I'm confused about use of the array.filter() method at 2:40.

It's correct that the callback function needs to return a boolean value, but what's happening in video at 2:40 is that he returns an integer instead. This is fine because javascript can be quite helpful without you asking it to be.

Javascript knows this callback should be a boolean, so when his function return, say, the number 4, javascript will do something called type coercion and try to convert the value to a boolean value.

For example;

• Boolean(4) => true

• Boolean(undefined) => false

Here is a implementation of a filter function:

Array.prototype.customFilter = function (callback) {
    var arr = [];

    for (var i = 0; i < this.length; i++) {
        //Here we call the passed in function on each element
        //to determine if we should include it or not
        if (callback(this[i])) {
            arr.push(this[i]);
        }
    }
    return arr;
}

var arr = [1, 2, 3, 4];



var newArr = arr.customFilter(function (n) {
    if(n % 2 == 0) {
        //Instead of returning n
        //You could return true - that would give the same result
        return n;
    }
});

console.log(newArr);

Hey Dave! I see what you mean. This seemed unusual to me too. It's seems far more natural to use the syntax below. To each their own I guess :).

$scope.todos.filter(function(todo) { return todo.edited; })