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JavaScript Iterables

Objects that can be iterated over with for..of are called iterable.

Technically, iterables must implement the Symbol.iterator method.

Iterating Over a String

You can use a for..of loop to iterate over the elements of a string:

Example

for (const x of "W3Schools") {
  // code block to be executed
}
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Iterating Over an Array

You can use a for..of loop to iterate over the elements of an Array:

Example

for (const x of [1,2,3,4,5] {
  // code block to be executed
}
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JavaScript Iterators

The iterator protocol defines how to produce a sequence of values from an object.

An object becomes an iterator when it implements a next() method.

The next() method must return an object with two properties:

  • value (the next value)
  • done (true or false)
value The value returned by the iterator
(Can be omitted if done is true)
done true if the iterator has completed
false if the iterator has produced a new value


Home Made Iterable

This iterable returns never ending: 10,20,30,40,.... Everytime next() is called:

Example

// Home Made Iterable
function myNumbers() {
  let n = 0;
  return {
    next: function() {
      n += 10;
      return {value:n, done:false};
    }
  };
}

// Create Iterable
const n = myNumbers();
n.next(); // Returns 10
n.next(); // Returns 20
n.next(); // Returns 30
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The problem with a home made iterable:

It does not support the JavaScript for..of statement.

A JavaScript iterable is an object that has a Symbol.iterator.

The Symbol.iterator is a function that returns a next() function.

An iterable can be iterated over with the code: for (const x of iterable) { }

Example

// Create an Object
myNumbers = {};

// Make it Iterable
myNumbers[Symbol.iterator] = function() {
  let n = 0;
  done = false;
  return {
    next() {
      n += 10;
      if (n = 100) {done = true}
      return {value:n, done:done};
    }
  };
}

Now you can use for..of

for (const num of myNumbers) {
  // Any Code Here
}
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The Symbol.iterator method is called automatically by for..of.

But we can also do it "manually":

Example

let iterator = myNumbers[Symbol.iterator]();

while (true) {
  const result = iterator.next();
  if (result.done) break;
  // Any Code Here
}
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