3. Advanced Loops #

Created Saturday 18 July 2020

Aren’t for/while enough? #

No, because they:

• Work on iterables, but with a potential for off-by-one errors.
• Don’t work on objects.

1. forEach - array #

• Purpose - iterate over an array.
• Since ES5.
• Syntax

  myIterable.forEach(f); // f(ele) for all elements, returns nothing
  
  f = function(a){};	// a  = element value
  f = function(a, b){}; // a = element value, b = index
  

• Arguments passed to function, in order:
  1. Element
  2. Index, only for arrays. OPTIONAL
• Gotcha - no way to break or continue

2. for in - object #

• Purpose - iterate over an object.
• Iterates over fieldNames, not values.
• MAID(in-enumerate object)

  let timeSpace = { place:'Earth',  era:'Phanerozoic'};
  for(const ele in x)
  	console.log(ele);
  /* fields not values
  place
  era
  */
  

• Cons
  • Works only for one iterable - array.
  • Iteration will include functions, in addition to indices of the array. See.
  • Loop variable is the index instead of element for arrays.
• Note: No provision for index as 2nd param.
• Advice - avoid with iterables.

3. for of - any iterable #

• Purpose - iterate over any iterable.
• Solves the problem of iterables in for in. Is a generalized forEach.

  // arrays
  for(const ele of [1,2,3])
  	console.log(ele);
  
  // set
  let s = new Set(['H', 'A', 'C', 'K']);
  for(const i of s)
  	console.log(i);
  

• To access array index, run the loop for arr.entries(), which is an array of [index, value] 2-arrays. Use destructuring for concise code.

  let s = ['H', 'A', 'C', 'K'];
  for(const i of s.entries())
  	console.log(i[1]);
  /*
  H
  A
  C
  K
  */
  
  // better - destructuring
  for(const [i, j] of s.entries())
  	console.log(i)
  /*
  0
  1
  2
  3
  */