Console Learner — An interactive JavaScript course.

2.6 Loops & Iteration

JavaScript supports four statements for looping (iteration):

for (init; test; update)
    statement
This for syntax is really only a short-form of the following while syntax:

init;
while (test) {
    statement
    update;
}
for (variable in object)
    statement
The variable expression must evaluate to a variable name or declaration. It is assigned to each of the object property names (in order). 
for (variable of iterable)
    statement
The variable expression must evaluate to a variable name or declaration. This is similar to the for...in loop, but works for Array and other iterables (not covered here). 
while (condition)
    statement
While the condition expression evaluates to true, the statement is executed. The loop terminates when condition evaluates to false. 
do
    statement
while (condition)
The statement is executed for each pass through the loop. After each pass, the condition expression is evaluated. The loop terminates when condition evaluates to false.
>>>
var arr = ['a', 'b', 'c'],
    obj = { a: 1, b: 2, c: 3 };


  

    
>>>

    
for (var i = 0; i < arr.length; i++) {
    print(i + ': ' + arr[i]);
}

  

  

  

    
    Looping over the entries in an array is achieved by
    stepping a variable from zero to the last index.
  


  

    
>>>

    
for (var key in obj) {
    print(key + ': ' + obj[key]);
}

  

  

  

    
    Looping over the properties in an object is similar, but
    uses a slightly different syntax.
  


  

    
>>>

    
for (var key in arr) {
    print(key);
}

  

  

  

    
    Since arrays are also objects, the array indices can be iterated
    as properties. Note that they are returned as strings when
    handled this way. And the length property is
    hidden (not enumerable).
  


  

    
>>>

    
for (var val of arr) {
    print(val);
}

  

  

  

    
    The more compact way to iterate over an array is usually the
    for...of loop (added in ES6). It is less prone to
    mistakes, but wasn't available in all major browsers until 2015.
  


  

    
>>>

    
while (true) {
    print('stopping the loop...');
    break;
}

  

  

  

    
    The break and continue statements
    are used to stop or proceed to the next step in a loop.
  


  

    
>>>

    

    
    
        
    
  







  
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