how to pass a variable by ref reference in javascript

Question

I have a scope variable which has false value by default.

I want when I pass this variable to a function to modify it's value to true.

but this can't be done since the passage of params in JavaScript is by value.

this is a simple code of what I'm tryin to do:

myapp.controller('PersonCtrl', function ($scope) { 
    $scope.step = false;
    change($scope.step);
    console.log($scope.step);
});

change = function(step){
  step = true;
}

jsfiddle:

http://jsfiddle.net/SNF9x/177/

how can I solve this ?

pass the $scope instead of $scope.step if possible, else just set $scope.step to the return value of the function. — hylimR
– hylimR, Commented Apr 20, 2016 at 14:18
@HarishKumar In any non-trivial application, global variables quickly become cumbersome and prone to conflicts — James Thorpe
– James Thorpe, Commented Apr 20, 2016 at 14:20

Nick D · Accepted Answer · 2016-04-20 14:18:24Z

5

You can pass the object holding the primtive to your change function:

myapp.controller('PersonCtrl', function ($scope) { 
    $scope.step = false;
    change($scope);
    console.log($scope.step);
});

change = function(obj){
  obj.step = true;
}

You could event pass the name of the variable you want to change:

myapp.controller('PersonCtrl', function ($scope) { 
    $scope.step = false;
    change($scope, 'step');
    console.log($scope.step);
});

change = function(obj, prop){
  obj[prop] = true;
}

answered Apr 20, 2016 at 14:18

Nick D

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Reinstate Monica Cellio · Accepted Answer · 2016-04-20 14:18:29Z

3

You can't pass variables by reference in JS, but you can pass objects by reference and then modify the properties, like this...

myapp.controller('PersonCtrl', function ($scope) { 
    $scope.step = false;
    change($scope);
    console.log($scope.step);
});

change = function($scp){
  $scp.step = true;
}

jsfiddle:

http://jsfiddle.net/SNF9x/178/

answered Apr 20, 2016 at 14:18

Reinstate Monica Cellio

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5 Comments

anon Over a year ago

I would recommend against starting the parameter name with a $.

Reinstate Monica Cellio Over a year ago

I would not, since the original variable uses a $ prefix, but it's neither right nor wrong and just preference.

anon Over a year ago

I'm thinking that it would cause confusion; generally (or at least in my experience) $var means it's global. If it isn't, in a larger method it may end up being confusing.

Reinstate Monica Cellio Over a year ago

As I said, it's a preference. I personally use _ to prefix global variables, and $ if it's a jQuery object. In this question I simply left it with the prefix it already had. This really isn't a point for discussion though, as it's nothing more than a choice.

anon Over a year ago

Oh, I must have missed those last three words. Fair enough.

Alex Char · Accepted Answer · 2016-04-20 14:40:46Z

1

In your OP you try to change the value of a primitive type:

var o = {
  b: true
};
changeValue(o.b);//pass a primitive type to the function
console.log(o.b);//print out true because is primitive type and all primitive types pass by value

function changeValue(b) {
  b = false;
}

One solution will be to pass the object $scope to the function:

var myapp = angular.module('myapp', []);

myapp.controller('PersonCtrl', function($scope) {
  $scope.step = false;
  change($scope);
  console.log($scope.step);//prints out true
});

change = function(obj) {
  obj.step = true;
}

fiddle

edited Apr 20, 2016 at 14:40

answered Apr 20, 2016 at 14:18

Alex Char

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