CODEWITHSUNDEEP
javascriptarrayspredicate
Huy Tran
Huy Tran

Reputation: 1902

How to write the Array.prototype.every() method in Javascript

As practice, I want to write a function all() that works similar to the Array.prototype.every() method. This function returns true only if the predicate supplied returns true for all the items in the array. 

Array.prototype.all = function (p) {
  this.forEach(function (elem) {
    if (!p(elem)) 
      return false;
  });
  return true;
};

function isGreaterThanZero (num) {
  return num > 0;
}

console.log([-1, 0, 2].all(isGreaterThanZero)); // should return false because -1 and 0 are not greater than 0

Somehow this doesn't work and returns true. What's wrong with my code? Is there a better way to write this?

Upvotes: 0

Views: 1031

Answers (4)

Huy Tran
Huy Tran

Reputation: 1902

You can only stop a forEach() loop by throwing an exception. Just use a normal for loop instead.

Array.prototype.all = function (p) {
  for(var i = 0; i < this.length; i++) {
    if(!p(this[i])) {
      return false;
    }
  }
  return true;
};

If any method other than Array.prototype.every() is allowed to be used, then you can: 

Array.prototype.all = function (p) {
  return this.filter(p).length == this.length;
};

Upvotes: 0

Ori Drori
Ori Drori

Reputation: 191976

You can't break out of an Array#forEach loop by returning. Use a for loop instead.

Note: This is a partial implementation of Array#every to demonstrate the return issue.

Array.prototype.all = function (p) {
  for(var i = 0; i < this.length; i++) {
    if(!p(this[i])) {
      return false;
    }
  }
  
  return true;
};

function isGreaterThanZero (num) {
  return num > 0;
}

console.log([-1, 0, 2].all(isGreaterThanZero));

Upvotes: 2

Arnav Aggarwal
Arnav Aggarwal

Reputation: 789

The other answers are slightly wrong. The callback you pass in should be called with 3 arguments: the current item, the index, and the entire array. This is how the native Array.every, and indeed most native array functions, work. Your callback can choose to use these arguments but most of the time it doesn't.

Array.prototype.all = function (p) {
  for(var i = 0; i < this.length; i++) {
    if (!p(this[i], i, this)) {
      return false;
    }
  }

  return true;
};

function isGreaterThanZero (num) {
  return num > 0;
}

console.log([-1, 0, 2].all(isGreaterThanZero)); // should return false because -1 and 0 are not greater than 0

Upvotes: 0

mehmet baran
mehmet baran

Reputation: 646

return in foreach function is not returning value from your function. You can code like this. 

        Array.prototype.all = function (p) {
            for(var i = 0; i < this.length; i++){
                if (!p(this[i])) 
                    return false;
            }                
            return true;
        };

        function isGreaterThanZero (num) {
            return num > 0;
        }

        var result = [2, 3, 4].all(isGreaterThanZero);
        console.log("result", result);

Upvotes: 0

Related Questions