I've been working on an angular project I inherited and I can't contact the original author. There is this watch expression I'm not sure of and I was wondering if someone could help me understand the code:

$scope.$watch(calculator.value, function(newVal, oldVal) {
    if(newVal !== oldVal) {
        i = newVal;
        updateCalculation();
    }
});

What confuses me is this line of code: newVal !== oldVal. Any idea why one would need such a check?

I've been working on an angular project I inherited and I can't contact the original author. There is this watch expression I'm not sure of and I was wondering if someone could help me understand the code:

$scope.$watch(calculator.value, function(newVal, oldVal) {
    if(newVal !== oldVal) {
        i = newVal;
        updateCalculation();
    }
});

What confuses me is this line of code: newVal !== oldVal. Any idea why one would need such a check?

• javascript
• angularjs

• Not familiar with $watch but isn't it just checking if the value changed? – MinusFour Commented Oct 13, 2015 at 14:34

3 Answers 3

The $watch method of a scope will always execute the callback at least once. The first time it does the new and old values are the same.

$scope.$watch(calculator.value, function(newVal, oldVal) {
    if(newVal === oldVal) {
       console.log('First time watcher is executed');
    }
    if(newVal !== oldVal) {
       console.log('Watcher called because values changed.');
    }
});

The sample you gave states that it will only execute updateCalculations() when the values have changed. Not the first time the watcher is called.

The $watch will get called for each turn of the $digest loop even if the value hasn't changed. So I can only assume that updateCalculation() should only be called if the value changed.

1. There is a value on the scope like $scope.calculator.value = 0;
2. Watch will call with every digest but might be this value is not changed
3. So match old value with new value and call the method if only there is any change