaluate A
3. evaluate B
4. activate function ternary_operator
5. assign to l the result of the function

Javascript ternary operator is lazy: that means that the condition is firstly evaluated, then just one of the other expression is evaluatued, gaining in terms of performance.

var l = C ? A : B;
1. evaluate C
2.1 if C is true evaluate A
2.2 if C is false evalutate B
3. assign to l the expression evaluated

score 1 · Answer 2 · answered May 31 '17 at 15:48

a=b?c:d;

In this case the ternary operator does not check for a=b but rather for b. Thats because the assignment operator has a higher operator predescendence than the ternary operator, for a good reason.

(a=b)?c:d;

In this case the ternary operator really tests for a=b. In this case it will store b in a and return this value.

Javascript's ternary operator semantic

2 Answers2

Lazy evaluation