Implementing double parentheses in the source code of an AngularJS input directive using JavaScript

Question

Implementing double parentheses in the source code of an AngularJS input directive using JavaScript

I came across some unfamiliar syntax in the angularJs source code. Can someone provide an explanation for the expression within the parentheses of logical operators and dependency injection in "(inputType[lowercase(attr.type)] || inputType.text)(scope, element, attr, ctrls[0], $sniffer, $browser, $filter, $parse);"?

Here's the context:

var inputDirective = ['$browser', '$sniffer', '$filter', '$parse',
    function($browser, $sniffer, $filter, $parse) {
  return {
    restrict: 'E',
    require: ['?ngModel'],
    link: {
      pre: function(scope, element, attr, ctrls) {
        if (ctrls[0]) {
          (inputType[lowercase(attr.type)] || inputType.text)(scope, element, attr, ctrls[0], $sniffer, $browser, $filter, $parse);
        }
      }
    }
  };
}];

javascript angularjs syntax

Answer 1

Answer №1

When making a function call in JavaScript, the specific function being called is determined by the initial part of the code:

(inputType[lowercase(attr.type)] || inputType.text)

This concept is easier to understand with a concise example:

var functionToExecute = ...;
var obj = {
  goodbye: function(name) { alert('Goodbye ' + name); } 
  hello: function(name) { alert('Hello ' + name); }
};
(obj[functionToExecute] || obj.hello)(name);

If functionToExecute == 'goodbye', it will trigger obj.goodbye(name); if functionToExecute == 'hello', it will execute obj.hello(name). However, for any other value of functionToExecute, the default function called will be hello.

Answer 2

When making a function call in JavaScript, the specific function being called is determined by the initial part of the code:

(inputType[lowercase(attr.type)] || inputType.text)

This concept is easier to understand with a concise example:

var functionToExecute = ...;
var obj = {
  goodbye: function(name) { alert('Goodbye ' + name); } 
  hello: function(name) { alert('Hello ' + name); }
};
(obj[functionToExecute] || obj.hello)(name);

If functionToExecute == 'goodbye', it will trigger obj.goodbye(name); if functionToExecute == 'hello', it will execute obj.hello(name). However, for any other value of functionToExecute, the default function called will be hello.

Answer 3

Answer №2

Perhaps a different perspective will help you grasp the concept:

var action = inputCategory[lowercase(attribute.category)];
if(!action) action = inputType.text;
action(scope, element, attribute, controllers[0], $sniffer, $browser, $filter, $parse);

|| is known as the logical OR operator. When used in expr1 || expr2, it will return expr1 if it can be interpreted as true; otherwise, it will return expr2.

Answer 4

Perhaps a different perspective will help you grasp the concept:

var action = inputCategory[lowercase(attribute.category)];
if(!action) action = inputType.text;
action(scope, element, attribute, controllers[0], $sniffer, $browser, $filter, $parse);

|| is known as the logical OR operator. When used in expr1 || expr2, it will return expr1 if it can be interpreted as true; otherwise, it will return expr2.

Answer 5

Answer №3

This could possibly indicate that inputType[lowercase(attr.type)] refers to a function named inputType.text. The second set of parentheses likely contain the arguments for the function.

Take a look at this basic example of the inputType object in action.

var inputType = {
    text : function(scope, element, attr, ctrls, sniffer, browser, filter, parse){
        //perform tasks
    },
    button : function(scope, element, attr, ctrls, sniffer, browser, filter, parse){
        //perform tasks
    }
}

If the value returned by attr.type is not "text" or "button", it falls back to using the text function and executes it with the specified parameters.

Answer 6

This could possibly indicate that inputType[lowercase(attr.type)] refers to a function named inputType.text. The second set of parentheses likely contain the arguments for the function.

Take a look at this basic example of the inputType object in action.

var inputType = {
    text : function(scope, element, attr, ctrls, sniffer, browser, filter, parse){
        //perform tasks
    },
    button : function(scope, element, attr, ctrls, sniffer, browser, filter, parse){
        //perform tasks
    }
}

If the value returned by attr.type is not "text" or "button", it falls back to using the text function and executes it with the specified parameters.

Answer 7

Answer №4

Unfamiliar with the specific location in the angular source where this code resides, I am unsure of the exact whereabouts of the inputType object containing various functions for different input types, with its default set to text.

The following code:

(inputType[lowercase(attr.type)] || inputType.text)

makes use of the logical OR (||) operator. This operator returns the left operand if it is true; otherwise, it returns the right operand if the left one evaluates to false or undefined.

Essentially, this code looks up attr.type in the inputType object. If found, it returns that function; if not, it defaults to the function assigned to inputType.text. The arguments passed to this resulting function are enclosed in parentheses.

Using the || operator in this manner provides a convenient way to establish default values for incoming arguments. Another alternative you may have encountered is the ternary operator:

var func = inputType[lowercase(attr.type)] ? inputType[lowercase(attr.type)] : inputType.text;
func( scope, element, attr, ... );

In this scenario, the || version appears more sleek, especially when dealing with straightforward true/false conditions.

Answer 8

Unfamiliar with the specific location in the angular source where this code resides, I am unsure of the exact whereabouts of the inputType object containing various functions for different input types, with its default set to text.

The following code:

(inputType[lowercase(attr.type)] || inputType.text)

makes use of the logical OR (||) operator. This operator returns the left operand if it is true; otherwise, it returns the right operand if the left one evaluates to false or undefined.

Essentially, this code looks up attr.type in the inputType object. If found, it returns that function; if not, it defaults to the function assigned to inputType.text. The arguments passed to this resulting function are enclosed in parentheses.

Using the || operator in this manner provides a convenient way to establish default values for incoming arguments. Another alternative you may have encountered is the ternary operator:

var func = inputType[lowercase(attr.type)] ? inputType[lowercase(attr.type)] : inputType.text;
func( scope, element, attr, ... );

In this scenario, the || version appears more sleek, especially when dealing with straightforward true/false conditions.

Implementing double parentheses in the source code of an AngularJS input directive using JavaScript

Answer №1

Answer №2

Answer №3

Answer №4

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